Air Tight 300B Flame Out

Red plate, from the sound of it. Is there a failure on your bias board?

@carlsbad2

+1

Yep, occationally a tube goes out. Hopefully it did not take out a protective resistor... but it may have. That is why they are there. 

Thanks all.

I have basically the same conclusion. I am thinking that the tube failed (arced), and the smoke was from a fried resistor. 

Rather than put in another tube, thinking that having the amp checked out by a tech is the best corse of action.

Some amplifiers use a sacrificial resistor to protect the amplifier when a tube fails.  In most instances, this is a simple fix although stressful when it happens to your $15k+ amp. 

@gareents I've seen what test pilot said.  But smoke is not normal.  I put a wrong tube in an amp, blew a resistor, replaced it and was making music again the next day (had to order the resistor).  In this instance, the burning smell was more noticable than the smoke.

The good news is when there is smoke, there is usually evidence of what failed--blackened, burnt, or paint discolored--so you can easily figure out where the problem is.  I'd open it up myself and look for the overheated component.

Now the bad news would be if the smoke was from a transformer.  failed tubes can take out the output transformer.  the good news is that replacement trasmformers aren't as expensive as you would think (in most cases.  My amp has hand wound transformers from Bulgaria) and they are not hard to replace. 

Tubes don't often short although they sometimes do.  More often they "red plate" where the current goes up in a runaway positive feedback syndrome.  This usually starts with either too high a bias current setting or a faulty autobias control.  Your initial description didn't sound like simple redplating.  Were there sparkes inside the tube or was the plate simply glowing red?

Finally, I've heard rumors (I don't want to spread rumors, so I am just calling this something to consider) that WE 300B tubes have been counterfeited.  I don't know where you bought yours but consider if they might be fake.  OTOH, I've also heard they have a fairly high infant failure rate which is covered by the warranty. 

All of this discussion is because you mentioned smoke.

Jerry