Hot glue works well ---- until it doesn't. A friend once hot-glued new mids and tweets onto the plastic speaker grilles on the front dashboard of his Saab 900 --- they came undone on a hot summer day and fell down inside the dash!. Silicone adhesive is a better choice for securing those caps!
never change the way you make these. Most here want to see start to finish including all the mistakes. Great repair. Only thing I would ditch is the hot glue as it is too rigid and can break off if the amp is handled roughly. Silicon is far better, flexible and heat proof.
I STUART IT PHIL MANNING. can you help i have a small valve amp ; it work well on my dim bulb teater 2x40 watt bulbs ,but if i plug it in to a 240v socket it go very quiet; may be it the filler cap; what do you think''the tubes are EZ80/ECC83/EL84' this data from the 1960;
Stu, Uncle doug has a couple of videos on how the internals of tubes/valves work, including the screen and control grid. Very worth the time spent watching. In short, controlling the flow and direction of electron stream going to the plate. I think this was your best one yet. I really like these old amps.
H Hello Mr. Stuart. Jack Mathews here In Salisbury USA.Are there other UK amp tech like you? Do they have a RU-vid channel like you. Yours is the best!And can I please ask you for the layout that you have for this particular amplifier Did you are working on in this video.?Thank you for the great help.And thank you for the schematic Schematic
@@jackmatthews2840 Hi JAck I think I am the only UK guitar tech with a RU-vid channel. If you go to the end of any of my vids you'll see a short end screen where you can see an email address for me. Can you email me via that and I'll see what I can send you. Thanks
Yes it doesn;t ALWYS work though particularly when you're trying to retro fit it to an existing amp. As you probably know it's a huge, complex an esoteric issue!
Fantastic amp for sure. I think most anyone could have found the resistor getting 🔥 hot (good humor). Once again, you present the issues in ways this novice can understand; Thanks for the drawings. This all makes a lot of sense; it is nice to know these things as it helps to understand what is going on. I'm curious, how hot do you like to keep your soldering iron?
Nice work Stu. I had one of the earliest PA100 on the bench a few years back that had 2 output transformers, yes really! They couldn't source a 100watter at the time of design so they used 2x JTM50 tranies in series, strange but true
Yep , leave in all the frustrating diagnosis stuff. I know exactly how it is to work on these faults and how long (days) it can take sometimes .Great job , well done .
What a great video Stuart ! For those who want to know what these amps sound like go to youtube and watch Free - All Right now - live at the Isle of White Festival 1970
I appreciate the time spent and techniques used to solve this problem. It may not be pretty but that is exactly what a tech does to properly repair a problem amp. Many thanks.
Looks as though the transformer cores are in fact oriented well. Quite likely heater to cathode induction in the preamp tubes.. Solution /elevate heater voltage or run DC heaters
Yes I've had SOME success with that but it usually only reduces the hum by 50% or whatever. Turning the OT can sometimes work miracles, if there's room.
Thanks allot for the videos, very informative! I’m guessing you have discharged the caps beforehand, but on these can caps the body is a negative, and in series connection the bottom once can still hold some odd 200v, don’t they?
Hi Yes I would have discharged all cap at the right time. If you have 2 caps in series and you discharge the top cap positive to ground, you will discharge both. The bottom one won;t hold its charge.
I'll be honest. I don't think you really fixed the hum problem I think you just put a band aid on it. I think the real problem that's actually causing the hum is still hidden.
Still haven't quite finished watching the entire video, but it occured to me that, since the amplifier is basically a mixer with separate preamp tubes for each input channel, the owner could do a little tone/gain shaping just by using different 12AX7-family tubes with different gains, *without having to make circuit changes*. Maybe a 12AU7 for one channel, a 12AYY or 5751 for another channel (these two types were used in some Tweed Fenders), and the stock 12AX7 for another input? Or, if each input only uses one triode (one half of a 12AX7 per input), try a 12DW7 (aka 7247) which is half of a 12AX7 and half of a 12AU7 combined in one channel ---- this would give stock gain on one input and lower gain on another. No circuit changes and its easy to go back to stock 12AX7s if one doesn't like the results. Of course, the phase invertor tube in this Marshall would likely still remain a 12AX7 for best results (though many amps use a 12AT7 or 12AU7 as the phase-invertor/ driver). I have a BIG Webster PA amp that uses individual 5879 pentodes (similar to EF86's but lower gain) for each mic-input channel ---- I am thinking of changing a couple of the inputs to other tube types, 12AX7 and the like, though that would require both circuit/component changes and socket rewiring....
