Marshall Valvestate 8100, No Dirt!

[AtomicMassUnit]

I'm working on a 100 watt valvestate 8100. The clean channel works, but the dirty channel is totally clean and really quiet, and with the overdrive 2 engaged, it's still clean but a little louder. I changed out a few of the switching op amps and some of the Tl072s, but no luck. Any ideas?

[teemuk]

The amp should generate a wealth of distortion already when the signal hits those clipping LEDs so - the way I see it - you don't have much to test, about two gain stages. Assuming you already switched those OpAmps in them the next thing you should do is to find out if they produce their intented gain or not. This should already point you pretty close to the source of the fault. Likely there's a failed component, an intermittency (eg. solder joint of, say R1, failing would reduce gain dramatically) or something else along those lines.

[Jazz P Bass]

Are you seeing the "clipping LED's" illuminate on the dirt channel?

[AtomicMassUnit]

I am not seeing the LED's light up.

The normal Channel has plenty of volume, the dirty channel has almost none on OD1, and OD2 is a little louder, and both overdrive settings have no dirt at all.

IC's 1-5 have been changed, and all are showing the right +15/-15v on them.

The tube itself is getting the right voltage on the plates and grid of the 2nd stage (have swapped a new tube just to make sure it's not that).

[Enzo]

What voltages are on the tube cathodes? That will tell you how much the tube is conducting.

FInd the problem, rather than replacing parts. APply a strong signal to the input, then trace it throug the amp. What signal level is at IC1-1, IC3-5, first grid of tube, first plate of tube, then cathode of second half of tube (cathode follower), then IC4-6 and IC5-5. Just because teh ICs may be good doesn;t mean the signal is getting to them or passing through.

Though teh level is small, do the dirt channel tone controls ALL work? An open treble control or for that matter open post volume would kill signal levels.

[AtomicMassUnit]

All of the controls are working, and all the pots check out to the right values.

The tube is getting:

Pin 1-195
2-127
3-127

6-127
7-0
8-.65

I don't have a scope, so I'm going to try using a clip in line out device I made to try and see where the signal is getting lost.

[AtomicMassUnit]

Update: I am getting a good level lining out at r11, and it's losing a lot of signal before r34, so the problem has got to be in the IC3 stage. Changed the IC just to be sure. All resistors for that stage are reading good.

[Jazz P Bass]

All of the controls are working, and all the pots check out to the right values.

The tube is getting:

Pin 1-195
2-127
3-127

6-127
7-0
8-.65

I don't have a scope, so I'm going to try using a clip in line out device I made to try and see where the signal is getting lost.

Figure out why pin #2 & #3 are at such a high potential.

[AtomicMassUnit]

The earlier readings are with the tube installed.

Without it, they are:

1-398
2-394
3-0

6-398
7-0
8-0

[Jazz P Bass]

O/K.
Good fault finding pulling the tube.
So find out why you have 394 Vdc on the grid pin (#2)
It should read zero volts.
Cannot help with the why.
You have the amp in front of you.
I can tell you, it should be obvious.
A solder bridge from Pin #3 to 2?

[oc disorder]

What about C54 (where are you ?)um shows my age stupid tv programs from my youth
but back to the point if that was shorted the voltage from the second cathode would be present.
See if you can measure a voltage across it or as its neg f/b perhaps remove it temporarily.
Won't cost much to change it .Measure that 1k at the first cathode too with the power off!

[AtomicMassUnit]

Thanks for all the help guys! It's fixed.

I was getting a good level on one side of R11, then losing it on the other side. The gain pot measured ok (1.3m), so I resoldered the connections there, and that fixed the problem!

Jazz, the schematic has the grid connected to the plate of the other triode, so that's OK.