Tech help needed - runaway tube heating to glowing red

[Zoner]

I had my Simul-Sat combo at a local shop on consignment for a while, changed my mind about selling and when I got it home it was not happy. It barely worked (very little power, cracking and popping sounds) and I noticed one of the power tubes was absolutely glowing red-hot. It only got worse when I switched it into simul-class and repeated with a replacement tube installed as well. When I picked it up from the shop I noticed the internal speaker was not plugged in and there was nothing else plugged into the speaker outputs at the time, but of course the store staff said it had been plugged into an extension cab or the internal 12" the whole time and that it worked "perfectly" whenever someone tried it out. This only occurred with the outer right-hand tube if looking from the rear. Do I have a fried tranny due to no load during operation or could there be another simpler (cheaper) fix? I know diagnosis via remote is tough, but any help would be appreciated. Thanks.

Z

 

[RussB]

The amp needs to be looked at by a Tech...

I could speculate, but its just a guess.

 

[Monsta-Tone]

If your Power Tube is glowing red, you probably have a Bias problem.

Try changing all of the Power Tubes. I have had amps that had glowing tubes, but the tubes that were glowing were not the problem tubes.

This is a good 1st step. If the problem goes away, you have a bad tube in there somewhere.

If the new tubes glow, turn the amp off immediately and get it to a tech.



I don't want to scare you, but you may have a bad Output Transformer as well. I just repaired a DC-2 and it had a similar crackling noise problem. At lower volumes it was fine, but the second I cranked it, the noises got really bad.

One of the Signal Input wires on the Output Transformer was burned nearly all the way through. It made nice pretty sparks.



Usually, you can run an amp for a little bit with no speaker plugged in and not hurt anything. Most amps (good ones) use fairly hearty transformers.