Great idea! But the owner of this amp is one of the largest vintage Marshall collectors in the UK. He prefers to leave them as close to original as he can. Nice pension one day!
It is possible to temporarily run the preamp heaters from a lantern battery. If the hum/buzz dissappears then rectifying their heater supply would solve the issue.
Hi there. Enjoyed the video. The screen resistors are 5 watt not so much the current but the heat. Yes heat goes up but that area gets hot before it's able to travel up as it's fairly sealed in the tube socket. That's why it's 5w though. I have a few Marshall's and those things can cook at the power tubes.
To take advantage of the benefits of a pentode or beam power tube it is necessary to run the screen grid at a lower voltage than the plate. Most guitar amps don' t do this because the design becomes more complicated and expensive. Essentially the screen and plate are strapped together running as a triode. The bigger screen feed resistor is an attempt to lower the screen voltage. For a linear amp it is critical to keep the screen voltage steady which of course it won't but rather race up and down with the load induced variation of the plate. A beefy choke before the screen supply helps to isolate the screen voltage..
you probably didn't need that new ground wire there, the signal ground of that coax was soldered to a terminal that is screwed to the tube socket, that terminal most likely was the bad connection. the best fix for that is to solder it directly to the chassis instead of just the bolt.
Hello Stuart, Would it be possible for me to get a copy of this layout please? I believe I have found a schematic for it if you are still interested? Joe
Really helpful video! Showing your thinking and methods of troubleshooting provides great insights. I was wondering why you didn't remove the 4 volume knobs and make sure they have a toothed lock washer and the nuts were securely tightened to the chassis. This is could possibly have eliminated the need for the wire you added. I have a Jimi Hendrix Super 100 reissue amp, and it is very similar to this JTM100 Super PA. I was astounded to see the KT66 plate voltages running at 540 VDC! But this is how these amps were designed. A bit scary to work on :)
Good point about the pots. I'll remember it for next time. Yes 540V is insane. Mind you, just speaking to a guy today who used to work on 135KV equipment, so that puts it into perspective!
The errors and red-herrings are fascinating. Frustrating and time consuming for you thank you for the honest coverage. I wonder if those little earth tags were all screwed down tight and making a clean connection? I'm sure you gave them a tighten. The first valve amp I made I used thick copper earth cable from some house wiring as a buss wire thinking it would be well earthed and avoid any buzz. The amp motor-boated loudly, I spent hours (days) trying to stop it. Eventually I rigged a temp thin earth wire and it stopped. My soldering iron was too puny to properly solder that thick heat sinking copper wire and I had dry joints to ground. Doh!!! Got a beefier iron and reflowed, sounds lovely now. :)
Well done. Those oscillation problems can be real pigs to solve. good trick is to reverse the phase of the output transforer primary. That sometimes fixes it.
Masterfully done, as usual. I wish I lived closer to you, then I could be your apprentice an learn amp repair from an old school master, bring you tea, clean the shop, etc.
Thanks! YEs I'd be willing to do that. On of the reasons I do tyhe vids is to pass on as much knowledge as possible. That, and the $100k a week I get from TouTube...
Hello Stuart, watching your video's i've noticed you used various values of FT capacitors,can you tell me where you buy them from? the suppliers i know have limited stocks here in the UK thanks
Fun video. My story to the theme of high HT: I recently repaired an German el34 Amp from the 70s (I suppose). That Monster has 800V HT. Yes, I mean it. 800 Volts. I only found one old el34 datasheet for these conditions but they do exist in theory and, as I can tell now, in real live. Four el34s running completely maxed out to their limits. Won't last long, I suppose. But it's a terrific bass Amp. Sounds crappy for guitars, but the best sounding bass Amp I came to know so far.
