Two Six Pac's On The Bench

hi everyone and welcome to another episode of mr. Carlson's lab on the bench behind me today I have 2a es6 Pack audio amplifiers so that's a total of twelve l34 or 6c a7 output tubes between the two amplifiers that's a whole lot of output tubes so the story on these amplifiers goes like this a very good friend of mine purchased these some time back he purchased them used and paid a fair amount of money for them he didn't hook them up because his sound room wasn't set up at that time so he finally got his sound room setup hooks the amplifiers up and says Paul these things don't sound quite right he says one of them has quite an audio Bowl hum and he says maybe you can just go through both of them and make sure they're up to spec so that's what we're going to do together today we're gonna go through these amplifiers diagnose them find the issues and fix them now there's not a whole lot of information out there about these amplifiers so we're gonna also run some tests on these amplifiers today we'll take a look at some distortion specs then we're going to reverse engineer this amplifier I'm gonna draw up a schematic and we'll check out their design see how well they did so let's get started here's a look at the front side of the 6-pack monoblock amplifiers and I do admit they look pretty impressive with all these output tubes here imagine they get pretty hot as well so these OPA tubes are el34s or 6 CA 7s this tube here is an IL 84 vacuum tube now in many other amplifiers the el84 is used as an audio output tube itself what it's doing on this chassis at this point I cannot tell you because I haven't drawn up a schematic and I haven't reverse engineered these yet now there is some talk online about what this vacuum tube does I don't want to repeat that without knowing for sure so we'll talk about the tube functions after I drop a schematic and reverse engineer these things this tube here is a 12 ax7 there is nothing written on the chassis here because you're supposed to be able to try different tubes in the socket and experiment with the sound here's the thing with this particular tube this tube is most likely going to be the phase inverter this is a push-pull amplifier and there are two triodes in here if you're going to be experimenting with this tube you want to know the state of the tube that you're plugging in here to experiment with for example if you're buying a used say mullard Telefunken or maybe something fancy with a Bugle Boy on the side of it and you're plugging it into the chassis and experimenting with the sound unless it's a brand new tube you don't know whether the two triodes within that vacuum tube are matched here's the thing with phase inverter circuits the phase inverter is the heart of any push-pull amplifier and any mismatch in these two trials within this vacuum tube is going to introduce Distortion here's the thing with an amplifier like this you have switchable feedback here so you can turn the feedback right off or you can turn it to negative 10 DB with the feedback off there is no feedback in here to reduce distortion and to you know so basically widen out the response so you want to start off with the lowest distortion that you can get so you want to have a matched tube in the phase inverter socket if you don't have a way to match or find out what the triodes are doing inside your used vacuum tubes your best bet is to plug in a brand new tube now I'm going to let you in a little secret here if you don't know this already a lot of these fancy tubes like the mauler to tell functions and and other bugle boys all those kinds of vacuum tubes come out a lot of while we would say expensive tester way back in the day now here's the thing with those used vacuum tubes this tube here 12ax7 say you're replacing this with a used 12 ax7 all right the 12ax7 has two triodes inside it now if you're going to be putting a used tube in here and say it came out of a an old oscilloscope or signal generator or something like that a lot of the time each triode within that vacuum tube does a different service so one triode will be worked harder throughout its service life than the other triode meaning that one trial it will probably be a little bit more tired than the other one that creates a big imbalance imbalance introduces distortion and this is very very common in tes Keir a lot of the times the triodes in the vacuum tube are used for very different functions so keep that in mind using used vacuum tubes anywhere else in the amplification chain is absolutely fine for the phase inverter again it's the heart of a push-pull vacuum tube amplifier you either want to use a brand new tube or you're gonna want to know the state of the triodes in your used vacuum tube basically you want a very thorough test on that vacuum tube now down the road together we're going to design a vacuum tube matching device a very simple device it'll probably just use a bunch of LEDs and things like that to indicate how close each triode is to each other and we'll get into that down the road that greatly exceeds the scope of this video here so we'll design something like that down the road so what else can I tell you about this amp here let's go back to the feedback here so the feedback is really a personal thing when you switch the feedback off you're gonna have a peak in the response pattern so for example say there the peak is around a female vocalists voice alright so say the peak and the response pattern is right around her voice what it's going to do is it's going to make that female vocalist sound like she's standing out from the rest of the instruments in the band when you click the feedback on it flattens the response pattern or makes it more flat let's talk about when it'll explain it like that makes it more flat so it gets rid of that peak maybe now it's just a little bit of a bump where you know her voice is alright what its gonna do is settle her off and push her back into the rest of the band that's because it's flattening out the response pattern so many people like no feedback and that's really the effect of putting feedback on and off and of course adding feedback lowers the distortion within the amplifier and we'll take a look at this amplifiers distortion here in a little bit we'll run some really extensive tests on one of these amplifiers here and take a look at what they can do we'll run it through it paces there so so let's see what else can I tell ya on/off switch here light filter capacitors in the back here can I see those on camera I can see those on camera so these are the filter caps right here they're placing them far away from the vacuum tubes here because the tubes get hot and these these filter caps have got a plastic coating on them that'll shrink if they get too hot so they've placed away from the el34s and again I imagine that these things would heat a small room quite nicely when they're just sitting in idling power transformer in the back so this is creating the B+ for all of the tubes here so there's going to be quite a bit of high voltage under this chassis if you're unfamiliar with vacuum tube amplifiers I strongly suggest you read up on them and learn about high voltage what this big transformer here does is it takes what comes from the wall and steps it up to about 400 plus volts so it makes what comes out of the wall much more deadlier than it started out to be so there's some pretty deadly voltages under the chassis zuv vacuum tube amplifiers so if you're experimenting with any type of a vacuum tube amplifier know that and be very very careful and if you're following along you're doing so at your own risk be careful this here is going to be the audio output transformer and chances are this is going to be placed on a 90 so that these two transformers do not couple with each other so the audio output transformer will listen to the power transformer and you know they'll be induced hum in the in the speaker lines so what they do is they take the Transformers and place them 90 so that they don't couple so well nice thing about this amplifier is it's probably placed on a 90 I'm imagining it is and they have a shield on this transformer as well so the shield again is separating the Transformers you can see on this chassis they're extremely close together so whenever you're designing any amplifier you really want the audio output transformer to be as far as possible from the power transformer and you want the audio amplification chain the vacuum tubes to be as far away from the power transformer as well so the further you can get these things from the power supply the better so the audio output tubes not so incredibly particular about of course you don't want them to be too incredibly close but when you're dealing with your high gain stages in the front end of the amplifier they need to be away from this big fellow right here let's see what else can I tell you about this here I think that pretty much covers this so what I'm gonna do now is get rid of one of these things off the bench here and we'll take a look at the backside and I'll explain what it does now I don't know which of these amplifiers is humming and you know which one isn't sounding quite right so what we'll do is we'll do some experimentation and find out which one is making the noise without taking any covers off so I'll show you guys how to determine whether the hum coming from an amplifier is from a vacuum tube or from a power supply so that's coming up here quite shortly this is the back side to one of these 6-pack monoblock amplifiers this is the audio input jack here so standard RCA type Jack this is a quarter inch Jack here this is used in conjunction with this control here to set the cathode current to all of the el34 tubes here again el34 6c a7 whatever you prefer in order to set the bias on one of these amplifiers you do need some form of an external current meter that external current meter would get plugged into this jack here and what happens is there's a switch inside this