Marshall JCM800 2205 help needed

[mickeydg5]

I am not sure it is the set of power tubes.
Voltages need to be taken around the tube sockets to see if there are any faults.

@gibson17
If you are not tech savvy and able to deal with high voltages in the chassis then bringing it to a amplifier technician may be in order but you did give a plate voltage.
Was that by way of a multimeter?

 

[gibson17]

Thanks for all the replies, this got put on the back burner as I had a lot of household issues that took precedent.

Here's the schematics I was using:

https://www.drtube.com/schematics/marshall/2205prem.gif

https://www.drtube.com/schematics/marshall/2205pwrm.gif

I did switch the power tubes between each other, and the problem followed the tube swap. The crazy thing is the tubes are fairly new and were biasing fine and then had a runaway where I couldn't lower them. It seems as if I witnessed a tube fail right before my eyes. I am fairly competent with safety and mechanical issues. Theory I need ALOT of work on. I'll get there slowly. Right now I am just a monkey with decent soldering skills.

I did solve the problem with everyone's help. I do take all of my measurements with a multimeter. I used a probe to get plate voltage to avoid slipping and taking out myself or letting out the magic smoke. I cleaned all tube sockets and retensioned them. I cleaned all the grounds. I cleaned all input jacks. I found some cold solder joints and reflowed at least half of the main board. There were actually quite a few failed joints on the front of the board where the bank of pots are located.

Here's where I went wrong: I thought by testing for mutual conductance that I could definitively rule out a tube as good or bad, all of the tubes tested well above the threshold for good. So in my mind, the tubes were fairly new and they were bought as matched and I tested in a calibrated Hickok TV7D/U. After your help and suggestions, I pulled out every tube in the amp and installed new el34's and 12ax7's. I was able to bias up within 2ma of each other and all noise issues went away. Because so many actions were performed, I am unsure what cured the noise issue. I do know that changing to a new set of tubes fixed the bias issue so it's very likely that the noise was generating from there.

I wanted to come back and report so if anyone is reading this in the future, the thread has a conclusion and doesn't just go dead. I hate that! Thank you again for all of your help and knowledge.

 

[XTRXTR]

What year was this 2205 made? I'm just curious.

 

[gibson17]

What year was this 2205 made? I'm just curious.

Sticker says '89

 

[Pete Farrington]

The resistor on R59 was burned and failed open. I replaced that with a new resistor

Here's the schematics I was using:

https://www.drtube.com/schematics/marshall/2205prem.gif https://www.drtube.com/schematics/marshall/2205pwrm.gif

R59 looks to be the 10k 1W HT dropper resistor between the screen grid and LTP supply nodes.
I can’t think of any valve failure modes that would cause it to overheat so heavily.
And I can’t think of any poor connections that would do it.
The most common cause of such a failure is a short somewhere, or a failing HT cap, most likely C44 which decouples the LTP HT supply node.

With the amp idling, if you put the red probe tip to the C43 end lead of R59, and the black probe tip to the C44 end lead of R59, what V DC do you measure?

 

[XTRXTR]

Pete beat me to it I was going to say the filter caps may need replacing.