I have a Grunow 661 That came to me essentially untouched, and not working for a very long time. I checked the tubes and re-capped it along with a new power cord. I also sprayed contact cleaner into the volume control. It sounds great and tunes with really good sensitivity through the BC and SW bands. Made me happy right away. The problem is that after it's on for a minute or so, the volume drops very slightly along with a very slight distortion. At first I thought maybe a tube was going bad after heating up, but I re-tested them, leaving them in the tester to warm up for a couple of minutes before testing. All were good. The tube lineup is: (2) 6D6, (1) ea. 6A7, 42, 75, and 80. If anyone has any ideas regarding the volume drop and distortion, I would certainly appreciate it.

Might be a problem in the grid biasing resistors- when they heat up, resistance values shift and so does the tube bias voltage. This set uses a somewhat unusual grid-biasing scheme where the loudspeaker field is in the transformer high tension center-tap circuit, so it is also part of the grid bias circuit. (Hope that the speaker field isn't the problem.) My recommendation would be to take DC voltage readings of the audio amp grids when (1) the set is playing good; and (2) when it isn't, and then go from there if you find that there is a difference. Hopefully, it's just a resistor shifting its ohmic value excessively as it heats up.

:Might be a problem in the grid biasing resistors- when they heat up, resistance values shift and so does the tube bias voltage. This set uses a somewhat unusual grid-biasing scheme where the loudspeaker field is in the transformer high tension center-tap circuit, so it is also part of the grid bias circuit. (Hope that the speaker field isn't the problem.) My recommendation would be to take DC voltage readings of the audio amp grids when (1) the set is playing good; and (2) when it isn't, and then go from there if you find that there is a difference. Hopefully, it's just a resistor shifting its ohmic value excessively as it heats up.
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:I had looked at it too much. I'll check from that aspect. Thanks!
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:Might be a problem in the grid biasing resistors- when they heat up, resistance values shift and so does the tube bias voltage. This set uses a somewhat unusual grid-biasing scheme where the loudspeaker field is in the transformer high tension center-tap circuit, so it is also part of the grid bias circuit. (Hope that the speaker field isn't the problem.) My recommendation would be to take DC voltage readings of the audio amp grids when (1) the set is playing good; and (2) when it isn't, and then go from there if you find that there is a difference. Hopefully, it's just a resistor shifting its ohmic value excessively as it heats up.
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: I found the two 500k resistors went way off after warm up, replaced those, but still the sound was not as good as it should be. I started replacing tubes, even though they had all tested good. The 42 was the ultimate culprit.
I appreciate your help. Thanks, Bill:

Sort of supports the old adage that "a tube tester can always identify a bad tube, but it can't always identify a good tube".

:Sort of supports the old adage that "a tube tester can always identify a bad tube, but it can't always identify a good tube".

CV:
I used a tube tester very seldom in my early career when all we had was tubes, I preferred to replace the tube. Of course, in a radio station, replacing a tube to fix a problem was done on an expidite basis.
Lewis

::Sort of supports the old adage that "a tube tester can always identify a bad tube, but it can't always identify a good tube".
:
:
:CV:
:I used a tube tester very seldom in my early career when all we had was tubes, I preferred to replace the tube. Of course, in a radio station, replacing a tube to fix a problem was done on an expidite basis.
:Lewis
:

Maybe it was grid emission - high-time audio output tubes can accumulate a little cathode coating onto the control grid. Then as the tube gets hot enough the grid emits and bleeds off some bias - but I think it usually takes several minutes to heat up enough to show that.
Eddie

:::Sort of supports the old adage that "a tube tester can always identify a bad tube, but it can't always identify a good tube".
::
::
::CV:
::I used a tube tester very seldom in my early career when all we had was tubes, I preferred to replace the tube. Of course, in a radio station, replacing a tube to fix a problem was done on an expidite basis.
::Lewis
::
:
:Maybe it was grid emission - high-time audio output tubes can accumulate a little cathode coating onto the control grid. