Smoke or Burning Smell

There's good news and bad news. The good news is that it's easy to find the problem, at least the thing that is burning. Just unplug the amp, open up the chassis and look for what's burned, charred, or overheated looking. That's what's causing the smoke/smell.

The bad news is that in almost all cases, the part that is burning is a power handling component. These are ALL expensive. Worse, in some cases, the part that is burning is not what is causing the problem, and you still have to find what else is faulty.

In some ways, having smoke coming out of the amplifier is kind of a deviant, hard-headed version of having a fuse blow - something is eating too much power, it's just that the fuse for some reason is not blowing. This is especially suspect if the fuse blew, and you didn't have another of the right rating, so you stuck in a higher current rated fuse.

Possible causes are:

• Failing/shorted output tube - this can overheat the output transformer and/or power tranformer. More rarely, it can also overheat the choke, but usually the transformers go first.
• Improperly biased output tube.
• Failing bias supply on fixed bias amplifiers
• Failing cathode resistor bypass capacitor on cathode biased amps.
• Failing/shorted rectifier tube (or solid state diodes - they do fail, if rarely) can overload the power transformer as well as killing the power filter capacitors by letting AC through. A failing filter cap or shorted output tube can pull so much current that it overloads the rectifier tube, too.
• Failing power filter capacitor. These can sometimes get hot enough to literally explode or burn, as well as just quietly overloading the power supply and popping rectifiers and power transformers.
• Failing power transformer.
• Failing power filter capacitor
• Choke with a "soft" short between winding and core
• Failing output transformer