Marshall JCM900 Help

[marcumh]

Trying to get this old JCM900 (EL34) up and running. I have about 6 schematics and none of them match exactly, but I think I've figured out most of the differences. The problem (that remains unfixed) is that it's only putting out about 2-3vac. I am running a 1k tone into an 8 or 4 ohm load to get this value.

With tubes in and all vol/gain down I have about 490 volts on the plate, approx 484 on the screen, and -47 on the control. I have tested the OP xformer with a variac and it seems to be ok,...I have also stuck in an OT that I had laying around with the same results. Using a scope and a 1k tone I can trace my signal onto the control grid and it looks fine. I do have signal on the screen as well (I don't know if this is normal), but I compared the findings to a 50w dual 6l6 amp I have, and there was no signal on the screen of that one when running in the 1k tone,....so not sure if there should be signal there or not.

I have poked and prodded what I believe to be the obvious things,...grid resistors, bias circuit etc,...but I am missing something for sure.

Thanks for any guidance,.....

[Enzo]

Six schematics? Perhaps if you told us the MODEL number of the amp, we could select the correct drawing. JCM900 is a line of amps, like Chevrolet is a line of cars. JCM900 includes models like 2100, 2500, 4100, 4500. As well as the -01 or -02 when in combo.


Got signal on the control grids and it looks fine? What is its amplitude? A nice looking signal that is only a couple volts will result in a very puny output.

What happens when you feed the signal to the FX return? And make sure the loop level controls are up.

[marcumh]

Enzo, sorry for the holes in my story. It is a 4100 (PC0112 JMP50C screened on the PCB). I'm not sure how to give you the answer to the amplitude of the signal,..so I am going to improvise,...hopefully in a way that makes sense. The signal at the input jack is .309 vac, the signal at control grid is 9.82 vac. I see that doesn't work because the grid vac changes with gain and master changes,...so I don't know how to answer that amplitude question. Signal into the fx return yields the same result as going into the input jack.

[Jazz P Bass]

If indeed a 300 mv signal (which is pretty 'hot') was fed into the front of the preamp circuit, the phase inverter circuit should be making alot more than a 9 VoltAC signal (which is the amplitude of the signal) to the output tube grids.
No way is a 9 volt signal going to drive the output to full power.
(On a 50 watt amp, a 50 volt signal would be more like it.)
You will need to follow the signal from the input jack through to the output tube grids to see where it is dropping off.

[Enzo]

Jazz covered it, amplitude just means the amount of signal in volts. Of course it changes with the volume setting. You set it up for what OUGHT to be full output then see how much signal is ACTUALLY getting to those plates. In a case where we are getting very little output to the speaker, then I;d say what we are asking is what is the largest amount of signal you can get to the power tube grids.


First, isolate the problem a little. Turn the amp on with controls up as normal, and turn the reverb well up - reverb pan connected. Now rock the amp or bounce the reverb pan. We want to make the reverb springs crash. Is the noise loud or is it also very diminished like everything else? (The reverb is pretty far along in the signal path.) Got FX loop jacks? PLug a signalo into the return jack - still real weak or does that sound OK? And testing the other way, plug a signal into the regular input, and run a cord from FX send to some other amp. How does that FX send sound coming out over there? Strong or still has the problem?

This is a hybrid amp, plenty of op amp ICs strewn in with the tubes. SO are both +15 and -15 supplies OK? And are they actually reaching the ICs?

[marcumh]

Thought I'd put this one to bed. After putting on and off the shelf a couple of times, it turned out to be the OT. OT tested ok with meter and variac, but despite passing these tests, it was bad.
Just for fun, here is a pic of one of the exploded/melted tubes it had when it came in. It's rockin' now though