Air Tight 300B Flame Out

@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

What is the cause of red plating, could be bad tube or loss of bias in amp, don't assume faulty tube. One way to test for this, try a cheapo 300B. But then you likely took out bias resistor or possibly something else in amp.with the smoke issue. I recently experienced a WE failure, in my case loss of vacuum, entire tube encased in opaque blue haze followed by a single pop. Since these were new tubes I was watching very carefully at powering up, was able to immediately shut down amp. No smoke in my case, but burned electrical smell.

In order to determine whether I had damaged anything in amp I next powered up with some cheapo 300B tubes, same pop. Immediately shut down amp and made the call perhaps my Mullard rectifier tube was the issue. Replaced with another NOS Mullard, no issues with my cheapo or Psvane Acme for several weeks now. Have the replacement WE, I'll cross my fingers and try again this weekend. So question is did WE take out rectifier tube or did rectifier tube take out WE? Loss of vacuum far different than red plating, my take is WE took out rectifier.

I hae a cheap pair of 300B electoharmonics.  Let me know if you want me to list them for sale.  Audiogon will kick us both off if we try to do a sale without using the classifieds.

@carlsbad2 ​​​​​@sns  @testpilot 

Thank you for your responses.

The little smoke, but strong smell, was only from beneath the 300B tube that went bad. Probably an indication of fried resistor? No discoloration on the tube socket or on the tube pins.

The tube did not show any sparks, nor a red plate. Rather, inside the tube was a roiling aurora borealis of beautiful blue and green plasma throughout the entire tube. Series of crackles and pops and a final louder pop through the speakers, no noise from the amp itself.

The AirTight ATM-300 uses auto bias.

I purchased the WE tubes from Uncle Kevin at Upscale Audio. Assuming they are authentic.....

@carlsbad2  Thanks for the offer of cheap 300B tubes, but I have some other 300B tubes myself. However, given the smoke and burning smell, I am reluctant to put in another tube. Think I will just bite the bullet and have the amp looked at by a good audio technician. Turns out we have one here in the Sacramento area.

Thanks again to you all.

    -GAR

Gar,

Based on that detailed description, I think you'll find a smoked resistor.  Possibly another component but resistor is most likely.  

I'll let others comment on the tube glow you saw.  the blue is generally electons escapong the plate and decelerating releasing a blue photo.  but I'm not sure how to correlate this to a failure. 

I like manual bias just because autobias circuits sometimes fail.  Not saying that is what happened but it could be.

Glad to hear there is no question of authenticity.

Jerry