1966 Marshall JTM-45 Mk II in for Repairs......A Rare and Wonderful Gem !!

[Music] so [Music] well greetings out there on youtube land and welcome to today's blockbuster video featuring the contents of this very large and cumbersome not to mention heavy box it actually required the use of the hand cart to get it here into the workshop so let's uh without further ado open this jewel up and see what surprises await us within well it looks like we're confronted with the old box in a box ploy here the inner box has been insulated from harm all the way around by these air bags except on one end and let's hope that that one didn't suffer any damage as a result of no real padding okay without further ado let's move those bladders away and let's take a look at what's in that inner box well after adjusting my truss uh i've managed to hoist the massive inner box up here onto the workshop tabletop and looked in the bottom and lo and behold two packs of treats for casey jack and ollie we really appreciate and also a little hint here as to what we can expect in the large inner box and we see here a bulging ended power cable so all you marshall fans uh better get ready because it looks like we're going to have something special to check out and repair well after carefully removing about three square parsecs of bubble wrap and plastic wrap with bandage scissors lo and behold this is what lurked within the box and for all you marshall fans out there i think this is going to be a moment to remember because it is a jtm 45 mark ii and from what i can gather from this type of script on the front the gold uh plexiglas front control panel and all this is probably from late 1965 maybe early 1966. now jtm 45 marshals are of great both financial and historical value in that it was the very first amp that jim marshall ever created now he started out with the regular jtm 45s like in his kitchen on his kitchen table building them by hand but once he developed a market for these jewels he had to get the equipment and space and all to move into some sort of mass production and this as a mark ii would have been uh one of the early amps that were produced with a little bit better facilities and equipment and also it's my understanding that this particular amp was the very first to have the circuit which is designated as a model 1987 circuit okay that number does not refer to any date or time of manufacture it's just an arbitrary number the 1987 is considered the lead amp for regular guitar use and there were also amps made at the same time for base and pa use that had different circuits there are ways to tell the exact vintage of these based on the type of script that you'll see up here and this script has been changed but i think it's pretty much the way that the script should have been on this particular model we're going to go over every square inch of it inside and out so uh let's take a look here at the front with all the the gold piping in place there's a little disruption in the gold plexiglas off stand by an indicator now look at all the tone controls presence base middle treble high treble and what's kind of strange is we have loudness 1 and loudness 2 which are going to uh control the volume then of this pair of inputs and this pair of inputs now why i say unusual is that this knob seems to have two purposes high treble and loudness for the first channel but in reality uh if you look here at channel one and channel two this is the channel that favors uh the treble frequencies and this uh favors the normal frequencies okay so uh it's as if the tone controls are over here on the left and these are simply the two volume controls for the two very different channels now i think this harkens back to the fact that as we all know the marshall circuit was originally based on the fender 5 f6 a basement circuit and in that circuit we have a bright channel and a normal channel and uh in this case the bright channel is called high treble and we have our normal channel now i i think originally the bright was probably for uh regular guitar use and the normal was for bass guitar use so now let's take a look at a couple schematics to see if we can understand how the difference in tone was achieved between the two input channels first in the basement you'll see that the bright channel comes in here to the first triode of the 12a y7 and then it proceeds to the volume control well uh a .0001 microfarad cap provides a bypass for the high frequencies to get around that volume control and that produces the brighter tone in this channel now on the jtm 45 let's look first at the normal channel no matter which jack you plug into you're going to be feeding the grid of the 12 ax 7 first triode the output from there will come around here to a 1 meg volume control pot which sends more or less of the signal to ground and then what remains will be sent here around the loop and into the first triode of the second amplification stage but when you plug into channel one or the high treble channel you feed the second triode grid here of the 12ax7 and the plate output comes around here to a one meg volume control and so far it's the same but there are two bypass caps this one here uh which is uh 100 picofarads will bypass the volume control and then this one here which is uh 500 picofarads will bypass this resistor so as you can see the high frequencies are allowed to bypass both the volume pot and the resistor and proceed on to the same triode of the v2 or 12ax7 second stage of amplification now originally possibly uh channel 1 was for guitars and channel 2 was for bass guitars um you could look at it that way but i think a more practical application of this difference is channel 1 for humbuckers channel 2 for single coils okay this tendency to favor the treble frequencies then we'll kind of clear up any muddiness that might exist in your home book or pickups or as many people do simply jump or channel one and two on your marshall amp and get the best of both worlds now while we're in this area let's talk about a second uh feature of marshall amps and that is i think most will agree they're about the loudest amps on god's earth for