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I have a Kondo Ongaku and just got a pair of NOS GE 211 tubes, tested and burned-in.They seem to work fine, I have not had the bias fine-tuned yet.
One oddity: on power-up the right channel has a slight audible oscillation for about 1 second. Flipping left-right channel tubes keeps this effect on the right channel. I have not flipped the preamp tubes so it might be the combination. Also the oscillation might remain at ultrasonic frequencies, I don't know.
Other tubes (the original GEs and a pair of new Kondos) do not have this effect.
Why is this? Would it be solved by precise biasing (hard to believe, the Ongaku does not push the tubes hard)?
Edits: 06/05/16 06/05/16Follow Ups:
I would start with the simple stuff first. I agree that the 211 socket doesn't need tensioned like an octal or 9-pin socket, but have you cleaned the contacts? Even if they look fairly clean to the naked eye, you might be surprised at the amount of crud that can accumulate. Take a Q-Tip and swab the inside of one of the contacts and see if it comes out clean or some varying shade of gray (with power to the amp disconnected, of course).
Secondly, by your statement "haven't fine-tuned the bias yet," is it safe to assume that you have manual bias and that you didn't make any adjustment before putting the new tubes in? If you have a lot of hours on the old tubes and have made adjustments over the course of time, your bias may be set way too high for the new tubes which will likely require a lower setting at the outset. I'd check that also.
Let us know how things go....I'm interested to hear what caused the problem and if/when you find a solution.
The Ongaku does not push the 211 tubes very hard, idle current is about 60mA. Bias is a pain to adjust and I have only had it done by a tech - you need to put this 85 lbs amp on the side, use a high-voltage probe, etc - so only an authorized tech did this once for me. So I have not really used any worn tubes in the amp.
So couple more observations: The tubes look pristine as I said, very large getters, don't look burnt out at all. I have two other tubes which work fine: GE 211 that came with the amp, and new Kondo tubes. None of this produce the initial oscillation. Additionally the oscillation happens in the right channel even when I switch left and right tubes.
What I will try is switching around the preamp tubes (ie left to right). This is a bit harder as I have to remove the preamp tube cover, but I will try it when I have a sec.
How does the silvering look on the tubes that squeak? Is anything visually different in terms of construction between those and the others?
--------------------------
Buy Chinese. Bury freedom.
Tubes are pristine. Very large getters, they look true NOS. Also boxed as NOS with original (aging) data sheets, etc.
I've built SET amps with 211s, but never had any issues with instability. Assuming this really is an oscillation, it's possible power supply decoupling has deteriorated over time. You may have capacitors that are no longer performing as intended, and the gain of NOS tubes is simply revealing the need for replacement. That's just a guess, of course. My suggestion is that you find a reputable shop that can diagnose this.
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Buy Chinese. Bury freedom.
The 211 sockets are such that this is not a problem. And this doesn't happen with other 211 tubes.
Clean & tighten the sockets?
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