Marshall JVM410 problem

[Rod]

Hello! I have a JVM410H that is loosing gain after some minutes of being on, the gain starts fading away until the amp almost has no signal. i Have measure the anode voltages and i have real low voltages, (check attached image) what can be the problem?

[Jon Snell]

What do you mean by the gain is fading away?
Where are you measuring the signal and comparing it to the signal source over time?

[Rod]

After some seconds of being on, the amp has volume but it slowly stars to loose gain, i would described as it seems like the gain knob is slowly going to zero, but not completly on zero, because there is still a weak signal.

Also the voltages at the anode dont seem ok, they seem to low.

[J M Fahey]

Do they *stay* at that voltage or they are slowly rising or falling to some crazy value?

Post schematic, you may be losing signal somewhere else but can´t suggest anything "blind".

[Rod]

It seems to me that voltage drops and falls from higher values to this low values after some time of the amp the being on.
I attached the JVM410 schematics to this message.

[Jon Snell]

Check the send and return sockets. Place a guitar lead in between them and test again.

[Rod]

Jon Snell, i have the same problem after test the amp wtih a lead between the send and return sockets.

[Jon Snell]

What we have to do is measure the amplitude of the signal with an oscilloscope and a signal generator. If you are equipt with those items of test gear, monitor the signal on the grid ofV5A ... does it reduce? If it does, move to the anode of V6 or grid of V7A. Does it reduce there, pinpoint the area of the fault and move in.
The connection on the PA in Return is a good place to monitor, just before the power section.

[Rod]

Hello again Jon Snell, i do have a osciloscope and signal generator, i will check it out, thanks for your help.

[g1]

The anode voltages that you marked in the 1st post, is that when the problem is occurring? They don't seem all that low (80V plate), there are a lot of voltage droppers in the supply.
Attaching the first schematic from post #6 as that link is broken.

[Helmholtz]

A 160V drop across the 150k plate resistor means a plate current of 1.07mA. That's about 25% higher than typical with a 1.5k cathode resistor.
Might indicate a leaky/bad bypass cap.
Cathode voltage might give some clue.

[Chuck H]

I was thinking something similar to Helmholtz. Though I didn't have a detail on it yet (because I haven't opened the schematics or seen any notes). But gain loss with voltage shifting almost surely means some failure is messing with the operating conditions of one or more preamplifiers.

It might not even be safe to operate the amp other than for test measurement purposes at this time. So no firing it up to see how it sounds before this gets fixed.

[Rod]

A 160V drop across the 150k plate resistor means a plate current of 1.07mA. That's about 25% higher than typical with a 1.5k cathode resistor.
Might indicate a leaky/bad bypass cap.
Cathode voltage might give some clue.

Are you saying that the problem can be a cathode bypass capacitor rigth?

[Helmholtz]

Are you saying that the problem can be a cathode bypass capacitor rigth?

Yes, please measure cathode voltage.

Then lift one end of C52 and see what happens to tube cathode and plate voltages - and how this affects your problem.