their wattage and um how does that happen when they're based on a 5f6a baseman why are they so much louder well let's talk about a couple good reasons for that the first preamp tube in the basement is a 12 ay7 it has an amplification factor of about 40 the first preamp tube in the marshall jtm 45 is a 12 a x 7 with an amplification factor of a hundred so right off the bat in the first stage of amplification you have uh a tube in the marshall amp that has about two and a half times the amplification factor of that in the basement amp also recall that these early basement amps used 5881 output tubes which is sort of like a 6l6 gb not a particularly high output tube whereas the marshall used either 606 gcs or kt66s as are used in the amp that we've received both of which have a much higher output wattage than 5881s so even after fender changed over to 606 gcs in the basement they still had the relatively low amplification factor first preamp tube compared to the 12ax7 in the marshall jtm45 okay so uh i think these feet have been added to it but the cabinet the toll x the handle absolutely extraordinary i think for an amp this old it's just in beautiful shape let's turn it around and take a look at the back before we turn it around i thought we might have just a quick little tidbit here about where the jtm came from on these very first marshall amps it's jim and terry marshall it's just it's their initials and the number 45 refers to the rather conservative estimate of the output power of this amp in rms1 in a business where the tendency is to way over rate your amps just like the horsepower and american muscle cars marshall actually was rather conservative here and i have no doubt that if you hooked one of these up to the gear necessary to measure output wattage that almost all of these could make at least 45 watts of rms power as you can see one of the original knobs has lost its little kind of gold anodized hubcap out here on the front if any of you out there in youtube land happen to have one of these little gold discs and would like to sell it please let me know and we can see that the rear is every bit is nice or nicer than the front okay the toll x is perfect the gold screen to allow ventilation is in perfect shape see the label and it's kind of a shame they don't have an address because in late 65 i believe marshall moved to hayes in middlesex england and that would give us a clue if that address were on here we know that it was late 65 early 66 but i i really think there's no doubt that that's exactly what it is we have here on the control panel and this is white i don't know how it's going to look in the video but it's simply white and appears to be like a plexiglas or bakelite material it's not shiny it's kind of dull and we have here a two amp fuse the bolgen mains type of receptacle we'll take a look at that remember the power cord came with this we've got since these were made for use all over europe and the united states the transformer can accept a 110 115 120 220 240 volts of ac supply and depending on where you plug it in you pull this jumper out and put it in to the right two holes and you'll get the proper uh voltage for your circuit the output transformer also came with a jumper so that you could use either 8 ohm 16 ohm or 100 volt and i'm thinking that's one of those constant voltage types of speakers uh where you use the amp uh like for a pa system something like that heaven forbid okay it was in the 8 ohm jumper position and i'm going to put it back into that just a second there it is and we have the quarter inch output jack for the speakers just a single output jack also everything is in a real shiny black finish it's not corroded steel as we're kind of used to seeing this is probably the serial number and it's 1954. a four digit serial number and a low one at that so with all that said uh let's unscrew this rear panel and take a look at what's inside rear panel has been removed and you can see that the extruded gold probably aluminum ventilation screen is in just beautiful shape we've got some cracks here from drying of the wood over time remember it's really humid in england and then when they send these to less humid areas the wood will dry out and crack very typical uh you can see that quite a bit of what looks like i don't know what contact cement or weather stripping adhesive has been slathered on here to hold this white panel on the rear of the chassis the chassis itself has no affinity to our magnets so it's obviously aluminum uh and you can see where all the tubes have been removed and packed back here uh so let's get them out safely and then we'll proceed with our inspection of the amp chassis right all the packing and tubes and all have been removed and i can see one of the reasons this is such a heavy uh amplifier head and that is look at the size of the transformers in this jewel output transformer power transformer and filter choke you see these really snazzy spring loaded retainers and we got a nice pair of vintage kt 66 output tubes we have three vintage 12ax7s and we also got a pair of tube dr kt66s which are closely matched in case we need to replace the vintage pair we also got a really nice letter here from the owner of the amp he introduces us to the amp the only day codes he could find were on the mustard caps which thank heavens must still be intact which point to 1966 no codes on the pots we'll see uh two apparent problems volume control for channel two has an area about halfway up where the volume cuts in and out uh turned up past this pointer down below seems fine let's hope there's not a damaged area on that carbon track inside the potentiometer we'll disassemble it and then inspect it to see okay the uh he tried to clean the pots but there was no way to get into them second problem is red plating a couple of years ago i noticed a degradation and sound found a bad power tube i replaced it only to have the same thing occur again okay what does this sound like to us those of you watched or videos know that coupling caps are a really common cause of uh kind of an acute onset of red plating okay in other words amps working fine no