jack that switch gets opened and what it does is it disconnects the cathodes of all of these el34s from the chassis and routes the cathodes through your current meter and back to the chassis again so that allows you to read the cathode current of all of these el34s in order to adjust the bias you slowly advance this control to the desired cathode current and for this amplifier they specify 220 to 230 million and I'll go over how to set this here in just a little bit one thing that's of paramount importance when you receive one of these amplifiers if it's being used or you know it's not brand new out of the box they say that the bias is factory preset I would even you know be worried about that what you should do before you even plug anything into this am so no a/c is applied to this take this control and put it to its extreme counterclockwise position what that does is that puts the most negative voltage on the control grids of these vacuum tubes so basically you're turning these vacuum tubes off when you have this at its extreme counterclockwise position once the amplifier has been warmed up for a set period of time for argument's sake we'll say ten minutes is a nice comfortable time to let everything stabilize what you're going to do with your current meter in line and plugged into this jack is advance this control until you reach your desired cathode current at that point what you do is basically you can shut the amplifier off remove your jack and then the bias is set at that point and then every time you turn your amplifier on it should be correct if you ever change any vacuum tubes and after the amplifier has been running for a while it's always a good idea to every now and then just check to make sure that the bias is okay now the reason that this is so incredibly important is just say this is up here like this and you turn the amplifier on what happens is is the cathode current to these vacuum tubes is going to be extremely high there's a chance it's going to blow the fuse that's hiding down in here and if it sets at a really high current for an extremely long time that is hard on the power transformer now you there is no way to set the bias control on an amplifier like this by ear or anything like that you know you can't say that oh it sounds better up here and then it sounds here this has to be set with a meter it's very very important because you can damage the tubes and you can also damage the transformer and take the fuse out if for some reason the the current on these tubes the cathode current is too high so always keep that in mind this is a very very important control now somebody on this amplifier has put a knob on this you can see that there's a knob on here I believe from the factory there's just a shaft protruding from the back of the chassis even that I'm not really comfortable with when I've designed an amplifier like this any kind of a monoblock or any other type of amplifier the control is always countersunk in the chassis or there is some form of a retainer not on the actual shaft so that you can't move that shaft inadvertently now I don't know why they have the shaft protruding from the back and they didn't use a countersunk you know control in here basically if the control is countersunk you stick a screwdriver in advance it and it's safe you know if you're touching the amplifier or anything like that you can't bump the control you know and you know basically take the thing out of adjustment and the bias control on these amplifiers is you know it's a it's a real crucial thing and I'll show you what I mean here when I'm setting the bicep I'll show you exactly how to set the bias up here in a little bit so this control is very important to have sitting in the right position and again if you change any vacuum tubes on here or even move one around anything like that you know if you're handling them taking them out of the chassis it's always a good idea just to check the bias again make sure everything is okay over here we have the output jack so this just runs to the speaker we have a switch on the bottom which selects the impedance we have a fuse over here now it says tube fuse what this exactly does inside here at this point again I haven't drawn the schematic so I'll determine this in a little bit I have an idea of what it does but again I don't want to say anything until I've determined it's it's function so we have a fuse here and there's also a fuse hiding in the bottom down here so don't forget about this one right here if for some reason you turn the switch on and this fuse looks okay the fuse in here could be shot so what I'm going to do is turn that to its extreme counterclockwise position I'm going to apply power to this amplifier and I will show you how to set the bias next this is how you set the bias on a sixpack monoblock amplifier I have my current meter attached to the bias jack here now this is an older panel meter and it works well with the camera you don't need to use an old meter like this if you have a modern digital multimeter that has the ability to read current you can use that as well the first thing you want to do is make sure the bias adjustment is completely counterclockwise so it's at its top that point turn the filaments on let the vacuum tubes warm up for a set period of time again ten minutes would be fine let everything stabilize I'm not going to wait that long for the adjustment here I'm just going to give you the idea of how this works and then we'll move on from there now you'll notice that the vacuum tubes will draw just a little bit of current even with this completely counterclockwise that's absolutely fine if you were to turn this on and it went up to 250 milliamps with this completely counterclockwise you would definitely have some form of a problem either with a vacuum tube or the by circuit itself so this is going to settle off here in just a moment all the cathodes in these el34s are glowing nice and orange they sure do have a lot of cathode area okay so now what I'm going to do is advance this until this reads 220 milliamps let's roll this up I'll get it close it looks to be close to 220 now it can be 220 to 230 s absolutely fine at this point after that 10-minute warmup period you would shut the amplifier off remove this jack and the bias is set just make sure that you do not bump that control in any way shape or form and that's just how easy it is to set the bias now to give you an idea of how sensitive this control is if I move this just a little bit more over watch what happens to the current here so this is at a boat wall just a little bit past 12 o'clock if I move that towards one o'clock I can bring this rate to 300 mils no problems right very quickly now brief excursions like that isn't going to hurt anything but that was only one o'clock this control can go all the way down to five o'clock so you can imagine I'll just turn that back down to 220 or there are boats it's close enough so you can imagine what would happen if this was completely clockwise and you flipped the switch once the filaments came the thing would draw heavy current the fuse would go away and the amplifier would go dark so a lot of bias adjustments on guitar amplifiers and things like that only have a limited range so that the owner really can't cause too much damage if the bias adjustment is way off there is no limited range with this bias adjustment so you can basically turn these tubes right on so not only is that hard on the power transformer and everything else it's also hard on the windings in the audio output transformer as well so you can see how very important it is to have that bias adjustment in the right area so what I'm gonna do now is shut this off adjust the bias in the other amplifier and get this ready to test so we can find out which amplifier is humming both amplifiers are attached to the oscilloscope this amplifier here is the upper channel this amplifier here is the lower channel so what I'm seeing on the oscilloscope right now is indicating a bunch of different issues I'll just zoom on into the oscilloscope and explain what I see so we can clearly see on the upper channel here that there's quite a bit of noise on that 120 cycle ripple on the bottom channel you can see that there is 120 cycle ripple but it is quite a bit quieter so this amplifier here is zoom back out so this amplifier here has got some noise happening as well as ripple but they both have ripple now we can see that the lower channel if we take a look at the level of the ripple now there are no loads attached to these amplifiers so this is just the scope probes attached to the audio amplifier outputs you see on the lower channel we have between say seven and nine millivolts of ripple the upper channel the amplitude is a little bit higher and it is quite unstable because of all the noise on the signal so we're probably dealing with between ten and twelve millivolts of ripple something like that on the upper channel now since we see on the bottom channel we have a relatively stable readout for frequency it says 120 Hertz what does that tell us well that tells us that this is power supply hum in both of these amplifiers in an amplifier of this size they would not have half wave rectification they have full wave rectification in both of these amplifiers and in any large amplifier they're going to have full wave rectification whenever you have full wave rectification and you see 120 cycle hum that means that you have a power supply issue so if there was 60 cycle hum present say it was a 60 cycle signal on the oscilloscope there and it said 60 cycles instead of 120 we would know that that hum is most likely coming from a heater to cathode leak or something like that in a preamplifier tube or there's something wrong in an amplification stage when you see 120 cycle hum it's almost hands down there's a power supply issue so we know that both channels have power supply issues there's a hundred and twenty cycle hum in both of these channels here now I'm not sure of the