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Diagnosing a major issue with my 300b SET

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Three days ago, I experienced something I've never heard in four+ decades as an audiophile. The system was warming up when all of a sudden, an extremely loud hum began emanating from the left channel speaker. It was so loud the frame of the left speaker (PureAudioProject Quintet 15) was vibrating uncontrollably. Nothing short of turning of my Airtight ATM300 amplifier stopped it. When I turned the amp back on, everything worked find, at least for the rest of the night. However, this happened again two days ago and again yesterday.

It made no difference whether music was playing, the volume level on the preamp, if the preamp was muted or not, or even if I turned down the attenuators on the face of the Airtight amp. Crazy loud hum!

Prior to this incident three days ago, the system was sounding fantastic.

My amp is not new, at least 10 years old, built by the father, Atsushi Miura. I use a matched pair of Western Electric 300b tubes (not vintage).
That said, if this is tube issue, I've never heard a tube die like that!

Any suggestions as to what I'm experiencing? I'm guessing it is the amp, but I really don't know.

Any suggestions greatly appreciated.

Follow Ups:

Update and Thank You - Eflatminor 17:44:02 06/30/23 (1)
Thank you all for your suggestions. I am happy to report that I'm an idiot! Well, not really, but it turns out one of the tubes was failing. I've never heard a tube fail like that, but there you are.

Good news, I just received a replacement shipment of GE 12BH7As, matched and balanced. It is sounding GLORIOUS.

Thanks to Joshua at Tubes Unlimited out of Texas. I appreciated his advice and we had a great conversation about all things SET-tubes related.
- RE: Update and Thank You - Carlsbad 04:56:28 07/03/23 (0)
  Late to this thread but based on it happening during warmup and your description of the issue it sounded to me like heater circuit was shorting intermittently to signal circuit. I was going to suggest swapping tubes to between channels and once the problem moved, you'd know it was a bad tube.
  
  Jerry
You've got some good suggestions- here's another - Ralph 10:40:55 05/12/23 (0)
If the attenuators of the amp make no difference, the problem is downstream of them.

Try removing the input tube and let the amp sit there. If its properly designed this will be no problem. Obviously it won't play, but the question is will it do the same noise?

Tubes can develop internal shorts such as a cathode/filament short; if they have AC filaments the AC signal can be amplified by the amp.

It sounds to me that you have two options here since you don't sound like working on electronics is your forte. The first is to replace your signal tubes in that channel (not the output tube) and see if that sorts it out.

If that doesn't then it needs a trip to the shop to put things right.
RE: Diagnosing a major issue with my 300b SET - Ian L 22:08:44 05/03/23 (0)
Do you know if the bias for the tubes is adjustable on the ATM300 ?

If NOT, my first suggestion would be to swap all three tubes from each channel to the other channel. If the noise changes sides you know it's one of the tubes.
Intermintent problems are the worse.... - Rod M 20:25:49 05/03/23 (0)
It could be so many things, a partially shorted interconnect nearly anywhere or the amp and tubes, a cold solder joint, whatever.

My first thought is to change out the amp and see if all goes well. At least you solved one problem.

Or, swap all your cables from right to left and see if that changes. then swap back one at a time.

It's just a process of elimination which is a bitch.

-Rod
RE: Diagnosing a major issue with my 300b SET - Chip647 12:08:46 05/03/23 (0)
Have you ever plugged a preamp into an amp with an RCA interconnect and the positive pin for the first interconnect contacted before the ground shell? You get a strong hum through the amp and speakers. When you push it all the way in the hum stops.

It could be something like that, a loose ground. I would suspect the chinese preamp or the interconnects before the ATM300. Try different interconnects with a different preamp/source. If, with this different upstream chain, the amp still passes a hum it could either be a loose ground in the amp or a power supply cap that is intermittently failing. Additionally, it could be a tube, but less likely.

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