problems and then it starts to red plate something's going wrong inside and almost always it's coupling caps why because we know that the coupling caps being capacitors uh will block the passage of direct current so the b plus cannot get past them to the grids of the kt-66s okay if they can if the b-plus can get to the grids the grids will become increasingly positively charged which is like putting your foot on the accelerator and the plate current will just run away within the tube red plating and eventually destroying the tubes themselves okay so i think that's what happened he plugged in a new power tube and it failed almost immediately because it was subjected to the failure of the coupling cap now apparently both of the tubes red plate well that makes sense one coupling cap goes bad the other one's probably going to follow suit fairly soon okay so we're going to look at that he says the amp has been recapped well if the original mustard caps are in it apparently that was just the filter caps and even that was years ago okay other than that it's original except for the logo we already saw that which was replaced when i got the ant 25 years ago and the big cushy rubber feet that we spotted as being not original but probably a whole lot better than original okay the rest of this is just good luck do your best and keep me posted so i think we know exactly what we have to focus on here and i think to get started uh the first move should be to remove the screws that are holding in the chassis and extract it from the cabinet the extraction of the chassis is hampered just like it is in fender chassis removal where the nuts that hold the chassis to the retention screws are not captive they just spin freely so for each one of them you have to reach in put your finger against that knot and keep it from spinning really kind of a shortcoming but let's face it it would have been real hard for them to weld those nuts to the aluminum chassis the four screws uh are out the chassis has been removed we see that there is an aluminum plate at the bottom to shield the circuit probably from rf interference the interior of the cabinet shows that i don't see any patching or repair work that it's i guess remained in sound condition all its life which is really nice okay let's set this cabinet aside and take a look at the chassis and here's the chassis and all its glory with the spot here for the gz34 in this case 5a r4 rectifier tube okay these very early ones were tube rectified then they went to solid state and stayed that way i see no markings on any of the three transformers those of you who are experts i'm going to show you the transformers and you see if they appear original to you i bet they are but it's really hard to tell these very early amps in this case this one particularly has almost a homemade look to it okay this is fairly thin aluminum and it's kind of soft and lumpy and i've never seen anything like this but the ends are reinforced by what look like cast pieces of solid aluminum that are inserted on either end and then screwed in place to give structure and and all to the kind of flimsy aluminum sheet chassis as you can see the those pieces are on either end okay they're very solid and probably confer a lot of strength to an otherwise rather flimsy chassis okay uh let's flip this over and take a look at the circuit before i flip it over let's take a close-up look here at this eerie and i mean that as in lake erie not weird eerie electrolytic can cap it has sort of a what looks like anodizing but it's not it's a blue a kind of a plastic film around the outside and um it's says 32 microfarads at 450 volts okay now i'm not sure if that's an original cap or original style cap we'll have to check into that but it is unusual and i thought you might get a kick out of seeing it up close okay now probably what you all been waiting for is the interior view see the input jacks all the potentiometers with the bus grounding wire mustard tone cap we see here that several of the capacitors have been replaced with what appear to be asian caps they might be okay but maybe we can do a little better metal film resistor great big sprig adam okay uh nice wiring on this the turrets nicely twisted filament wires and they're color-coded so you can preserve the polarity of your filaments that may be original and that definitely isn't and neither is that kind of glaring to me okay the it i believe that the electrolytic cap that we saw on the surface has been disconnected that it's doing nothing and that these are the replacements for it i honestly think from an aesthetic standpoint this is a very valuable amp and i really think i can aesthetically make the repairs a little easier on the item so when dealing with an amp of this caliber to me cosmetics are as important as the actual technical aspects of the repairs uh these look like pretty grubby pots here i haven't seen ponce's grubby since the last truck stop bathroom i visited but these jewels are going to need some cleaning especially that one that has a segment apparently a troublesome break in the carbon track okay so we've seen the inside and the extremely inconvenient fuse here that needs to be replaced imagine from time to time and um let's get started one part of this amp is just a disappointment and i think that is this wretched bolgan three wire power cord i'm going to construct a really nice looking original style power cord using this receptacle and a much more modern three-wire cable okay i've liberated that bulgan receptacle from those really wretched wires and i dug through my stash of old 1940s jukebox parts and found this uh type of receptacle here and i've bent the little pins out and i'm removed the bakelite discs and the wire now it looks to me like with these tabs bent in just the right amount they can engage the threads of the bulging receptacle and this can screw on and actually be i think very much like the original uh plug back that came with the original power cord okay prepared the new three wire cord with the short leads soldered and bent at 90 degrees so that they can go into the holes in those screw terminals and one other