Sovtek 5881/6L6WGC physical noise

[htrd]

Well, I'm back with my favorite Fender Twin Amp, Pro Tube series. It just can't be a good and trustworthy amp.

This time I have a random high-pitched squeal, coming from the 6L6 tube (coin-base Sovtek 5881/6L6WGC, that are old good 6P3S-E). Squeal is physical, no loud, heard not from the speakers, but from the tube itself. Usually it appears after the powerup, after the standby, when HT hits the tube. It can disappear by itself a few minutes later, or after a slight hit at the side of the combo. And can reappear again later. Problem follows the tube, not the socket. Soundwise everything is fine, just this random squeal from the chassis.
So the question is - should I just ignore the problem or there can be some bad consequences and it will be better to pull out the tube? Quad is really fresh, 30-40 hours at max, so the greed is strong in this one.

Here you can hear the trouble (sorry, in Russian, I'm just pointing the bad tube). Speakers are off, dummy load engaged.
https://drive.google.com/file/d/1sY8...usp=share_link

Thank you!

[Helmholtz]

Can't hear a squeal.

[htrd]

2,96KHz and 8,88KHz according to analyzer.

[Mick Bailey]

I hear it. Because tapping the tube stops it, and the fault follows the tube, there's perhaps a mechanical process at work caused by a loose element within the tube itself. There has to be a feedback loop to maintain the oscillation and maybe there's something else going on with that particular tube's characteristics that will maintain the oscillation in the absence of an audio signal.

If you hold the tube by the pins close to your ear, does it rattle more when you tap it compared to a 'good' tube?

You can't really fix a problem tube. You could make electrical changes to the amp itself, but I think it's a wrong approach to compensate for a fault by altering something else that's good.

[mikeydee77]

I hear it.... I would say whistle in range of 4khz.

Does it persist if using a speaker? Some output transformers seem to make a noise when on a dummy load. Maybe that vibration triggers the tube to resonate?

[Helmholtz]

Ok, using good headphones I can hear it.

[htrd]

If you hold the tube by the pins close to your ear, does it rattle more when you tap it compared to a 'good' tube?

You can't really fix a problem tube. You could make electrical changes to the amp itself, but I think it's a wrong approach to compensate for a fault by altering something else that's good.

I will check tomorrow. I don't want to fix it, I have the fresh spare quad, I'm just interested - is it safe to use it? Don't want to damage the amp if it will go nuts.

Does it persist if using a speaker?

Yes. Not always, though, I've wasted about 30 minutes before the video to get it oscillating.

[Mick Bailey]

I've come across a new tube making a noise on its own and with no other audio issues and left it alone with no further problems. Of course, if there is something loose inside the tube it may eventually become microphonic and give an audio feedback through the speaker, but any tube can do that.

[htrd]

If you hold the tube by the pins close to your ear, does it rattle more when you tap it compared to a 'good' tube?

Tested all four. Yes, it looks like the bad one rattles a little more, a bit longer and zingy sound. Also slightly lower in tone.
Something is definitely loose inside.

[Chuck H]

Microphonic power tubes are a bigger problem for combo amps than heads. If you have another amp that uses this tube type, but is typically plugged into a separate cabinet you could put your alternate tubes in the Twin and relegate that tube set to the less sensitive duty.

I did a 2xel34 amp some years ago that needed a tube that could withstand high-ish voltage and could also be in a combo cabinet. At the time the JJ's wouldn't hold up to the voltages so I went with the Ruby selected Shuguang tubes (el34B). Those are good sounding el34's that can take higher voltages but I had to buy three sets to get a pair that wasn't too microphonic to use in a combo cabinet. I don't remember either problem before this last fifteen years or so with power tubes being soft on voltage spec or microphonic right out of the box. We're learning to live with what we have and I'm glad to have tubes to use but I still long for what we had.

Mick is right that any microphony in that tube will just get worse with time. This will be exacerbated by use in a combo cabinet. If the tubes came with a warranty and you're within that period you should at least try to do a return. The seller will probably make it hard in my experience.

[htrd]

Funny thing is that I used this Twin mainly as an amp into a reactive loadbox and noticed this sound after the silent evening session with headphones.
I have 7 tube amps and a backup pair of 6L6 is more than welcome, my only concern is the safety of amps. No tube is worth the risk of damaging the OT for example.

[Helmholtz]

... my only concern is the safety of amps. No tube is worth the risk of damaging the OT for example.

An OT is stressed by either:
- High current, causing excessive wire temperature. May melt wire insulation and lead to shorted turns in extreme cases.
- Very high voltage peaks (say>2kV) causing internal arcing resulting in carbonized wire insulation - also causing shorts.

Neither of the two seems to be happening in your case as the speaker noise is low.
If you have a scope you might check for additional ultrasonic oscillation.

[mozz]

Did you try to reflow the solder in the pins, maybe adding a bit more?

[Chuck H]

Did you try to reflow the solder in the pins, maybe adding a bit more?

That's a really good consideration. It struck me when I read it that one should try to recognize and not rule out all possibilities. But...

Then I remembered that earlier in the thread htrd mentioned that the noise followed the tube to different sockets. So it's less likely to be mechanical noise from the socket at that point.

[Helmholtz]

I thought he meant resoldering tube pins - not socket lugs?