[morzh]

OK, update.

Today 6AU5GT came, bought 4, all NOS.
First I duplicated the oscilaltion die-out, to make sure I still have it.
At the same time I put the voltmeter on the B+ to monitor it.

In about 3 minutes one of the wavecrests started to flatten, I looked at the B+, it was going down. From 195V it gradually got down to below 160V and this is where it really falttened and I turned it off.
Then I replaced one tube (I had 6AV5GAs there) with 6AU5.
Same story happened again.
So I replaced the second tube and started watching.

Once the oscillation started the B+ as expected started going down a bit, got to 193V and then climbed back to 195.5V. The osciullation was stable.
I waited about a couple of minutes, and got the marker to mark the last 6AV5 tube I replaced as bad. At the same time I noticed the B+ started dropping. It kept dropping, when at about 186V I noticed one of the wave crests starting to flatten. My mood fell through the floor all the way down the sewer, and the voltage kept falling until it reached 181V. The lower crest had flattened noticeably now, about 10% when the dropping stopped, and the voltage started very slowly climbing back. When it reached 185V the lower amplitgude got back to normal, abd the voltage kept climbing. It climbed very slowly to 195V, over a few minutes. Now as I am writing this it is sitting at 197.6V and still going up very slowly.

I am not sure what this could be. I even thought of electrolytic capacitors as I reformed them and did not replace. But the DC was very clean when I looked at it before, so I doubt it is them.

I am puzzled but no it is working fine and I am gonna take it.
I will probably try to reinstall the old tubes one more time to see if something happened to the power somehow, but....

Dunno.
In any event it seems to work now.



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More.
Pulled one of the new tubes and inserted one of the old ones, the one that I replaced first without any result.
For a while it worked normally, then over 5 or so minutes the voltage dropped to 175V at which point the amplitude was 2/3 of initial but the signal was symmetrical and looking good.

Then I replaced it with the second old tube, and the initial scenaro repeated itself, when the oscilaltion got distorted fast and the voltage dropped also fast.

I am back to the new tubes, seems stable.

PS. The output tubes run very hot, you cannot touch the base, it is hot so I have to use something wrapped over it to pull it.
Don't know if thios is normal for these, I have no experience with this kind.

PPS. Not even sure the tube is bad. Could be some incompatibility. I am sure irt could work in another device.
But here's using tube tester for you - it tests absolutely normal, same uMhos as the other tubes and no gas.

PPPS. The oscillator is still running on the new 6AU5GT tubes and the B+ is very stable at about 196.5V (should be 190V but then is at 115V).