thing that makes this cap ideal for this purpose is the inside is heavily insulated with cardboard so there's no chance of a short against this that might shock you now being very careful to observe the polarity of the receptacle i've inserted the three power cord wires into the screw terminals and secured them now i've crimped those fingers over the threaded raised area of the bulging receptacle and all we need now is a grommet back here to to take up the space between the hole in the metal cover and the cord the grommet fills in that empty space and uh now we have a really nice safe and solid plug to connect our amp to the ac wall socket now as a final touch because i have a feeling the original was probably painted black i sprayed it with some semi-gloss black so now we've got a really nice looking plug for a really nice looking amp well now one of my favorite segments of any video is opening up gifs from our very generous viewers in this case a gentleman named mike mendito received over a short period of time four separate packages here so god only knows what's in them uh jack is very interested here comes casey to do some serious sniffing and from their reaction i can tell you this is good stuff okay so let's start opening them up well how about this for an absolutely fantastic haul first off a pair of channel lock crimping pliers i pretty well worn out my old crimping pliers and these are way better so that's going to really come in handy three different gauges of crimp on butt joint connectors for those of you whose butt joint isn't quite connected automatic wire stripper cutter a color display i believe guitar tuner a huge container of cat treats which should last about a week and a half with these voracious beasts and then the prize of all time a vehicle super pro okay that's all that it does right there you can stop the video and read i'll have to go back to school i think and learn how to operate this jewel it looks like you use it for diagnosing and repairing electrical wiring in vehicles so it's really going to come in handy because i do a lot of that okay so the card that came with it said happy birthday from mike mandido and sadia rodriguez so thanks so much to you too uh your generosity and kindness are unexcelled okay we really appreciate it thanks so much and jack of course always enjoys that packing material thank you all looks like jack is enjoying the shipping container i know we'll be enjoying these two very soon [Applause] since i find this kind of diagonally inserted modern capacitor to be very visually annoying here and it just doesn't fit in to the circuit i've been giving some thought to actually drilling a hole in the old worn out eerie capacitor and inserting a 33 microfarad capacitor and to test my theory i grabbed an old mallory can cap and drilled an 11 16 hole down to the center of it and you can actually press the capacitor down inside i'll have to put an extension on the ground lead so that i have two leads coming out the top of the capacitor actually the bottom of it and then those can be wired into the circuit the outside of the can cap would be visible to all looking into the back of the amp so it would look original and when you looked in at the circuit instead of seeing that you would just see the bottom of the can cap with two wires coming out and going to the circuit i think i'm going to give it a try i think this um the age and and specialness of this particular amp require maybe uh some extra procedures here okay so let's see how this works out before we start two final little uh points here as you can see the new capacitor will press down inside the old one and also you can see from this bubble here that there are internal problems in this capacitor which uh did necessitate its removal step one i drilled out the rivets that are holding those two terminals now we're going to start with a small drill right at the center and then increase the diameter of the drill bit so that we gently uh increase the size of the hole that's going to be drilled down into this electrolytic capacitor well the process was a little different with the erie capacitor it reminds me of when they would mummify people back in egypt and they would extract the entire brain through the nose with different tools well that's pretty much what i did i now have a nice empty can i'm going to insert a piece of foam that takes up the outside diameter here and then press my new capacitor down in the center and then create a new cap for the capacitor i'll show you uh in just a second or two how it looks when it's done i had a change of plans with the stuffing procedure here on that 33 microfarad original erie cap that sits right up here in full view on the chassis and i was going to use a 33 microfarad at 450 um asian sourced uh capacitor and i thought well you know those kt 66 is operated around 450 plate vaults and probably higher now and that seemed a little shabby to me so i stepped up to f and t which i think is a better quality capacitor 30 microfarads at 500 volts now the price i'm paying for this uh better cap is that it's much larger in diameter so i had to use a rotary file in the drill press to open the hole up to allow this cap to fit in snugly okay so i'm going to do that and then we'll have the two leads coming out here through holes in this cap that i produced that i will then glue on to the capacitor and make it look even more original all right i've soldered on an extension to the negative or grounded lead from the f and t electrolytic cap and now it's time to press all that down into the uh erie can and then attach the lid the f and t cap is now nestled happily within the empty eerie can i cut a few little blocks of foam to put around it to keep it pretty well centered and stable now it's time to glue the lid on we know that the longer lead here is going to be the negative or grounded lead there was no need to be concerned with it being a bare wire inside the can since it's all going to be grounded anyway okay the one we worry about is the positive lead and it comes straight out the center and here we have our reconditioned erie electrolytic cap so let's