Ground loops aren't high impedance, they're *low* impedance, or low resistance anyway, at least where audio frequencies are concerned (RF may be another matter). According to Ohms Law, where resistances are low, the actual current passing through the chassis from one ground point to another is HIGH, and the ripple or eddy currents in the chassis end up modulating the audio, causing audible hum. Unfortunately most multimeters have poor resolution for measuring resistances of less than an ohm, and the meter probes and cables themselves can easily have point .2 to .5 ohms of resistance. Sometimes trial and error is the easiest way to find the dodgy grounding..... By the way, what you call "croc" clips, we call "alligator clips"; perhaps because we have far more alligators (thriving populations, actually) in the southern US than crocodiles, which nearly went extinct here (the largest remaining American Crocodile population in the US lives in the outflow water of a nuclear reactor in Florida! 😯). I actually saw a large alligator or crocodile swimming in the surf off of Ft Lauderdale Beach in the early 1980's! The UK, of course, has no such beasties....
Not sure if I said 'high resistance'; but if I did, I meant high compared with zero ohms. Yes, trial and error. Check out the next video I post with very similar hum problems!
Grounding issues can bedevil the best tech and manufacturers often contribute to the problems with poor layouts in their designs. Here is a video by a local Memphis tech who explores the where’s and whys of grounding schemes along with some different approaches to their implementation and a reference to a more detailed explanation by Merlin Glencowe (aka the valvewizard): ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-ZySW1bq_nY0.html
@@stuartukguitarampguy5830 Alternately I have also had regular problems with the switching contacts of the jack sockets becoming highly resistive, as they are exposed to the environment. I now have rubber bungs for the unused jacks. I spent about 2 years working on a simple Hughes & Kettner valve amp which I kept coming back to because I couldn't locate the problem. It turned out to be the jack socket for the extension speaker being dirty creating an intermittent noise problem. Crazy but true.
@@SlowfingerJC I totally believe you. My very first port of call is FX send/return jacks. I have had literally dozens of amps with this exact problem. Either noise or intermittent sound.
Great channel. If I may point out a possible tiny slip. That big brown job between the filter caps looks suspiciously like a Hunts capacitor. It really needs to go, it may explode tomorrow or in 30 years time but they generally leak like a sieve anyway. Cheers, Steve.
Hi Steve Thanks for that. The problem I have is that this particular customer is a major collwctor and likes amps left as original as possible. You're right though.
I cant help to notice that the pots are close to the negative bias supply circuit. I assume it can induce hum on unshielded wires? Nice headscratching video.
I've regularly come across screen grid resistor drift and failure which would be due to the max load current at full volume for example or valve failures, so it makes sense to overrate them. Great fault finding vid. I've now placed this scenario in my brain for the future. 👍
This might be a stupid question. What would happen if you took that black wire and put the other end to the earth on the power lead as it comes into the chassis? Would it make any difference?
Probably not. I'm just working on another Marshall with similar hum issues. I'll post soon so keep an eye out. I can only describe my results as bizarre. The TINIEST change in where you decide to earth can either kill the hum or double it. Hard to explain but do try and watch. Hum loops are total PIGS to fix.
That PA amp is in great condition for it's age.. I used a Laney Super 100 PA equivalent in 1970 . Consensus of opinion back then was that it made a fine sounding guitar amp ! Great repair and seeing you get through the whole process, warts and all, makes for a great learning experience.
Perhaps the individual screen resistors are rated at 5W because they will be getting very hot as they are soldered to the valve bases. In other words, they're rated at 5W because of their physical position, not because of the electrical power they dissipate.
At 20:00, the glare reflecting off of the voltmeter display makes it very difficult to read the voltage figure on the meter, especially as the voltage bounces around. You may want to consider angling the meter a bit so that your viewers can see it more clearly.
