Oscilloscopes For Audio 101 - Part 4A Single Ended Tube Amplifier Troubleshooting

[Music] please pause the video and take a moment to read this important safety message hey welcome everyone part four here in our oscilloscopes for the audio bench finally we're going to actually uh test out some equipment and learn how to use an oscilloscope what i've decided to do is this is part 4 in this series and i think i'm going to have a part 4a a part 4b a part 4c and each of those will be using a different amplifier i thought i'd start out with a super simple little single-ended amplifier then i would do a push-pull tube amplifier and then the third one i thought i would get into a solid-state unit of some type and that would give me the flexibility if i wanted to add a part d or e later i could but i think my original scope here is going to be a b and c what we've got here is just a super simple uh started out its life as a magnavox console amplifier and um there's a thread out here and i'll put a link below that uh dave gallips had made a post a few years back in 2015 where he had modified one of those to actually use some modern transformers here and um you know made some modifications to the feedback and the circuit and various things and made a nice little two channel left and right stereo single-ended 6bq5 amplifier here looks like it's got a 5y3 or something here on the front end for the rectifier and then it runs they either run a 12ax7 or a 60u7 depending on how you wire up the heater voltage here for this so super simple little amplifier not a lot to it someone has taken this one and put it into a new chassis and has swapped over and using a triad transformer i actually bought this unit and you guys may have remembered i showed a repair of it i bought it off of audio karma wasn't working went in figured out what was wrong and at this point i don't even remember what that was but but we got it working so i thought i would just use it on the bench today okay a couple things i'm using in the filming of this video just so you're aware um the same little bnk precision unit i put it back on my bench i'm going to be using the 2246 for this one i thought the push pull one part b we would use the digital um regular scope but i'm going to be using the 2246 here to match up with it and then i'm going to shift over and i've got camera mount here for the oscilloscope and camera amount downward so we're actually going to mount this uh this little amplifier upside down in this and i'll be right back okay if you'll notice i'm using the little bnc adapter piece i showed you in one of the earlier videos the little stereo to mono adapter here uh just a you know one end of this has a pair of rca's on it and the other is the 3.5 millimeter and i'm just feeding it here into the oscilloscope the other side of this i'm just simply going to plug into super simple little connection there for that okay let's talk a little bit about the dummy load so dummy load wise this is a dale 250 watt 8 ohm resistor non-inductive okay and the reason i like that is you know 250 watts is a lot by the way these things are rated when they're heat sinked so without heat sinking it probably doesn't go all the way up to 250 watts something south of that so i really don't have to worry about you know if i had a pair of these sitting here what to do this is exactly what i have mounted i have two of those mounted up here inside of this little box matter of fact if i pick it up and show you on the inside you see two of them it's the exact same two resistors as these now this this is a 50 watt and you know what this is a 5 watt amplifier so i could easily use this 50 watt resistor right here instead of this big one matter of fact it makes it a lot easier to clip leads on to let me show you just a little bit of the challenge of what you run into with working with the the bigger units like this and this why i have it wired up inside of this box and all soldered up and uh and whatnot but you know when you go and you clip one end on here and i'm just using a uh banana jack cord here right and then we're gonna come over here and we're gonna plug this into the back of the amplifier okay so all i've got now out of the amplifier going across my dummy load but then i want to be able to measure it with an oscilloscope probe so i'm going to plug my oscilloscope probe into port number one up here and i'll come down here and i'll clip onto the red lead right here but i've got to establish a ground on the other side of that or i should so i'm going to end up using my little clip here from my oscilloscope to actually do that right so i'm going to clip on here and then when i go to the other side here guess what i don't have enough length to get over there and do what i need so i'm having to use a little jumper here to kind of extend it it just gets a little clunky right it's all i'm saying at the end of the day but i've got it all wired up now so what i've got is i've got this and just like you hook up a speaker i'm just connecting up to this unit and now at this point when i play the music through this it just goes into the non-inductive load here instead of into a speaker it's not all that hard like i said probably would have been easier to set up with this got the function generator turned on feeding into the amplifier i'm coming out of the amplifier