install it up here on the chassis and wire it into the circuit here you can see where that replacement cap has been just double stick taped in here hanging over the hole where the original capacitor was let's get this out of the circuit paying attention to uh the positive leads and where the negative was grounded all right i removed the replacement capacitor and these two wires will be soldered together and the positive lead will go to their junction the negative is going to ground back here on this uh little grounding tab also i'll be honest with you this just sticks out like a sore thumb to me this bright blue 0.047 microfarad cap it's between pin 8 of the rectifier and ground it's in the line the b plus line to the standby switch and i think it's there to absorb the pop when you flip the switch instead of the b plus arcing across the contacts of the standby switch it will go in here to the cap okay so it will absorb that pop hopefully so let's replace that with an older more appropriate looking 0.047 cap here's our eerie electrolytic cap up back in its holder on the chassis surface now let's flip the chassis over and wire it into the circuit so now the two replacement caps have themselves been replaced by a innocuous looking brown drop .047 microfarad cap and our re-stuffed eerie electrolytic and you can see that the wires pass right along and connect to the positive terminal of the now f and t 30 microfarad cap and that the negative end is grounded to the same tab as the 0.047 i'm in the process of installing the tubes so that i can turn the amp on through the current limiter and check up on the status of the circuit the first one is a mullard 12ax7 and then one i'm not familiar with a brit mar how about those potatoes made in england okay the third one is the sovtech i might try to scare up a suitable third 12ax7 to keep up the british tradition but for now it looks like at least two of our three uh preamp tubes are a first quality all right the tubes are all in place i have the eurotubes bias probes positioned under the kt66 which are based the same as a 606 or 6v6 so i can use them uh to bias these output tubes i've got the signal generator putting in about 600 cycles per second and i'm hooked up to the shop speaker system so let's turn it on and see if number one the uh output tubes red plate and number two if there is a dead spot in the volume control i believe it was for channel one okay so that'll be the two things we'll be looking for all right the tubes are heating up and before i take it off of standby let's take a quick look at kt 66 tubes a lot of you probably aren't familiar with them they're like a sort of like a high voltage 6l6 the power out if it's at about 500 plate volts is around 50 watts in fixed bias okay the normally you expect a negative dc bias of about 67 uh volts being applied to the grids but you can see that this being called a 45 watt amp is certainly within the realm of believability plate dissipation is about the same as a 6l6 gc in that 25 watts they say is a good working maximum but that 3 30 watts is the absolute maximum the pin out is the same as a 606 or 6v6 fortunately okay so let's take a look at this and let's estimate what we think would be an appropriate plate current for these kt 66s say let's say they're operating at 450 volts okay we want uh 70 of 25 watts is what they said was a good comfortable maximum so that would be 17.5 watts and if you did go to 30 milli watts then uh times 0.7 would be 21 watts so let's try to stick around 17.5 maybe 18. looks to me like around 40 milliamps because that times the 450 plate volts is going to give us around 18 watts so we can expect then around 450 and 40 milliamps when we turn this on okay i'm going to come off a standby and remember we're looking for around 450 on the bottom and up on the top row we want to see a little less than 40 milliamps oh good lord well i think we can see why they're red plating they're close to a hundred percent over um not right at all also that's brought the plate voltage down i'm going to turn this off and uh since i thought initially it might be the coupling caps but you're not going to have two coupling caps fail exactly the same and this was quick so i think it's the negative dc bias circuit that is faulty and it and it is not uh providing sufficient negative dc bias to suppress the plate current down to like 38 39 milliamps okay so let's flip this joule over uh drain all the electrolytics and take a look at that negative dc bias supply okay i have all the tubes pulled out so that they can suffer no harm and i'm connected to the grid of one of the kt66s i've got the amp on and we see that it's receiving negative 43.5 volts a dc and as i recall the tube handbook said negative 65 was more like it okay so this may be just grossly inadequate to suppress the plate current let's check what the grid negative dc bias of voltage is on the other kt66 and it's a negative 43.8 so it's almost the same uh it sure seems to me like that's the issue here now one thing that i discovered and this amp is on so i'm not going to stick my finger in here is a little trim pot right up here that is in the area that feeding the diode for the negative dc bias supply i'm going to move the trim pot a little bit and see if it alters our bias voltage okay i've got my trusty wooden stick let's move that trim pot and see what happens okay i'm going counter clockwise and it lowers the bias voltage now i'm going to turn it clockwise and it gets right up there to where the tube manual tells us it should be let me set it right at 65 or as close to it as i can get okay uh let's plug in the tubes and see now what our plate current is here's a close-up of that little trim pot it's all metal i've never seen one quite like it and i have no idea if that's original or if it's been added on at some time but it's sure an old-fashioned looking uh component uh it makes me think either the amp came this way or it was modified very early on okay but it's now set to around negative 65 volts dc for a bias voltage let's plug in the tubes and check our plate current all right the tubes are in place and they're warmed up let's go off of standby and see if our plate current looks a little more acceptable well now it's real low okay we've got our voltages come way up these are inversely related okay the higher the plate voltage lower the plate current so i'm going to get my stick and let's adjust that little trimmer until we get uh let's say uh 39 milliamps okay let me use my little uh wooden stick and move the trimmer up until we're right at about 39 milliamps well that looks pretty good i think that's an average of about 39 and a pretty good match and also look at that the plate voltage is right at 450 it's right where it ought to be i think we've solved the red plating issue um and all and it looks like these tubes are in pretty good shape i'm going to leave it on now and make sure that they're stable which will tell us something about both the tubes and the coupling caps but i believe that uh this biasing issue is resolved okay uh about 15 minutes now has passed remember it was 40 and 38 before well it's still right around 40 and 38 so i believe then the tubes are stable the coupling caps are in good shape and i think we're good to go now it's time to test that volume control according to the letter it was the volume pot for channel 2 that caused all the grief i've got a 50 cycle tone here and for those of you who can't stand 50 cycle tones once you take off your headphones or turn down your volume let's see how the volume control works well it does have a really bad uh area there about what one third of the way up [Music] to a little over two-thirds volume really nasty uh it sounds to me just like a dirty pot with no disrespect to my local truck stop bathroom but let's try now channel one well not as bad but it's still got it so what i'm going to do is pull out both of those pots disassemble them and let's see what they look like inside and if we can clean them up you know before i go to the trouble of pulling those parts out and disassembling them um another thing that can make a noises like that on a pot is if the coupling capacitor that protects each of the potentiometers from the plate voltage of the preceding preamp tube if these are leaky and if they're letting dc get in here into the volume pot they're not really supposed to see any voltage to speak of but if they have the dc voltage coming in here as you move the wiper you can cause a static sound okay so let's just go ahead and turn this on and see for a second if we have dc reaching the potentiometers both of the volume controls through the coupling caps and what i'm seeing here is no steady negative dc voltage being applied um as you can see it's jumping around down to zero if it's there it will register and it will never go down to zero so i i guess it is that they are dirty and need to be cleaned so let's get started well after removing six wires from the uh channel two pot which was the noisier the two i have it out and it is a pretty filthy mess and it also looks like it may have already been opened okay so that doesn't bode well for uh this type of repair but let's go ahead and open it up again and take a look okay it's a part and as you can see it is a slimy mess looks like a whole bunch of probably deoxide or some other type of chemical has been sprayed into it um those are the little fingers that make the continuity um well you know what i'm going to do is is clean it up with a non-oily cleaner and see if the carbon track shows any type of damage or scratches or anything like that here's that one mega ohm track that goes around the inside of the pot and it looks really clean to me i don't see any issues when i test the resistance between this prong and this prong it's showing one mega ohm as it should so i'm going to put it together with just a very light lubrication and see if it works a little more smoothly okay that cleaned pot has been installed remember this was the really noisy one now amp is on everything's ready to go [Music] no static at all smooth as glass i don't doubt uh that it did make static you saw what a filthy mess it was inside uh you know i'm guilty of this too people just it's easier to spray some some cleaner in there and wiggle a knob and hope everything's okay but over time all those contaminants just kind of agglutinate inside the pot and you end up with the mess that we had with this one so now i'm going to take this one apart do the same thing and put it back in well both of the volume control pots have been cleaned and seem to be working just fine now i'm going to focus my attention on this obviously overheated 2 watt 100 ohm resistor it's 2 watt for a reason obviously they recognize that it would be subjected to heat and it certainly has and by coincidence i happen to have an identical carbon comp resistor uh to replace it with so let's do and just to show you how bad uh these uh carbon comp resistors can be damaged by overheating uh you see here that the 100 ohm resistor now measures 225.5 ohms okay so it's way out of spec more than double the resistance it should right the new hundred ohm resistor is in place and uh i measured this 8200 ohm resistor that follows it and it's up at around a thousand ohms so it's way out of spec also so let's change this joule and now that second resistor has been replaced with one of exactly the right value but double the wattage so that it should be able to hold up better over time and while i was at it i decided to replace that blue metal resistor uh with a matching uh 2 watt carbon comp nos resistor i thought well if those had failed how about the bias resistors and sure enough on the preamp tubes uh both of the bias resistors were way out i mean like two or more times out of spec so i replaced both of them with all the resistors and caps tested and up to spec i went ahead and plugged in the amp and started applying stronger and stronger input signals and something really scary happened the channel two the high trouble channel developed a just an intense oscillation at relatively high volume levels now i'm telling you this rather than showing it to you because oscillations are scary they can cause a lot of harm and i did not want to destroy this output transformer or these output tubes what i did was i hooked up the oscilloscope and i started back here at the first preamp to grid and looked for signs of oscillation none none none okay we went through the phase inverter and when i got to the grids of the uh kt66s the oscillation would occur and would shut the tubes down this is probably what was responsible for the dead spot in the what the owner thought was the fault of the number or channel 2 volume control in reality i think the oscillation was developing and shutting the volume down and the tubes themselves when it got to that volume level okay so the cure uh i used something that fender used in their higher wattage circuits and that is i put a very low value capacitors to ground from each of the kt66 grids what this does then as as the oscillation begins it will take it to ground and stop it sure enough with these small capacitors attached i watched on the oscilloscope cranked up the channel 2 volume and as i get up to the area where the normal type sine wave would start oscillating terribly it was absolutely stable so i'm confident then that these two capacitors here have solved the problem i contacted the owner and asked him about the tube set that he had supplied and he said he would rather have the new tad tubes installed and biased and i don't blame him you might as well get the amp back with a new set of output tubes and have it properly biased so i did that so now we are all fixed up checked out on the oscilloscope and ready to go with our audio demonstration i'm going to have ollie and jack play some familiar tunes for us and some of their own personal compositions and we're going to try uh humbuckers single coils and p90s we'll do neck bridge and both pickups and we'll test channel one and channel two and give this amp a really good testing procedure so that you can see how it sounds with all different pickups and at all different settings on the two channels [Applause] [Music] so [Applause] [Music] [Applause] so [Music] so [Music] [Music] [Applause] [Music] [Music] [Applause] [Music] [Applause] you [Music] [Applause] [Music] [Applause] [Music] [Music] so [Music] [Applause] [Music] [Applause] [Music] do [Music] [Music] [Applause] so [Applause] [Music] [Music] [Applause] [Music] [Music] so [Music] [Music] [Music] [Music] [Applause] well i guess that does it for this video on the rare and wonderful marshall jtm 45 i hope you enjoyed it we're celebrating here in rusty's workshop that we finally reached the 100 000 subscriber level and we couldn't have done it without you all so every one of you who subscribed thanks so much for helping us uh finally reach a goal that we set i don't know six or seven years ago and really didn't have a whole lot of hope of ever achieving i'd also like to extend my heartfelt appreciation to all our paypal uh contributors and patreon patrons who have kept us on the air and advertising free for another month if youtube is putting ads into my videos i apologize on their behalf they're doing it without my permission and i'm not making a cent from it i also wanted to thank mike mandito for the fabulous gifts and our old friend and artist extraordinaire the jaster who sent us this magnificent oil painting of jack reclining on the windowsill okay it will soon be available for viewing at the louvre now this won't be the one in paris france but the one in paris texas and now it's time to introduce our part two video in which i'll share with you a pretty scary experience i had recently while repairing a super reverb amp okay you'll see why it's kind of scary and uh if it's ever happened to you perhaps my approach and solution to the problem will help you uh if it ever does okay so uh if that sounds interesting please stay tuned thanks so much well greetings out there on youtube land and welcome to today's big video which will be a change of pace for us instead of simply going through and restoring a blackface super reverb like you see before you we're going to address a very serious problem that this particular amp has and it's a problem that people have contacted me about and asked me questions about so first off let's see what's wrong with it and then we're going to follow a very logical pattern uh of addressing a different parts of the circuit in an effort to repair that problem let's discuss our setup here i've got the signal generator on and putting in a 600 cycle per second tone that way we'll know if the amp is alive or not if it works and we can also sort of judge its gain by cranking the volume control up and down and listening to how loud that 600 cycle tone gets okay i also have my eurotubes probes under each of the 6l6 gcs and most importantly i have the amp plugged into my trusty current limiter okay so let's go ahead and tr well before we turn it on i wanted to tell you i already tried to turn it on once and nothing happened i checked and the fuse had been blown okay so that's our first hint something's drawing a little too much current here okay let's see if we can get it to happen again or to give us some kind of clues as to why that's happening step one i'm going to turn on the main power you'll see then that our eurotubes probes are showing that the circuit is getting the 120 volts and the primary portion of the circuit okay now let's let it warm up for a while then we'll come back and flip the standby switch and watch in a mute amazement at what happens okay we've given plenty of time for the tubes to warm up now let's flip the standby switch off of standby wow [Music] oh well well now that i changed my underwear let's start to address what we just saw first off apparently there's nothing wrong in the primary circuit that's drawing too much current the moment that i engage the high voltage circuit however you saw what happened and thank god for the current limiter okay uh let's check to see if it saved the two amp fuse first i have to unplug the amp uh from the current limiter because i have no idea how this fuse holder is wired and secondly let's open it up take a look at our 2 amp fuse and i don't know if you can see in there but it's perfectly fine so our current