Marshall JCM 800 4211 Combo Blowing HT Fuses

[sohosteve]

The reason I was thinking of trying swapping out one valve at a time is to avoid the cost and hassle of taking it to a tech.
I only bought it about eight months ago and was told that it had just been serviced - no reason to not believe that as at the time it still had the tech's label on it with the original owner's phone number.
And though I realise that a thirty year old amp may well have developed another fault etc etc I would rather try to remedy the problem than take it to a tech, get charged £30 per hour to be told 'one of your output valves had died' - here's a bill for two hours work and the new valve'....

I think I'll take this into my own hands and try swapping out the valves one at a time until the volume stops fading away to silence.
If that doesn't work, then I'll take it to a tech, but I'll least I'll know that I tried the most (potentially) obvious fix.

Thanks for your suggestions.

One could do something like put one at a time in an amp and see what happens but that's a real hassle and certainly not cost effective to pay a repair tech to do for you.

[Tom Phillips]

... One could do something like put one at a time in an amp and see what happens but that's a real hassle and certainly not cost effective to pay a repair tech to do for you.

Read my post #14 carefully and you will realize that I meant to test one tube at a time in a known working amp. Anyway...that's just a suggestion for someone who has the equipment, the time and is willing to take the risk of damaging some equipment for the sake of testing potentially bad tubes. Sorting out intermittent power tubes from a set is a bitch. If the problem tube has gotten worse as you said then it will be easier to find the bad one using your method. Hopefully, there is only one problem to be found.

Long ago I spend an enormous amount of time sorting out a similar intermittent problem with an Ampeg SVT. That amp uses six 6550 Power tubes and the customer wanted to keep his GE power tubes in the amp if at all possible. In the end it turned out that three of the six power tubes had intermittent shorts. Troubleshooting a circuit with MULTIPLE intermittent components can be extremely time consuming.

Also note that I didn't say to scrap the old tubes without further consideration. The tradeoff is your time and the on stage failure hassle vs. the cost of a set of tubes. The swap out of all power tubes is just one step in the process and a side effect is that you may just end up with a spare set of tubes. Taking the amp to a tech is no panacea either. We frequently hear "I checked everything and the amp works perfectly. Found nothing wrong. Here's my bill" then days or weeks later the problem shows up again. The fact that the amp was serviced just before you bought it doesn't matter either. That was then - This is now. Everything is working OK until its not. Even brand new amps can have the problem you describe.

Hope you can find the problem.
I didn't realize that we had the same discussion last April until I scrolled back in this thread!
Cheers,
Tom

[sohosteve]

Thanks Tom,
Your advice is duly noted.
And yes, I'm much keener to see if it is a dying valve as it is (seemingly) no longer an intermittent fault - as soon as the amp gets up to full power it's sounds good for few minutes and then gradually fades quieter and quieter..
There are only four power valves - so not a major headache to run quickly through them.
And if that don't fix it I'll just have to get the b#stard to a tech - it is one heavy b#stard too!
Have a great weekend
Steve

Read my post #14 carefully and you will realize that I meant to test one tube at a time in a known working amp. Anyway...that's just a suggestion for someone who has the equipment, the time and is willing to take the risk of damaging some equipment for the sake of testing potentially bad tubes. Sorting out intermittent power tubes from a set is a bitch. If the problem tube has gotten worse as you said then it will be easier to find the bad one using your method. Hopefully, there is only one problem to be found.

Long ago I spend an enormous amount of time sorting out a similar intermittent problem with an Ampeg SVT. That amp uses six 6550 Power tubes and the customer wanted to keep his GE power tubes in the amp if at all possible. In the end it turned out that three of the six power tubes had intermittent shorts. Troubleshooting a circuit with MULTIPLE intermittent components can be extremely time consuming.

Also note that I didn't say to scrap the old tubes without further consideration. The tradeoff is your time and the on stage failure hassle vs. the cost of a set of tubes. The swap out of all power tubes is just one step in the process and a side effect is that you may just end up with a spare set of tubes. Taking the amp to a tech is no panacea either. We frequently hear "I checked everything and the amp works perfectly. Found nothing wrong. Here's my bill" then days or weeks later the problem shows up again. The fact that the amp was serviced just before you bought it doesn't matter either. That was then - This is now. Everything is working OK until its not. Even brand new amps can have the problem you describe.

Hope you can find the problem.
I didn't realize that we had the same discussion last April until I scrolled back in this thread!
Cheers,
Tom

[g1]

You could try a pair at a time to try to narrow it down some. With a pair, set the impedance switch to half of the normal setting.
Also, watch for red-plating when the power fades. It may give you a clue.

[sohosteve]

2nd Valve I swapped out cured the problem!
No more fading out and I've had it on for several frantic hours of fun and it still sounds all fat, tight and juicy!
It would seem it was ONE dying valve that was causing all the problems!
Happy days :-)