feeding in down here to the oscilloscope probe on one side for the sake of simplicity for me i fed the other side out of my little bnc cable up into my dummy load but i could have just replicated this again on the other side of the bench or beside of this for the other channel okay but you can see i'm getting i've got this probe set on one x right now i'm getting the exact same l that's left and right channel you're seeing up here there you go i thought it'd make it a little easier to see but i've got left and right channel both right here same amplitude same signal um you know scope probe and using a you know b and c connector over tight on the two ends of this thing exact same at rf frequencies could be slightly different story but at audio frequencies we're good to use uh things like bnc connectors for this type of thing all right let's get into the amplifier itself okay same as we were before i've got two signals being fed here on the oscilloscope left channel right channel and i'm just feeding out of the back of the amplifier into the dummy loads like we had talked about before and what i'm going to do is walk you through a few things that i like to do before i actually get into the amplifier with an oscilloscope probe itself right i'm just measuring across the outputs right now and i like to start out oh yeah by the way a tip never run an amplifier i don't care if it's solid state or tube without something connected to the outputs of both outputs on it or if it's a mono amplifier always have a speaker on it or a dummy load operating without can cause damage to your unit so it's easy to do when you're bench testing to hook up one side forget to hook up the other things of that nature all right so we got two signals here left and right measuring across the output is all we're looking at right now first thing i want to do is i want to vary the input so i'm reaching up here on my b and k precision and i'm just using the amplitude and i'm going down to as far down as i can go before i lose the trigger on the signal basically right and then you see there i'm starting to lose the trigger um and i didn't see anything different between the two okay but then i'm going to start cranking the amplitude up you guys seeing anything look at this notice the bottom one here notice the sharp kind of corners here on the sine wave notice they're sharp on the top but then you get to this one and it's all blurry and muddy here we've got some rounding going on here clipping of our signal on the bottom half here and you know there's a lot of things that could cause that it could be it could be out of bias could have some leaky coupling there's there's multitude of things that could cause this to uh to do this but you see here finally the second one will start clipping but the first one clips way way way i mean all the way back into right there i can tell a difference um so even about half power of this amplifier or less so let's take it back down then until we do have nice equal signal if we can get that the other thing i like to do is sweep the frequency now i am going to have to reach over here to the time domain to kind of change my time domain to keep up with what i'm doing here okay if you'll notice at some point my signal gets really flashy on me right i'm down to around 100 hertz right now okay i mentioned earlier there's this chop alternate button excuse me i meant to mention it earlier in the previous video and i didn't you can see here the difference so this is alternating between two signals here and it gets really hard to read that when you get way down here okay but the chop kind of takes a little bit of slices all along the way so one of them lets a full wave play out then it plays the next one below it this one does just a little bit all along the way of each one so if you're getting down into low frequencies here um you use that button to kind of help you out but uh you know as i keep going down here in frequency if you'll notice one of these is still kind of nice and clean the other one it's getting some ugliness on it right um which which means this thing's not as responding well on uh on low frequencies on one channel as it is the other and then i'll i'll take it the other direction right i have to keep keep adjusting this here on the right to kind of keep up with what i've got as i'm sweeping okay and i'm up to right there i'm maxed out on that scale so i'm gonna flip up to the 10k scale um so i was at four kilohertz there's six kilohertz um all right i'm at 32 kilohertz and this thing's still looking good i get way up here at 40 kilohertz and i'm starting to notice how they're squished a little bit together but the samples never designed to work at 40 kilohertz none of us can hear that anyway by the top end of the audio spectrum for a nice young individual might be around 20 kilohertz right here and this thing looks really good around 20 kilohertz you know my max hearing maxes out around 18 kilohertz so um but you can see we definitely have as we get into right and notice it's doing better on the high frequencies at a higher amplitude than it does down here at the lower frequencies on this uh it's higher amplitude here so we definitely got something different than one in one channel than the other and i believe this is affecting the sound of the amplifier as well because i played some some