HELP!!!!! old faithful mark IV busted for the first time :(

[scottieL]

I've had an early 90's mark IVa for three years now and up until now it has not given me a hint of trouble. Just turned it on and all I get is a very faint clean sound at any volume. turning the master just adds noise. switching channels makes the faint clean sound a bit louder.

I've ruled out my guitar, effects, and the loop. I'm inclined to think the issue lies in one of the preamp tubes. Any advice as to which one specifically might be broken would be greatly appreciated.

Thanks

 

[bluesbreaker]

Suspect your output tubes before anything else, as they are the most likely culprit.

If you have a spare quad of output tubes, replace all the output tubes and fire the amp up. If you don't have a spare quad, here's what you can do:

[1] Switch the amp to "class A" mode and see if the problem still happens. If not, its your inner pair of output tubes.
[2] Take your inner pair of output tubes and place them in the outer sockets and put the amp into "class A" mode. If the problem is gone, the tubes you had in the outer sockets need replacing.

If the problem still happens, suspect the preamp tubes. Get a known working 12AX7 and begin swapping it into different preamp positions V1 thru V5.

Report back if the problem still happens after ruling out the tubes.

 

[Rob Lockwood]

A few things that might help:- You could plug you me50 straight into the fx return socket. If you get a sound then a large section of the amp is ok ruling out power tubes and phase splitter V5 and half of V2. If you get a sound this way then V1 is the most likely problem but could still be one half of V2.

The recording output comes at the very end of the preamp so if you have a signal here in all channels the whole preamp is ok and the problem is V5 or power amp.

V3 is only used in the lead mode I think so not likely to be the problem. V4 is the reverb driver so again not likely to be the problem (unless the reverb level is well up).

As you have some sound, and switching channels makes some difference to that I think V1/V2 in the preamp are the most likely cause if its a tube problem. If you dont have a good spare tube you could swap these in turn for V3. If you get a clean sound but no lead then the tube that you put in V3 is the bad one.

 

[scottieL]

thanks for the help guys. it seems that whatever the issue was it has stopped acting up for the time being. I'm still thinking it may be time to send it in for a good servicing.