Musicians Roadhouse

Hi, a couple of weeks ago my ma100 head died, no noticeable pops or anything. power valves all looked pretty burned around the top, pre valves showed no signs of heat. got 4 new matched el34b tung sols. no vol still. checked the valves 3rd el34 looked burned around the top again. fuses still good, I have ordered 3 12ax7s to try the pre valves. any other ideas, thanks.

Probably best to post an image of your "burned" tubes. That can mean so many things, depending on the person saying it & the person hearing it.

Hopefully swapping out the tubes will fix the problem. If it does, I would suggest, pulling a tube & replacing it w/ one of your previous ones, until you find the bad culprit. Then throw that one away & keep the rest for spares...

When you say fuses, are you referring to the fuses by the power socket or the ones on the PCB itself? There are two on the PCB (one for each bank of power tubes). If one of those is blown you will definitely lose audio output and a bad tube (or tubes) can cause them to blow.

Also check the obvious....the effects loop switch on the back of the amp. If it's engaged and nothing is connected to the loop, you won't get signal from the preamp to the power amp.

thanks guys, got the new pre tubes in, working again, but with a lot less volume. I will bias it tonight as I put new el34s in as well, live isn't often easy.

Just in case a bias doesn't fix the volume problem: compare the pre-amp tubes to the old ones, specifically the tube in the V1 position. A pre-amp tube with less gain than the original will definitely cause a reduction in volume in the V1 position.

A good experiment would be to take the old tube that was in say V2 and pop it into V1 to see if the volume levels come back.

the bias adjustment sorted it a bit, the big problem was I forgot I used to have a eq in the fx loop, set that back up n its rocking hard, that extra gain sure makes these amps shine. ;-)

jamesfromaus wrote:the big problem was I forgot I used to have a eq in the fx loop, set that back up n its rocking hard, that extra gain sure makes these amps shine. ;-)

That would do it. Glad you got it sorted out.