music through it a while ago with it like this and i can definitely tell the difference between one channel and the other all right so we've got our amplifier here on the bench um i did fail to mention this earlier but this amplifier is plugged into my variac which then plugs into my isolation transformer which then plugs into the ac wall outlet so this this amplifier's chassis here is isolated from the earth mains earth ground in every way shape or form at this point so that creates a problem for me when i'm wanting to use my oscilloscope now if you'll remember earlier i was measuring with my oscilloscope across two things from one lead to the other or across the dummy load and i was able to read across both sides of it but if i'm wanting to use a scope probe with just one end and not have to connect up this little connection here i need to reference my chassis back to earth ground now i've looked at the schematic of this unit and i've also looked at the unit itself and i can tell you that the center tap on the transformer here is tied down to ground right here and i can also tell you that the signal path ground is tied to ground if you'll notice this right here is the ground between the two speaker lugs if you'll notice it's tied to a center tap here on this terminal strip that goes right down to ground similarly right here we're tying our our signal path to ground so the chassis of this amplifier is not different than the signal path chassis or ground um so i don't have to worry about using a differential probe in this amplifier on a bridged amplifier and and or android differential amplifier be a different story i'd be one of i want to be working in here with a differential probe but i'm okay working with this but i need to establish ground in some way shape or form so i'm going to take one end of a green alligator clip thing here i'm going to clip it on ground and on my oscilloscope here i am going to simply plug it in here now this will work fine for audio frequencies if i was working in rf frequencies i would need to abandon this long ground cord here and i would need to clip onto the chassis ground somewhere here and then use this unit and this is just a pain fortunately we're working audio frequencies you're not going to pick up that much noise through this chord here not enough to hinder the level of troubleshooting we're doing so now i'm free to use one hand keep the other hand remember off of this chassis and i'm free to start clipping into the circuit at various places okay now if you'll notice what i've got here two oscilloscope probes right um plugged in over here onto the oscilloscope and you could do this with one probe but i like using two and the reason being i like comparing left to right channels constantly so what i've done right here is i'm just feeding on the rca input where i'm feeding the signal into this amplifier so in other words pre-amplifier what i'm going to do is crank up the amplitude of the signal i'm feeding in i'm just wanting to make sure that's as far as it'll go that's all my uh small my function generator will feed in there what i'm validating is that my source isn't the cause of my problem okay maybe i've got a bad you know maybe this little uh unit right here was having a problem or was bad or maybe my cable was bad or something but i have validated at this point that i'm able to feed full signal into this unit and not lose any attenuation on one side or the other okay and what i've done next now is i have clipped on to the grid leads of this 60 u7 so i'm on still on pre-amplification but i'm on the other side of the voltage divider network and the grid leak resistor and the grid stopper resistor feeding into the 60u7 so i'm into the circuit but before the first tube and guess what what are we seeing here we're still beautifully matched signal on both sides okay what i've done now is i've came on the other side of and i just connected them one at a time i came on the other side of the plate of this tube so i looked up two two pin layout for that too but came on the other side of it and then i jumped on the other side of the coupling capacitors which connect directly to the grids of these two tubes right here so one comes straight across this coupling capacitor and i'm clipped on to it right here the other one comes across here ties off on this tab and then it comes down via this red wire and feeds to this point right here so now i'm feeding over on i'm post the 60 u7 okay the first stage of amplification and now i'm feeding into the actual output tubes and look at my signal up here now i can move them down on top of each other to compare them make them bigger to kind of get precise you can tell there's a slight bit of difference there between them i mean like a just a teeny little bit and all that typically is is the difference in maybe resistor values on this stage or potentially the tube itself one side's a little weaker than the other i've not actually measured this tube so um but that's hey this is way close enough for what we're doing here today so what does that tell me it tells me i've made it through the input through the first tube the driver tube here made it on the other side of the coupling capacitors and i still i'm still matched up quite well here right so it must be something to do with these output tubes okay and so let's move to