limiter prevented even two amps of current from flowing in here and further damaging the circuit okay so you can see the beauty and necessity of the current limiter alrighty now uh let's pull the tubes out flip this over and take a look at the high voltage circuit and see if we can detect anything wrong now before we start poking around in here i want to be sure that i discharge all of the electrolytics and i'll use this really snazzy tool that was made for me by a very talented viewer we'll go through and ground all of the positive ends of the electrolytic capacitors through a 100 ohm resistor okay so now there can be no residual voltage being stored in the circuit to test that i'm going to go ahead hook the voltmeter up and be sure so we see here at the initial point that the b plus enters the circuit we have a grand total of zero volts potential so the circuit is safe to navigate okay you have to do this every time that the electrolytic capacitors are energized let's do a quick sweep here and it looks to me like a circuit's been heavily worked on most of the blue molded caps have been replaced i don't see any areas of where you would see a lot of heat i want to wait a minute right here that's the screen resistor 470 ohm screen resistor for the second 6v6 now if there is a short in the screen within the tube it can cause exactly the symptoms that we've seen okay so uh and you know that these metal film resistors do not show this type of heat uh easily okay so this thing has been just through hell uh probably the fuse saved it from just burning up so i think we better test the second 6l6 to see if there's any internal shorts okay i've got the tube plugged into the uh hiccox 600a tube tester and over here we're going to test uh different positions here uh which will see if there's shorts within the tube okay you don't want that light to come on right there so let's go we'll go to position five nothing four three sustained elimination equals short within the tube we'll go on here to one so number three i don't really know exactly what position of the tube that's testing but we know it relates to the screen because that screen resistor overheated now if you don't have a tube tester with the short testing capability all you all i really had to do was just substitute another 606 for this tube and see if the problem went away if it did then there was something wrong with this tube we weren't exactly sure what it is when you use the tube tester and the short testing feature you can tell exactly what's wrong with the tube and it does have a short somewhere in the screen wiring also the tube's quite warm now um so what we'll do then is replace this tube with another now this tube it happens to be a jj which i've had if you've been watching my videos for long quite a bit of problem with okay uh i will not use jj tubes anymore if you love them great keep on using them and good luck but uh about the fourth or fifth time that one let me down uh i just i've decided i would rather try different brands okay just a word to the y's here okay so this tube is junk let's replace it before i replace the faulty 6l6 i'm going to replace the burnt screen resistor supposed to be 470 ohms and as you can see these metal film resistors really do hold their value okay but that dark burnt spot there was the giveaway that there was something wrong with the screen of the tube so let's install a similar but brand new and unburnt 470 ohm resistor okay here you see that the new screen resistor has been installed and i usually put them off to the side remember the amplifier chassis is hanging so that the heat will rise upward and just roast the socket and the resistor so it's best to keep your resistors out here a little bit out of the line of fire okay so now let's flip this joule over put in that replacement 606 and see if the problem is solved now it's time for our final test the old jj tube with the shorted screen has been replaced with a vintage 606 gc that i know to be a good tube signal generator is putting in the same 600 cycle tone i've been in a power on position now for long enough for the tubes to warm up let's go ahead and take it off of standby and the amp is working wow look at the difference in current flowing through the vintage tube compared to the jj tube okay you can tell that's not a very well matched pair but the point is even with that amount of current flowing the current limiter is not lit the amp is amplifying all is well in the world i think it's time to get rid of this jj and order a nice matched pair of tubes for this amplifier so just a quick review of what we learned here number one use a current limiter whenever you're plugging in any amp of unknown uh functionability okay they come in the mail i don't know what's uh right or wrong with them so i always plug them in and as you can see it saves a lot of grief um number two look at all the components uh especially resistors for signs of overheating they can give you a clue where that extra current is going if you've got a dead short situation like we had with this okay uh also drain your electrolytic capacitors every time you touch a circuit and only touch it with one hand when you do okay so that's about it uh i will put little uh excerpts like this in from time to time in videos uh whenever i think a good kind of like a teaching point comes up that i want to share with you all and i hope you appreciate it thanks for watching

Channel: Uncle Doug

Views: 64,356

Rating: 4.9770117 out of 5

Keywords:

Id: O65peHH0rU4

Length: 76min 59sec (4619 seconds)

Published: Fri May 28 2021

Reddit Comments

[u/S0undpl0t]

I love Uncle Doug. He and D Lab make up the majority of my screen time

[u/wearerobots138]

This dude is the Bob Ross of amplification.

[u/HV_Commissioning]

New one. Thanks!

[u/ELECTRICxWIZARDx]

Heckin' finally. Bout to dive in.

[u/RepresentativeOwl500]

I studied by watching Uncle Doug's videos and taking notes before I started my first amp build (5e3 clone) and it helped me tremendously.

[u/Son_of_Sophroniscus]

Thanks for posting this, never heard of Uncle Doug until now. Now I'm watching all his repair videos!