actual level of 120 cycle hum that these amplifiers have just from the factory from what I can see you know seven millivolts eight millivolts on one channel here is a fair amount of hum even with no load hooked up if I was to hook a a speaker up to that it would be quite audible in the speaker so just say I had three millivolts of hum that's getting down there but it is still quite audible in a speaker and if you have very efficient speakers that could be quite nauseating so there are some issues in both of these amplifiers here so what we're going to do is address the noisier one first we'll take a look at this amplifier here turn it upside down and check around inside so since I'm seeing 120 cycle hum and I'm seeing noise these two guys here are a suspect so we'll check out the capacitors I've removed all the screws on the bottom side here that's all it really takes to get in here as we can see there's some sort of a fluid leakage here so if they look on the other side we can see that there's quite a bit of fluid leakage there so that's on this side and as you can see on that capacitor down there it looks like it's been physically leaking this bottom half over there so there's quite a bit of discoloration on those pins there so what we want to do is test that cap so what I'll do is I'll grab a voltmeter over here so the first thing I want to do is make sure it's discharged now this has been off for quite a while at this point but you can never be too safe always make sure that it's discharged first and there's about half a volt on it no problems there there's another large can on this side here that I'm gonna have to check so this out of the way here I don't know their configuration so if that one's charged and this one isn't there could still be some pretty big issues so I'll just move this over here you're not gonna be able to see that because it looks like it's underneath an inductor here of some form so what I'll do is put this back over here again keep the light off that and I think I can get this way under there that's a nice thing about these probes here you can get the way under there and no problems point zero three of a hole so that's completely discharged now there are quite a few other caps in here as well I'm pretty sure that they're all discharged at this point we've got a little bit of a sleeve on it oh yeah no problems everything is discharged twenty-two hundred of 25 volts there will be absolutely fine all right so I know the caps are discharged so what we'll do just Center this up shot again huh that's looking pretty ugly so let's measure this capacitor here let's see if we can just measure it in circuit for a quick test get an idea all right 76 micro farad okay let's take a look at this one on this side here 518 microfarad around that so yeah what's down here I follow this line here goes over to this cap this one here runs out this one yeah so this is telling me this one here is actually running off the ground so that's running off to this ground they have a star type ground system down here you see that little star down there they're doing that to try and reduce hum so this is the negative side this runs off to this star type ground this runs over to this side here so as you can see this sides negative will run over to this sides positive and the negative of this capacitor would go here meaning that both of these capacitors here are in series not parallel so if these two capacitors are in series they're gonna have to be the same value and it's not going to be 70 some-odd micro farad it's most likely going to be close to that 500 micro farad that we see here so chances are what's happened is this capacitor has failed now the reason that that capacitor has failed could be a number of reasons since they're in series we've got a bleeder on them and sometimes they use this to try and equalize a small amount of the current as well so this looks like 100 K ohms across this capacitor here and there's another one over here chances are since these capacitors are in series when you first turn the amplifier on and all of the vacuum tubes in this amplifier are not warmed up chances are the B+ goes way way way way up so most capacitors this size here top out around 450 volts so I'm imagining that they have two capacitors here in series in order to get that voltage up so whenever you put capacitors in series you get double the voltage at half the capacitance so just say that this was 500 micro farad for argument's sake 500 and 500 would make 250 micro farad if they're in series if each cap was rated at 400 volts we would have 800 volts total so 800 volts at 250 micro farad so both of the caps have to be the same now usually what causes the cap to fail all right so this cap here is failed as chances are this capacitor here may be very leaky or it's pulling excessive current so when you first turn the amplifier on if this capacitor here pulls excessive current this one here might go over its rated voltage if this capacitor goes over its rated voltage it's going to let out the GUP so and that's probably what's happened here usually these capacitors are very good you can see that it hasn't leaked out of the vent at all but it looks like it's actually coming down the legs so you know it could be a number of reasons why this capacitor failed maybe even somebody over-tighten these and broke the seal on the base really hard to say at this point but I can tell you when two capacitors are in series like this they are going to be the same value and if at the factory that did spend any time with their capacitors they're gonna probably spend some time matching the leakage current both of these capacitors to make sure that they charge equally and of course these resistors help out with that and they also act as a bleeder so somewhat of a voltage divider across these capacitors so this capacitor is very bad so that's probably part of the reason why this is humming now in the other amplifier I'm not really too sure if that's going to be an issue in that amp as well I'll have to look at that in a little bit so I'll just focus on this one for now so what I'm going to do is replace these two capacitors here now I haven't drawn up the schematic for this amplifier yet and I plan on doing that very quickly here so in fact I might just do that before the next shot so what I'll do is I'll take this over to another bench spend some time with some paper and a pencil and draw this up and when I have this thing drawn up come back and we'll take a look at its design as well after I've changed these capacitors I've now replaced both of the capacitors in the power supply section of this amplifier you'll see that I'm using a more modern snap in style capacitor on each side the reason I prefer using this style of capacitor is because I can solder the connections directly to the bottom of the capacitor I much prefer a solder connection over a screw type connection you'll also notice that I've increased the wire size running to the capacitors in this circuit here all the orange wiring that you see here has been replaced that wiring is much larger than the original sized wiring here this wiring is also a high temperature wiring as well so now that both capacitors have been replaced the unit has been on for a while and it's biased let's take a look at the result on the oscilloscope screen now let's drop the ripple down quite substantially that's even lower than the other amplifier but there is still quite a bit of ripple there so about 4.8 4.9 millivolts and again that's with no load across the speaker terminals now if you have inefficient speakers that is not gonna be all that audio Bowl you got to remember when you hook the speakers up to it you know it's this what you're seeing here again is just you know wide open speaker terminals so inefficient speakers yeah that's gonna be pretty quiet yet if you're in a very quiet room you're still gonna hear that now this cannot be left in this amplifier like this at all because these amplifiers are powering up some clips corner horns and for those of you that are familiar with those speakers any kind of noise is going to show up on that especially when you have a monster-sized horn you know we're sitting directly at your head level when you're sitting in the audio room so it's definitely going to be you know it's gonna be presenting this let's put it that way so what we need to do is get rid of this this hum and a little bit of noise there very little noise it also cleaned up the noise quite a bit so we need to make this as low as possible now I'm not sure if this particular level is standard for this amplifier I know that if I plug the other amplifier and with good capacitors it's right around this so this is kind of interesting what we need to do is get rid of this and find out why this is there what you want to do is pretty much bring this to the absolute least amount of this that you can see on the screen you want to get that as close to a straight line as possible so that's what we're gonna work on next year what we're gonna do is take a look at the schematic as well I finished drawing up that schematic that was pretty time-consuming so I drew that up and marked down all the voltages and took some measurements inside here so we can get a good idea of what the engineers were thinking of when they put this together so we'll take a look at that schematic next go over that compare that to this amplifier and then we'll work on getting rid of this hum or reducing it and by doing that it should give you some good ideas if you're designing amplifiers on your own or if you encounter this problem in another amplifier where you can look to try and solve these issues all right let's check out this amplifiers design so first of all the schematic diagram that you see here is a direct result of what I found within this amplifiers chassis by no means is this a brand-new amplifier amplifier has been around for a while and it was purchased used by its owner which means that it may have had multiple owners in the past if any of those multiple owners in the past has modified this amplifier in any way shape or form since this is the amplifier that I reverse engineer to get the schematic any of those modifications are going to appear on the schematic so take what you see here with a grain of salt now I can tell you this if there has been any modifications done to this amplifier they were done very well so it's really nice and clean inside so let's start with the output section here first we have six el34s on this chassis which makes this thing a wonderful room heater you'll notice that all the plate lines are tied together so the plates are the upside-down T symbols in the vacuum tube symbol here or a note if you like they're all tied together and they go to one end of the audio transformers winding likewise on this side all the plates are tied together and they go to this end of the winding this is known as a push-pull configuration you'll notice on the audio transformer itself there's a bunch of leads that go nowhere just as NC and NC in the chassis here itself there are a bunch of leads coming out of the existing audio transformer that are just capped off so they folded over and basically heat shrunk that tells me that this audio transformer was most likely intended for ultra linear operation now that doesn't mean you need to use it for ultra linear operation it just gives you that option so normally if you were to have this in an ultra linear configuration these would be attached to the screen grids of the tubes here so the screen grid is the center most dots in these vacuum tubes here so they're not doing that because the screen grids are tied to the plate through 100 ohm resistors and that's known as triode connecting these pen codes so the reason that they have 100 ohm resistors in line with the screen grid to the plate is to stop the amplifier from randomly breaking into oscillation after all you did purchase an amplifier not a signal generator if for any reason this amplifier did break into oscillation on its own there's enough tubes and iron on this chassis to severely damage your speaker and your hearing as well so that needs to be taken care of you need to sweep the entire amplifier with a range of frequencies and make sure that you can't even kick the thing into oscillation that's another reason that there are 1k resistors on the control grids as well again to stop these oscillations a lot of the times when oscillations happen in an amplifier like this they're above our hearing range so what they do is they attack the tweeters and the speakers and they'll pop the speakers so what happens is when the amplifier oscillates at a very high frequency above our hearing range the capacitor in the crossover inside your speaker that's in line with the tweeter looks like a short circuit at that frequency and what it does is it fries the tweeters and of course it's also making your neighbors cats probably run for the hills as well so one thing to keep in mind if you're ever designing an audio amplifier you always want to sweep it and make sure that there is no high-frequency oscillations at least above our hearing range happening because it is quite common in this particular type of amplifier design now some tubes get excited a little bit easier than others you'll notice that all the plates on these vacuum tubes are just tied together on some tubes you can't do that you need a resistor in there as well for example tube that gets excited rather easily are the old 6l 6 G's not the GC version but the G's the G's look like an 807 with a without the cap on the top that's the old 6l6 design those vacuum tubes very easily oscillate up to 30 megahertz but basically no issues so becoming an oscillator in the audio Bowl range is basically you know a cakewalk for those vacuum tubes so something to keep in mind if you're ever designing an amplifier with multiple 6l 6 G's in it so they've obviously found that this won't break into oscillation so they've stopped this you know putting resistors in at that point there you'll notice that the suppressor grid the suppressor grid is the grid that's closest to the plate or the anode they're all tied together they actually tied each tube to the cathode I've drawn them connected together and just tied to the cathode line for ease of drawing here that's pretty standard the suppressor grid is inside the vacuum tube to suppress secondary emission so basically electrons spray from coming back off the plates and back in the tube again and creating noise so that's why they're there so kind of a nice design that the fact that they can actually have that in the vacuum tube triode connecting el34s is is actually kind of a bonus because if you just have triodes you know you can still get that effect happening whereas this you get that added effect of getting rid of that secondary emission so one bonus of running pen Toad's triode connected you'll notice that all the cathodes are tied together the catheters are tied together and they run here to this 500 milliamp fuse which is this views on the back of the unit here and then through a 10 ohm resistor to the bias Jack when you stick that jack in there that runs to your external current meter it breaks this connection and places the current meter in line with the cathodes to the chassis so you can effectively set the bias now one of the things that I do find odd about this design is this bias is basically from zero to full throttle a lot of vacuum tube audio manufacturers will not let that happen they'll only give you a certain range to bias your amplifier up and so basically if you turn the tubes on a little bit too hard you know it's not going to damaged too much because it's only a limited range this thing here you can bring it from you know basically zero to full throttle and it'll actually take a fuse out and possibly pop the line fuse which makes me wonder if that has been modified within this amplifier I notice that this 27 K ohm resistor here looks a little bit different than all the rest of the resistors and there's also a resistor on the bias pot that's just basically bridged out a circuit it's not really doing anything so it may have been modified there may be an actual limited range that these things came with and somebody took that limit off there so basically took the governor off the amplifier and allows you to move the bias anywhere you want now for my own reasons I purpose I really like that that that I honestly I don't want any limitation you know that's that's I would purposely set my own amplifier up that way put it that way now I don't like this bias control at all and even with this black knob off here I don't like the shaft sticking out so what I'm going to do is I'm going to replace those with some very high-quality allen-bradley controls as you can see this is countersunk so this year you just unthread and then you put this on the inside and put this through the chassis and this can be tightened up with a rather large allen wrench and then the controls countersunk so once it's set you can't bump it or move it and everything remains the same for a long period of time this is really handy because if you're gonna be you know playing with speaker cables or anything around the backside and you know even a speaker cable if it rubs up against this is gonna adjust the bias and if you don't have a current meter on here you don't know right so it might be running way over current well bad things will happen so you really don't want to have too much current there because you know you're you're putting a lot of DC current across the winding of this audio transformer and across the choke and you're pulling excessive current from that power transformer in the back so you know you kind of just want to run it where it's supposed to be run another reason why a lot of audio manufacturers will limit the range of the bias is because a lot of people think that they can just set the bias to where it sounds good well let me tell you this a lot of these amplifiers sound really good when they're drawing way too much current that's very hard on parts so that definitely has to be set with an external meter so all the gridlines are tied together this is the control grid here the control grid is the first set of dots right above the cathode here and that's what controls the tube that's where the signal goes so you can see that they're all tied together and they're isolated by these 1k resistors and that runs over to one triode in the phase inverter circuit here this is the entire phase inverter circuit right here and I'll start talking about this here in just a little bit same on this side runs over to this side and runs over to this triode in this 12ax7 now you can plug a bunch of different tubes in the phase inverter here and you know experiment with tubes again for just demonstration purposes we'll call this a 12 ax7 in this circuit so let's see let's make sure I'm not forgetting anything about the audio output section here so okay so for the bias line here we'll just talk about this quickly first the bias line which basically is what allows you to change the negative voltage on the control grids of all of these el34s runs down here to this by asset control and that's that control on the back and then of course we have a resistor which is again going to limit its range by changing this resistor you can you know add or take away from its range so this bias control is 25k potentiometer on the backside here is basically split off to both sides so there's a 68 K ohm resistor and a point 1 micro farad on this side and there's a 68 km resistor and a point 1 micro farad capacitor on this side we'll take a look at those here in just a little bit I'll do the schematic first here because if I move the schematic it completely changes the exposure level of the camera I'm gonna change this all over here again so we'll come back to that in just a little bit looking a little closer at the phase inverter circuit here so we have our 12 ax7 or whatever tube you want to plug in that socket here and they have a tube down here which they called a current source I would prefer to call that your current sink so this al 84 here is in the cathode line here as a constant current sink on the bottom portion of the tube now on the top side of this phase inverter circuit here you'll notice that we have a regulated high voltage supply so the v+ comes out of here and it just attaches to this point here we have an NPN transistor up here with two Zener diodes acting as a reference on the base here and we have that 100k ohm resistor running over to the base to turn the whole system on keep everything running properly now these are five watt Zener diodes and they're just kind of floating in mid-air and yeah they were going to get probably a little bit warm now with temperature these things move around so it's always a good idea to put a little clip around them and strap them to the chassis with some heat sink compound and that stabilizes things they haven't done that in here it's they're kind of floating in mid-air that's not so much of a big deal it'll probably move from say 318 to 325 volts while it's warming up and that's just because you know everything is moving around here so you know heat wise at any rate so in the amplifiers that I've designed where I've used you know a linear regulator that's what this is this is a linear regulator and this is acting it's basically a pass element the most quietest type of regulator put it like that so whenever I've designed these circuits I've always had a little it's aluminum clip and I put it around the body of the diode and tighten it to the chassis and it works very well to stabilize these up and when we get into some amplifier design here in the future I'll start talking about this and I'll show you some of those design tricks that stabilize these things little teeny things that you can do that really mean a lot for performance anyways back to this here so these circuits are relatively quiet as it is but in order to make sure that it stays quiet they've put a 1 micro farad capacitor here to ground now the two resistors on the plates of the phase inverter circuit here are very closely matched in fact they're so closely matched they're 27.4 that's getting pretty accurate 27.4 K 1% fifty parts-per-million resistors so basically what the designers of this circuit are doing is they're they're putting this together and what this tells me is they're saying okay this circuit is only going to be as good is the vacuum tubes that you put in it that's basically what they're saying the circuit is very very accurate it's got a regulated supply and very accurate resistors the next step is to get the best match triodes inside of a 12 ax7 that you can get there's a way of actually doing that you can run jacks onto the sides and do that and I might discuss that here in a little bit again down the road I'm going to design a triode match or so basically you can check out all of your tubes and see how closely match they are and if you go to a tube supply place that's nice enough they might even let you plug them in and try them out and choose your best tubes at any rate so we have our phase inverter circuit here and then down here we have a current sync now this thing here it looks like a very high impedance at AC but yet passes DC current and it's great for the circuit to keep everything in balance and that's the reason that they're putting this thing here and they're using an IL 84 to do this now the thing I don't understand is they put a regulated supply on the B+ but on the negative supply there's no regulation the negative supply can still move around maybe they found that it doesn't move around enough maybe they're expecting you to have stable line voltage or something like that I don't know if I was to design this this would definitely have a regulated negative supply so that would be another step in making this thing even better so something to think about if you want to improve your amplifier any rate we have our feedback tied to this grid here goes through 22 K ohm resistor the feedbacks which is this one here runs over here to the speaker jack in the back and of course we have our switch back here which is our speaker impedance selection switch let's see what else can I talk about here we have the audio jack in the back is actually lifted a little bit we have a 100 ohm resistor to ground here so this is not at chassis ground this is an isolated ground on this RCA jack in the back you can see it's lifted by 100 ohms something to keep in mind in the power supply here we have a very large transformer on the backside of the chassis and it's got two primary wine so in this configuration for 120 volts the primary windings are in parallel if you wanted to use this a 240 you would put those in series which is kind of nice it allows you to use this in Europe as well all you have to do is just change that configuration on this side here we have our high voltage winding this high voltage winding has 2 1 in 50 408 in series in this string and another 2 in series in this string the reason they're doing this is to raise the PIV so basically what's going to happen is every now and then when you click the switch on the amplifier you're going to get a bit of an inductive spike happening if it exceeds the diodes maximum reverse of the PIV it's gonna cause the diode to change its state to a jumper and that's a bad thing when diodes turned into jumpers windings get very hot and fuses start to blow so by putting these things in in series like this you're basically doubling the PIV so it's just making things a little bit safer we have a 2 Henry choke over here and that's the large choke in the center of the chassis we'll take a look at that if you don't have the ability to read say you don't have an LCR meter the DC resistance is 30 ohms and that runs over here to our two large capacitors that are in series at the back here we have two 100 K ohm resistors that are in parallel with each capacitor basically just acting as bleeders and there will also act to stabilize any kind of leakage across the the capacitors as well so if one capacitor has a little bit higher or leakage than the other it'll it'll work to stabilize it now that's not a whole lot there but it'll help out a little bit now to give you an idea I have a box full of these filter capacitors the ones that we just looked at I tested about 20 of them in my box and matched two up that have very similar leakage so that they charge evenly so basically what happens is when all the vacuum tubes are cold on this chassis and you first click that power switch on there is no load being drawn so what that means at this point is where it says 400 volts here this goes up to 600 volts and sits at 600 volts until all of these large power tubes start to our current at that point it pulls that down to 400 volts that's the reason they have two capacitors in series like this if they didn't have that it would exceed the voltage rating of the capacitors and the capacitors would explode so it's very important so this saddles off around 400 volts very close to 400 volts now you'll notice that there's no filter capacitor on this side here if you were to add a small amount of capacitance here this voltage would go up quite dramatically so this inductor here is doing a lot of filtering here so it's trying to get rid of that ripple well notice down here we have a negative supply here for our bias setup so the diode is in reverse and you'll notice that the capacitors are all in Reverse you'll notice that the positive portion of the capacitor runs to ground so this choke here is 11 Henry's or this reactor is 11 Henry's the DC resistance is 496 ohms again if you don't have an LCR meter we have a 2 mic filter on this end here there's a 47 mic at 350 here and there's another filter over here that's kind of an odd setup I don't know if anybody's moved anything around here but that's the way that this amplifier set up it seems to work okay they're running the filaments of the 12ax7 off DC here so it's only half wave rectification got one diode here and there's a 2200 microfarad capacitor 25 volts that runs to the filaments over there now a filament voltage is a little high it measures up at six point nine volts supposed to be six point three in order to get a little bit more filtering you could actually put a resistor in line between the diode and this capacitor here and it would smooth things up even more if you wanted to get a bit more filtering you drop that down to six point three you'd be doing very well so that's there so we have one winding that runs three el34s and one el84 so they'll run all the il 34 and this one il 84 and then this other winding here will go to just the three L three el34s this one right here and this is running off of the backside of the diode on this filament winding so they're using it for AC supply and for a DC supply well let's see what else can I tell you about this I think that's pretty much it so I have to say this design is a really good design it it just is like I mean it's a really nice design and honestly the guys that designed this I give you a thumbs up you did great it looks good again the things I would have addressed you know are the you know the Zener drift you know and I would have regulated this negative line here a little bit but everything else seems to be very good so there you know there's a bunch of things that you know might be able to be moved around and the chassis things that I don't quite agree with at this point but other than that the design itself is really sound it looks like a really good design so all-in-all I am pretty impressed with it and it's bound to sound very nice when it's running again you know these things trout connected you know getting rid of that secondary emission there you know but you know the bonus of running a pen tote in a triode connection so yeah all in all it looks very good so next what we're going to do is we'll take a look inside the chassis here and I'll just briefly explain what where all the parts and pieces are okay I'll just show you where some of the parts are on the chassis here according to the schematic so anyways this transistor that you see at the top of the schematic right here is this transistor right down here this enter diodes are right down in this little boot down on the bottom you can see that little kind of a heat shrink boot down there they're sitting down there so those are these two diodes right here let's see what else here this is the filter reactor for the negative line this is the filter reactor for the high voltage so the 400 volts here this is the capacitor and resistor combination right here that is on the grid line here so the 68k and the point one micro farad that's what that is right there there's one on each side is that cap there and this camp here that's why I marked on the schematic times two let's see what else is down here this is the the rectifier diode that changes the the filament winding to DC for the 12ax7 here and this is the filter cap for that there on the side this diode that you see down here is the the diode for the negative bias so that's the negative bias diode right there and the other diet I just showed you is this one here in the filament filaments there and these are those two very high accuracy resistors here so this resistor here in this resistor here are these two that are in the plate lines of the 12ax7 there these are the coupling caps this coupling cap here and this coupling cap here or the two coupling caps that couple the signal into the the final section here to the el34 section these are all the the screen grid two plate resistors here hence the triode connection you can see them all here and let's see I think that's pretty much it the capacitors for filtering the the negative bias right here and one thing that I didn't draw in the schematic it's kind of interesting this resistor here see this is b-plus is 400 volts here right this is a 470 K ohm resistor and that runs directly to the LED on the face kind of an interesting connection the leds countersunk you know so you really can't be sticking your fingers on it if it popped its head off or something like that nothing don't happened common failure point for LEDs is they you know they kind of blow apart odd to that they're running that off the high voltage line probably did that to reduce hum maybe if they ran the LED off the heater line it induced some hum here or something like that kind of interesting running an LED through a 470 K ohm resistor off the 400 volt B+ line oh these are the the two windings that are just capped off so those are the the screen windings that would most likely put the audio transformer down on the other side of the chassis allow you to use it for ultra linear operation I should say and I think that is about it so next what we're going to do is we're going to take a look at the specs of this amplifier now we still have ripple in this thing I haven't addressed that ripple and we need to find out where that is but I think at this point the amplifier is pretty much supposed to be like that you know I did both of them are doing it and you know it's just it's it's kind of bizarre so I think that small amount of ripple is supposed to be tolerable so it could be wrong but you know it's it's looking pretty much that way so I don't want to change anything in here I don't want to modify the design before we check out its frequency range and look at Distortion specs and things like that also want to see how much power this thing puts out so we'll bring this thing up to full throttle and see what it does here let's see how many watts this amplifier gives us so any guesses the test frequency is 400 cycles the display you see here reads out directly in watts the amplifier is terminated into an 8 ohm resistor flowed the oscilloscope is here to show us when the amplifier starts to clip we'll back the amplitude down until the clipping goes away and that's the maximum usable output that we can get out of this amplifier so I'll advance the gain on the signal generator and let's take a look here we go bump this down once and as you can see it's just starting to flattop so that's the flat topping there if I advance it a little more you can really see gets pronounced so what I'm going to do is back this down until I have a nice clean sine wave right about there's about the limit so we could probably take it right down to both there so I'd say 52 watts so it's working great they specify the amplifier is supposed to do 50 watts and it's making 52 so it does what it's supposed to do here's an example of what crossover distortion looks like in a vacuum tube amplifier and this is mostly caused by an incorrect bias setting so basically if the tubes are turned off too much so the bias voltage is too high on the control grids you get this effect and it looks like it's splitting the sine wave up so what I'll do is I'll advance a signal generator and show you what that's about [Music] that's what crossover distortion looks like you can see it looks like you have a split in the sine wave here that is due to an incorrect bias setting so basically the the tubes have got way too much negative voltage on the control grids so what I'm going to do now is advance the bias on the amplifier to get rid of that I'm gonna have to go back and forth between this and the signal generator here so I'll advance this you'll see the amplitude rise because we're turning the tubes on more and you'll see that crossover distortions start to disappear so you turn the amplitude down advance this some more so it looks like it's almost gone at this point so I'll advance it just a little bit more I would say that that is a very nice safe zone so I'll turn this down looks good so what I'm going to do now is plug in the current meter into the back into our bias jack and take a look at the cathode current that's a whole lot less than two hundred and twenty milli amps isn't it it's 160 milliamps and we have no crossover distortion at 160 so this goes to show you how useful an oscilloscope can be you can actually adjust your amplifier using your oscilloscope to get rid of the crossover distortion and determine the right bias level the reason that they give just a generic bias level say 220 to 230 milliamps is because that's pretty much safe in all applications and most people don't have an oscilloscope by using an oscilloscope to set the bias in this amplifier in that there is a lot of a SafeZone there I could turn this down even more I advanced it quite a bit so by using an oscilloscope to set the Bison the amplifier not only are you gonna make your amplifier run cooler you're gonna make the vacuum tubes last longer it's just all around better situation a very quick mr. Carlson's lab modification to this amplifier any guesses how many watts it's gonna make now the amplifier is still tryout connected the ultra linear taps have not been hooked up so here we go so starts to flattop at about a hundred let's say about a hundred and five watts so there's a continuous hundred and five watts output right now just turn that down so any guesses on what I did to make it do that the modification took I would say 15 seconds anyways ok back to stock let's check out this amplifiers frequency response or useful frequency range so the machine you're looking at is a Stanford research systems model SR 780 and this machine is going to draw a graph and show us how well this amplifier is performing so how the machine works is it feeds a signal out into the input of the amplifier so into that RCA jack the signal goes through the amplifier and then the Machine monitors the output of the amplifier so there's two leads hooked up to this machine right now it monitors the output of the amplifier across an 8 ohm load and that is going to again show us how well this amplifier performs and shows us it's useful frequency range now I have this set up so that it's going to go just under its maximum power about the 49 watt level or something like that because I don't want to drive the amp into Distortion so we'll see how well it performs right up to its maximum power this is in the 0 DB position right now so there is no feedback in this position so if you own one of these amplifiers you might want to get out a piece of paper and take some notes here we go this will take a small amount of time as I say it's slowly sweeping a frequency range and then monitoring it and drawing a graph showing us exactly what's going on within this amplifier and I have it set to 300 points so it's pretty accurate the more amount of points I set the longer it takes to draw the graph so this is kind of a happy medium so right now the frequency slowly climbing from 10 Hertz to 100 kilohertz you may have heard that I'm not sure if the mic picked that up that was it sweeping they've range there okay so let's take a look at this amplifiers 3 DB down points so I'll just move the marker here so we'll say 13 point 6 Hertz is the start and then I'll move this over to the other side in the end it's 40 point 9 kilohertz that's at the 3 DB down points and what you see the top line there is just below full power here right I'll bring it to here and then you can see it's a 398 point one volts RMS rate up here so we get our calculator that's squared so we just go three 98.1 divided by our load which is 8 equals s 49 point seven six watts is what that made at that point again I have this set up so it'll just go under it's you know power before you know maximum power before it starts to distort so let's check it's 1 DB down point so we'll go 1 DB down and check out its useful power here so we'll say nineteen point six Hertz is the start and twenty one point four kilohertz is the end and that's it's one DB down point so not bad for no feedback at all that tells me this thing has got some pretty nice iron in it alright so the they chose a pretty nice audio transformer and it's not even hooked up in ultra linear so very very nice no feedback so that is the response 1 & 3 DB down so what we're going to do now is set this thing up with feedback and try it again so click this on so now it's in the minus 10 DB position little news I have to readjust the amplitude here so source and I'll go amplitude 4.3 okay put this back to frequency here again and we'll hit start here we go this is what it does with minus 10 DB feedback get the marker out of the way okay so let's check this three DB down point here so three DB down will be roughly three boxes down it's a little above the line so we'll say right there so with feedback the top end is seventy five point seven kilohertz I would say that's quite beyond our hearing range so that's a 3 DB down point and we'll go to the start right about there so it starts at twelve point seven Hertz very very nice so let's go to its 1 DB down point let's see well be nice to the amplifier so the 1 DB down point is sixteen point three six Hertz that's the start along here and the stop would be forty two kilohertz forty two point two kilohertz very nice so you can see the advantage of having the the feedback in it really really flattens a response and widens things up so if we were to give this a little bit more feedback it would make this even wider now the disadvantage to this is this has basically got one 12 ax7 in the front end and that's it the more feedback you add to the amplifier the more drive the amplifier needs so technically the more feedback you give this with this phase inverter the more def the front end of this thing is going to be you're going to need to really drive it as you can see I really needed to up the amplitude there just to to drive this thing up with the feedback on now again feedback is personal preference if you listen to an amplifier with no feedback again it'll make some sometimes the vocalists and things like that stand out and you'll click the feedback on and it seems to settle people back in the background again now that's because again it's flattening out the frequency response so it's making everything even so basically it's pushing them back into the band so that they don't stand out anymore so again it's all personal preference so all in all I have to say this amplifier performs extremely well and it's it's working nice like I mean it's you know the advantage of of having you know very few tubes in the front end as well right you know it's basically just you know that 12 X 7 and the input and then the output tubes right away so not bad so you can tell that they definitely did their research on this design so yeah very impressed let's see how much total harmonic distortion this amplifier makes at 50 watts so that's just a little bit under its maximum usable power before we can visually see any clipping and in a moment we'll also test it at about half power as well so this is with no feedback so it circuits wide-open and the test frequency is 1 kilohertz since there's no audio gain control on the amplifier I'll have to plug the BNC into the front of the machine here so it's you know as I'm talking right now it's not screaming away at 50 watts output that's a little bit hard on my load as well so here we go so I'd say point six point five nine about so say point five nine point six percent total harmonic distortion with no feedback at all most maximum power that's doing pretty good alright so get rid of the source there now we'll do is we'll test it with feedback feedback is on a source change the amplitude three back all right so this is what it's going to do what about 50 watts again with the feedback so here we go point one point one nine so we'll say point two percent total harmonic distortion with only ten - 10 DB of feedback not bad not bad at all keep in mind that this is just about at full power so next what we're going to do is test this at half power this is the amplifiers total harmonic distortion at 25 watts or very close to so here we go this is no feedback in this position feedback is set to zero dB so I'd say about 0.4 to 0.4 3 so let's try this with the feedback on you to adjust the source here okay so this is with 10 DB and about 25 watts again so 10 DB of feedback here we go so point 2 3 not bad now you'll notice that it looks like it's moved up a little bit from it's full power area this is common with amplifiers they move around a little bit within its useful range so if you recall at full power I think we had point 1 something so let's try it again so at full power with negative 10 let's go back to source 4.3 4.3 yeah so a little bit less distortion at full power interesting but this is pretty common when you're working with amplifiers run across this kind of stuff all the time moves around throughout its throughout its range of power there so let's try it at say 10 watts or something like that let's just uh let's just turn it down right now it's uh okay so right now we're at 2.8 watts and take a look what does it make it 2.8 oz nice and low there point zero eight percent so you have very efficient speakers you have very low distortion so I'll slowly turn this up again this is in the 10 DB position so it has 10 DB of feedback right now it's amplitude I'll just turn this up to will say try it at 10 watts it's a nice usable level so there's 10 watts of output right there it's 0.156 not too bad not too bad at all again this is with feedback so let's try this with no feedback so although to say 500 millivolts will start out there and I'll click the switch so we're at 3.6 watts here I'll just roll this up to 10 watts 10.0 watts let's check it out so that's with no feedback so just about 0.3 percent not bad 2.9 it's all at all for 60 el34s in their configuration it's doing very well again impressed with the design the next thing on the to-do list with this amplifier is to try to reduce the hum level present at the loudspeaker jack on the rear of this amplifier as you can see on the oscilloscope have about 5.1 millivolts RMS of hum present there and that's with no load attached across the speaker Jack's basically the only thing attached to the speaker Jack's is the oscilloscope now both of these amplifiers are going to be attached to some very large speakers and in a relatively quiet room that hum is going to be audio Bowl so we want to get it as low as possible that's one of the reasons why I invented that Carlson super probe is to we locate noises inch Assis and it's great for audio work it'll point out where all the noises are coming from in this chassis and it'll do it electrically so it doesn't have a microphone or anything in there it's just a little piece of wire exposed on the end and it's going to listen around and you'll see that here in just a moment this device here and all the plans and everything are available on patreon so you can build one of these things if you'd like there's printed circuit board plans and everything are there this is just one of my inventions I plan on sharing many more of my inventions on patreon as well because hey that's what this is all about sharing knowledge right so here we go I'm gonna turn this thing on I'll grab the probe I'm gonna click it into the noise position your so it'll stay quiet here as I'm moving things around so it's in the noise position right now which is technically designed for RF work and for picking up noisy resistors and things like that but we won't go into that right now so what you see on the tip here is a completely isolated tip you don't need to attach this into circuit you just point it at things and it'll listen to them from a distance for example I'll move this away so the cord isn't touching the tubes because they are very hot let's point this at the diodes and see what the diodes have to say so we can see that it's listening to the diodes here so that's in the noise position right there so we can follow noises around we can also differentiate between 120 and 60 cycle hum so we're going to hear in here 120 cycle hum 60 cycle 120 so it'll help you locate different types of hum now the reason that there's 120 cycle hum in this chassis is because we have diodes in a full wave bridge which is effectively putting all the bumps on the top of the line so in a sense we actually have a frequency Dobler happening there that's the reason that we have 120 cycle hum with full wave rectification so what I'm going to do is I'm going to take the probe off that noise detection and we're going to search around the chassis for the noise now we're going to hear that as well with this it'll be much more clear you can actually can hear audio in cables without even attaching this you just pointed at the audio cable and it'll listen in to an audio cable so we want to listen to where that 120 cycle noise is coming from because that's the noise present on the scope it doesn't really seem to be all that much 60 cycle hum in there so what I'll do is I'll point this at parts and really here those diodes working as the diodes are right here in the chassis [Music] now the blue lead runs from the diodes to this filter reactor over here and we'll see you can hear the filter reactor working now if we get close to the wire that runs into the filter reactor that's running from the diodes so the most amount of noise is going to be worth at the actual lead-in wire is into the filter reactor and that'll be here as I move the probe around the filter reactor you'll hear it gets quieter as I get to the output lead see that this is the lead that's coming out of the filter reactor well from what we see on the schematic we know that the filter capacitors are on the output side of the filter reactor so these two large capacitors here are on this side of the reactor so we can see that they're working as we get closer to the input side also we can tell that that blue wire is running to the center first and white the windings are running to the outer side here so you'll see here he gets quieter so we would know that that blue lead-in wire is going to the core side first and then the windings are working their way out to this outside here so if we needed to trace a broken wire in a reactor that would greatly help us out so you can see here we have 60 cycle hum that's the main power transformer right there now if I move this around the chassis I come to an extremely quiet spot right here now you'll notice on the schematic in the negative supply they have a filter capacitor on the input of this choke here and that's why this choke is dead silent this choke is acting more as DC resistance in this circuit than it is as a reactor because they already have 47 micro farad before the lead in wire on this choke that's why it's sitting quiet so you could almost replace that choke with a resistor and get away with it in fact I think you could alright so back to the searching here so we know that we have a very noisy component here so this is shielded all right so this is acting as core you can hear as a you can hear me rubbing there the probe on here pretty much sounds like a record needle so now as I move over to this side here I get to a quiet portion of the chassis this is the first where I should say the phase inverter circuit here it's pretty quiet here as I get towards the power switch [Music] now keep in mind this is at half sensitivity here I have to have this down because it's just going to get so incredibly loud and it's so sensitive that I can point to things from such a far distance here that it's going to get you know confused between the noise of the oscilloscope the switch mode power supply in my lamps and everything else around here so I have to have the gain set accordingly so at any rate we can see that this is a very quiet portion of the chassis but as we get closer to the reactor again on this side [Music] [Music] so you can see that we have a quite a source of noise right here now this is probably one of the noisiest components in this entire chassis and it's bolted right in the center very close to the very first audio tube here for the input so that's that 12ax7 phase inverter circuit so we have an extremely noise noisy component right here the input wire to this phase inverter circuit is right here to this first audio circuit right here so that's where this shielded coax comes in into this area here now we know that we have you know 60 cycles over here right this is the main power transformer that's how noisy it is okay this is how noisy this is so what does that tell us this could be closer to the tubes than this thing so what I'm going to do is just shut this thing off I'll get rid of the super probe here so what we can see is this thing is making a lot of noise all right so we know that the audio transformer just from what we see in the chassis the wires are coming out of the audio transformer like this so on each side of this filter reactor here which means the windings in the audio transformer are going this way now which way are the windings going on our filter reactor well they're going this way so what's that going to do well that's going to pose a coupling problem now you'll notice that the windings on the audio output transformer are going the same way as the reactor here so they're going like this and you'll notice that the windings on this transformer are going like this now that's the proper way to design things that's the reason we don't see 60 cycle noise on the oscilloscope screen all right again winding this way winding this way that's the proper way of doing things unfortunately the thing in this chassis that's making a lot of noise is this and the windings are the same in the same fashion as the audio transformer on the other side so there's an extremely large chance that this is coupling through the chassis into that audio transformer and directly injecting this into the audio transformer or at least some of it we also know that this extremely noisy component is only I'll just get my pointer here because this thing is on so the first audio stage or the phase inverter circuit here where the audio circuit where the audio runs in comes through this coax here and goes through this resistor right into the grid well look at the distance that's it between the the 12ax7 in this filter reactor there's almost no distance here we have wires protruding right up near this noise and everything so this is telling me right now the noisiest component in the chassis this thing this thing is in the center of the chassis the windings are in the same fashion is the audio transformer on the top and it's close to the 12ax7 there are so many no noes here it's not funny technically this chassis should have been a little bit longer and this should have been further away from the audio transformer then this thing then the main power training so what I'm going to do next is I'm going to loosen this transformer off I'm going to extend the leads on this transformer and with a glove I'm going to hold on to that transformer and I'm going to move it around in the chassis and we'll see what it does to this hum level here we'll see if we move this to a different portion of the chassis if this is the solution to lower that hum level if it is this here very quickly and easily located the issue I've now removed the screws from the filter reactor and I've also extended one of the wires because it was pretty short so this way I can move this around the chassis and we can watch the result on the oscilloscope screen do not try this at home that reactor has over 400 volts on it and I do have some gloves on here - gloves - be careful again do not try this at home if there's any leakage between the windings and the core you get an extremely deadly shock so I'll just lift this up here and move this look at that from 5 millivolts RMS down to 1.3 1.4 now you can see when I rotate the transformer now we're up to 2.3 now we're down to 1.3 again as you can see when I move this around the chassis different areas it's noisier so now I can feel this thing buzzing seems buzzing really hard vibrating in my hands here so now what I want to do is find a nice area in the chassis to mount this thing so if I was to bring it over to here look at that this area right here we're down to 680 microvolt 640 and just move this around very gently until you find the area that it's at the quietest but right here so I would be mounting this in this area over here look at that 660 micro volts that's going to be inaudible in that speaker completely inaudible so basically it's just component placement so I put this back in its area here again look at that so that filter reactor is going to have to be moved to another portion of the chassis here and what I'll end up doing is moving a bunch of other components around and then mounting this thing most likely off the side with some standoffs or something like that so again I'll have to remove a bunch of these components probably move them over to this side and then the filter reactor will be this way in the chassis because that seems to be the lowest noise so what I'll do here is I'll reposition the camera and I'll get a closer view of this screen and I'll just do that again without this in view and you can see the result on the oscilloscope just a little bit better here's a better view of the scope screen so the reactor is sitting back in its stock area again and what I'm going to do is just lift it out of that area and bring it towards the rear of the chassis with my rubber glove on again do not try this at home there's high voltage present on this reactor so here we go I'll just lift it up now I'm just holding it in my hand here at the back of the chassis look at this we're already down to 2 point 2 millivolts so what I'm going to do is turn that 90 degrees now so I'll just turn the reactor 90 degrees that's just turning it 90 degrees 1 point 5 millivolts were down to already so I'm gonna move this to a quiet spot on the chassis there we go because we're down into the micro volts already so that would probably be the spot to mount it and right about there a little bit of movement so that's the area this reactor should have been mounted in or with an elongated chassis in a different spot completely so we can get the hum down to 640 micro volts RMS with a speaker attached to that that's going to be pretty quiet okay so I'll just put this thing back in its stock area again here just put that down here like so scope off here so I've got quite a bit of work ahead of me here I have to do this everything I've done to this on the other amplifier as well so I'll replace the filter capacitors and move the reactor around in that one I'm gonna have to move a whole bunch of components around to fit this thing in here properly because it's gonna be kind of tight the way the tube sockets are in there and everything like that so I have quite a bit of work ahead of me yet but at any rate there you have it thanks for stopping by the lab today I hope you enjoyed this video involving these two large monoblock amplifiers if you did enjoy the video you can let me know by giving me a big thumbs up and hang around there'll be more videos coming like this in the very near future we'll be taking a look at vacuum tube and solid-state electronics alike if you're interested in learning electronics in a different and very effective way you might want to check out my ongoing electronics course on patreon I'll have the link just below this video in the description so if you click on that link it'll take you right there if you do go there check out the community section there's many people sharing their projects there as well so until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 729,221

Keywords: AES Six Pac, AES Six Pack, Cary Six Pac, Cary Six Pack, Carlson Super Probe, AES Six Pac Schematic, Six Pac Schematic, EL34 Amplifier, Tube Matching, Valve Matching, Six Pac Specs, Tube Amplifier Repair, Six Pack Schematic, Design a tube amplifier, Bias a tube amplifier, crossover distortion analysis

Id: 6P4C9at5rV8

Length: 93min 26sec (5606 seconds)

Published: Mon Jan 29 2018