the other side of the output tubes okay so now what i've do is done is i've moved my scope probes over one at a time here to the little wire that feeds over to the red plug here um the positive terminal of the output and the same over here on the other channel and lo and behold check it out we're on the other side of the 6bq5 now and if you'll notice bottom signal looking nice and clean this top one got lots of squishiness to it so now you're sitting here going hmm i wonder what what's going on there we're going to try one more thing here last what i'm going to do is i'm going to go to the cathode of the output tube okay you know anything about cathode on a 6bq5 layout it's pin three so i'm gonna clip onto pin three on one side and i'm gonna come over here and i'm gonna click on to pin three on the other side of this tube okay now i want you to watch this keep in mind there is a voltage built up across the cathode right and it varies with the ac signal however you're trying to bypass some of that with the cathode bypass capacitor and you're trying to send it off to ground watch this you notice the top signal here stays rock solid and i probably inverted my probes when i when i moved them notice this bottom one here what is it doing up and down it's kind of and it's also having a hard time even even uh there you go but if you'll notice it up and down okay up and down what that's telling me is i've got a problem with my cathode bypass capacitor it is charging and discharging charging and discharging so i've got something wrong with the cathode bypass capacitor so what i've identified here is this coupling capacitor right here is leaky okay and now i'm gonna i'm gonna have to admit to you i caused this problem intentionally because there was nothing wrong with this amp to begin with i uh i simulated a leaky coupling capacitor here i'll show you here okay maybe you can see it i snipped it loose but i had put a 4.7 ohm resistor in parallel with this capacitor hidden behind it and that's what happens when you get a leaky capacitor is it builds up internal resistance otherwise known as esr equivalent series resistance and you'll start having weird stuff going on in your circuit because all of a sudden your capacitor is also acting like a resistor and instead it should be fairly transparent to an ac signal and let it flow on through and bypass the resistor in the cathode so your dc bias is established off of that resistor but the ac kind of flows around it through this capacitor that's why they call it a bypass capacitor but i basically made the bypass capacitor null and void here by putting this little capacitor i mean resistor in parallel with it simulating a leaky or bad um electrolytic capacitor let's test the circuit now that that's out of out of play okay i hooked it back up to the dummy load so i'm feeding out of this unit measuring across the um the dummy load resistors and i'm going to increase the input on this so we're back to where we started and if you'll notice i'm able to feed it all the way up here let me drop the scale down one time but you can see both of these they both start clipping out top and bottom at the same time which you're always going to have that you're always going to have when you overdrive the amplifier you will go beyond the swing allowed on the load line by the tubes either in the driver stage or in the output stage and at some point you will enter this state of clipping right and that's when you start having distortion come in but good gosh we're putting out a lot of uh a lot of uh power relative to this uh what we'll call flea wad amplifier but you get the point we've solved the problem it was as simple as uh getting the leaky capacitor ie what a bootstrapped with a 4.7 ohm resistor across it to simulate a bad capacitor and you can see how we chase that down using nothing more than a couple of oscilloscope probes um didn't even use a schematic i'll tell you what i'm going to do i'm going to take you back over and walk you through the schematic real quick just so you kind of know what we did all right if you felt comfortable with everything i did on the bench a while ago you may not need to watch the schematic walk through you're welcome to otherwise if you choose to drop off at this point i'll just say thanks for watching i hope you learned something i know i had fun making this video otherwise stay tuned probably another 8-10 minutes here to walk through the schematic and i thought i'd i thought i'd break it down this way so the items on here in red are the things that we did back here on the bench just a little bit ago if that had not solved the problem there would have been some next things i would have done with the oscilloscope and i thought i'd show you those and then there are probably some things that i would have done with a digital multimeter or might have been better to use a digital multimeter for than the um the oscilloscope so i've got those outlined as well so let's orient ourselves a little bit to the schematic down here at the bottom we have the power supply and there's only one power supply for this entire amplifier and they drew it once down here it's easy to identify you typically see something going to the wall outlet here you typically see a fuse power switch transformer here we have our rectifier tube then we have a clc network here for filtering the ripple off of this b-plus right here or high-voltage signal and then we've got another crc here that drops the voltage down a little bit and feed so this would feed like the 6bq5 tube and this would probably feed uh maybe the screens um and then there's some other dropping resistors up here in the top that would feed the uh the front end tube or the uh the driver tube anyway the top part of this schematic here which is let's just say this is the left channel they only draw it once on the schematic but yet in reality if you go to build this amplifier you have to put everything that's in this top part in the amplifier twice okay and so they just don't show it because it's redundant but this let's say this is the left hand channel there's also all of this again on the right channel okay so let's walk through here input rca jack this is where you know this is kind of the second place we went i'm going to go over here to the other side way over here on the speaker outputs this right here is what we were measuring now if you'll notice here the other side of the speaker terminals was actually grounded to the chassis here okay so i could have gotten by with actually only connecting the one side of the scope probe to the dummy load on the red the positive side and then the negative side would have carried back through the speaker wire back to chassis ground but sometimes these are floating so i went ahead and connected both of them anyway okay but this is where we started out and this is where we saw a difference between the left and right channel as we cranked up the amount of input over here on the left right so then next after we noticed there was a difference we came over here and we connected on this end right at the input right at the jack and we validated that nothing upstream right the it wasn't a problem with my cabling it wasn't a problem with any of the adapters it wasn't a problem with the um the function generator in any way we had equal signal coming into this point then this is this is a voltage divider here made up on the front end it's also kind of the grid leak resistor here and the grid stopper resistor going in here so i thought let's get on the other side of that just in case there would be a difference between these resistors right on one tube versus the other tube on the front end or one one half of the six cu7 versus the other half of the 60 u7 so we clipped on right here here then the grid the input of this six eu7 tube and we had the exact same outcome at this point in time so that that led me to believe i eliminated this it's all about the process of elimination and it's all about comparing left and right because then it's about knowing something is working right and something else is not and you're going to just chase that until you find it in the circuit all right so since my signal was good going into the tube then we want to look at the output of the tube okay and so we looked up the schematic here for the earth to pin out for the 60u7 and pins one and six are the plate and so we could have clamped on right here at the plate but then we're dealing with a lot of high voltage so we said heck let's just go on the other side of this coupling cap remember this coupling cap allows the ac to flow through but it blocks the dc okay and keep in mind right here we got 245 volts sitting here this is the other voltage divider i was talking about here the plate load resistor that feeds over here to this that drops it down to 118 volts but anyway we jumped on the other side of this capacitor so we're not we're not dealing with that big dc offset we're just dealing with the signal passing through and so we clipped right here onto the grid of the 6bq5 okay pin number two in this case so we clipped onto that and we noticed at this point we still had good signal so then look what all we've eliminated we've eliminated all of these components right here from being suspect and comparing the left channel to the right channel right and then we went over here on the other side and clipped right here we could have clipped right here on the plate right number seven here's the problem okay then you've got 255 volts of dc offset to deal with and most scopes have you know if you put if you push the little ac button for coupling they can handle 255 volts so it's not that big a deal we could have done that but it was just as simple for me to clip over here on the other side of the transformer either one would have worked but the bottom line is coming out of this we we didn't i mean we had the problem so i knew the problem lied somewhere between this red dot and this red dot and it's probably not the output transformer i could have easily clipped on the input here and told that and i probably should have clipped over here on pin seven of each of these just so you guys could have seen that the signal had the problem on the other side of the tube and we would have ruled out the output transformer you know i kind of jumped a little bit because i knew it wasn't the output transformer here and and it typically isn't the iron causing your problem right so what does that leave if we're good here up to this point this red dot and we're bad on the other side here there's not much left okay really all that's left to this tube right here is the voltage feeding it and keep in mind the same voltage is feeding both sides so if it was a problem with the power supply here at the 245 volt level or 245 volt we would have seen that in both channels more than likely okay unless it was just bad soldering or you know wiring or something of that nature so it's probably not this well that only leaves two components at that point this resistor and this coupling capacitor well i was trying to solve this with the oscilloscope and not use a multimeter so i didn't want to pull out an ohmmeter and measure that resistor instead i just clipped on right here right and i measured and watched what was happening in this circuit one of them was functioning properly the other one wasn't right and it was really easy from that to dissect it right down to these two components at that point you could have pulled out your own meter measured this and you could have measured some resistance with an esr meter across this capacitor that would have told you just what i found out we had a bad capacitor right so at that point you know we swapped out this capacitor i removed the parallel resistor i put in it that was making it leak pretending to be leaky here and um and we got our bias back in line and whatnot now if i had not been trying to demo all of this with an oscilloscope and i had just been showing you how to troubleshoot this amplifier there's a couple things i would have done along the way as well one i would have used a digital multimeter earlier on to check these voltages here i would have checked the voltage at the b plus i would have checked the voltage here going to the screen of this tube right i would have checked the voltage here which should be sitting around 5.7 volts which causes a negative 5.7 volts bias over here on the cathode and i would have seen that one of these probably sitting at 5.7 volts the other one was probably not sitting near 5.7 volts okay so i might could have figured out this a little quicker with a digital multimeter right here and i could have validated my voltages were in spec but let's say that it wasn't this capacitor and we needed to keep troubleshooting this amplifier what might i have done next to figure out what was wrong well really there's nothing left in this amplifier other than the feedback loop here right so this is a negative feedback loop comes across right here's a little bit of i think this is a zobel network made up here comes across goes through this scenario here comes down feeds back in here to the cathode of the original 60 u7 and it looks like there's 17 db of negative feedback there well i would have clipped my o scope here and compared left channel to right channel is there something along here causing me a problem right i would have clipped then on the other side of this 300 picofarad in this 10k over here to see hmm there's something there causing me a problem so if we'd gone through all that you know at that point i'd be left with you know probably go test these tubes could be a problem with one of the tubes um you know really weak or you know acting up type thing um you know beyond that i would probably just repeat my steps until i till i and fine tune in a little bit until i found the actual problem so all right guys i'm going to call this a wraps hopefully this is what you're looking for i'm trying to make these super practical not only showing you what i'm doing on the bench but also trying to walk you through the troubleshooting logic i use here hopefully trying to make you guys better uh troubleshooters yourself and i'm not saying i'm perfect uh i've just done a lot of this and i'll tell you this whole thing it would have taken me as long to put this amp on the bench and get it in the rack and then hook it all up and get all the test stuff set up um almost as long as it would to have take to solve the problem this is probably a 10 to 15 minute solve at the most here on the bench so you saw the logical steps that got you there very quickly all right coming up next will be a push-pull amplifier then i'm going to try to do a solid state i'm a ranch unit or something or another and walk through it give me some feedback down below is this what you're looking for out of this series if not i can course correct a little bit and adapt in the next ones oh yeah by the way in the next two i'm not going to drag you through the weeds that i did in the beginning of this one you know showing you the whole setup and talking about the isolation transformer in the ground and all that i'm just going to jump right in on the bench to uh what's the problem and how we're going to go about solving it so thanks for watching everybody hope you're having fun i know i'm having a blast we'll stay at it

Channel: Blueglow Electronics

Views: 11,983

Rating: 4.9810128 out of 5

Keywords: dynaco, heathkit, AR, audio research, cary, klipsch, marantz, luxman, knight, mcintosh, altec, sencore, tektronix, fluke, fisher, hh scott, western electric, akai, pioneer, sansui, harmon kardon, gibson, fender, mesa, 300b, single ended, tube amplifier, tube amp, solid state amplifier, krell, analog discovery, bottlehead, output transformers, transformers, hammond, alinco, atlas, collins, drake, dentron, elecraft, flexradio, hallicrafters, galaxy, gonset, icom, yeasu, hy-gain, kenwood, national, swan, ten-tec, passlabs

Id: KkJ_WOVWQWM

Length: 34min 19sec (2059 seconds)

Published: Thu Feb 18 2021