[morzh]

This is the same one I restored aboyt 1-1/2 years ago.

When I set it aside, it was working but when the volage in the Mains went above 115VAC, the reception would fade and become weak.

Today I decided to investigate.
Since it was fully recaped, I went straight to tubes and since 6A7 was changed before, I went past it.
The main suspect was 78 tube, the IF amp.

http://www.nostalgiaair.org/PagesByModel...013239.pdf

Plus, this is the only extra NOS tube I have from the whole setup.
I changed it, and, lo and behold, the symptom disapeared.

I put the old tube back and within 2 minutes it came back.

I rechecked the 78 on Hickok, it is fine, the TC is 1350.
I let it sit for a few minutes, no, no degradation.
And no gas.


I decided to take a look at AVC voltage. The grid of the 78 and 4th grid of 6A7 go to -2.9V or so first, then with the bad tube the voltage drops gradually rises to 0.7V. The sound weakens about 1.5V.

With the good tube the voltage stays about 2.8V all the time.

And the corelation with high AC votage in Mains - when I lower the voltage the AVC starts dropping again and the volume restores.

Well, it is 78. But...strange. Leakage at high plate voltage?

[Mondial]

Seems like the 78 is gassy or has grid emission. When the voltage rises it may reach the point where the gas ionizes and causes the control grid to go positive.

Tubes can develop some weird problems. I had a 39/44 tube that did something similar. Radio would play fine for a few minutes then gradually reduce volume. Found AVC line went from normal negative condition when receiving signal to slightly positive as it warmed up. Replacing the tube cured the problem.

[morzh]

Yep, same thing.
The tester does not detect gas, in fact if anything, this tube exhibits zero movement when the second Gas button is pressed, whereas it is normal to see up to 1/2 div movement and I usually see at least some.

Could be grid contamination.