Im jelous you dont shake like I do. You have a very steady hand. I dont know your age, but I am 66 and it forced me to pretty much retire because of my inabiliity to solder and dont trust myself with voltages north of 4000 volts. Thanks for another super video.
They were after the voltage rating, not the dissipation. Hence the use of higher dissipation resistors only being used in higher voltage areas, like the nodal dropping resistors and screen resistors.
I don't know if it's true, but I've heard that the *voltage* rating of modern carbon film and metal film resistors is often only 400 volts, and so updating the wattage will make it less likely that a resistor will fail prematurely just from normal use, without excessive current from a fault in the circuit.
@@goodun2974 you’d have to look at the spec sheet. I know that the Dale metal film half watt resistors are rated for 500vdc. Resistors become less reliable about their resistance the more voltage you run through them over their rating by generating a lot more heat. So, yes, increasing their dissipation will help significantly.
@@goodun2974 Yes someone else said that too. Its hard to authenticate these things though. E.g. It took me YEARS to find out why they added a standby switch. So many rumours and opinions. I expect you know, but if not let me know and I'll tell you.
Disclaimer: I've not actually worked on any Marshall's, and haven't watched the full video yet, so I might be barking up the wrong tree, but anyway: at 26:40, if the paperwork you've shown so far is correct, this amp was designed to run the 6L6, 5881 or KT66 family of *beam power* tubes, and NOT EL34 pentodes (aka 6CA7's, here in the States) as I would have expected for a British-market amplifier. Perhaps you're using drawings intended for servicing exported, American-market Marshall's? To clarify: Pin 6 on the sockets is often used as a "tie point" or terminal strip connection ---- typically for the screen resistor ---- in Fenders and other amps with 6L6-type "beam power" tubes that do not use pin 6 for any type of internal connection. EL34's, however, are Pentodes, and DO use pin 6 as a separate connection point for the tubes' internal "suppressor grid" (normally jumpered to ground, or the tube cathode, can't quite remember, since it's been a while since I've worked with EL34's), which is why merely swapping 6L6's etc to EL34's or vice versa will result in blown output tubes and possibly damage to the amplifier! (Unless the sockets have been rewired before substituting the other tube type, of course). The point being, a Marshall that uses EL34's will have, and will require, different wiring at the tube socket than one designed for 6L6's and their brethren, and therefore your drawing and layout diagrams might be incorrect for the amp you're working on, assuming it's a proper EL34 British Marshall.... Here in the States, high-power Marshall's were often supplied with, or converted to, running 6550 tubes (more or less akin to your British KT88's) because the American-made EL34's most easily available here weren't as reliable at 500 volts or more as your original Mullard or Genelec EL34's (with the possible exception of NOS Sylvania "fat bottle" 6CA7's). 6550's are generally considered interchangeable with KT88'S, and 6L6's interchangeable with KT66's, at least to the extent that they use the same pinout; although the ideal bias point may be different and the heater current requirements are also a bit different, but subbing one for the other is unlikely to cause immediate self-destruction. The KT designation actually stands for "Kinkless Tetrode" and those tubes are designed to be more linear and efficient, but that's a whole 'nother area of technical discussion!
@@stuartukguitarampguy5830 , will do! What year was this one built and what is the output power? I thought you had mentioned the Marshall Major which was supposed to be a 200 watt amp, with KT88's...does the JTM 100 model refer to the power output? That would be unusual for Marshall model- numbering, in my admittedly limited experience with them....
@@stuartukguitarampguy5830 , you're probably correct about the wattage, but usually Marshall uses weird 4 digit model numbers that are easily confused with the year they were actually built.
Marshall's ground setup always seems to get noisy.......I really liked watching your approach to fixing ground hums....I've got to fix a ground issue in a marshall clone amp that I built....basically the same thing....no matter what wires you move..etc...it doesn't go away...but that's marshall for ya.....thanks for the video...I learned alot
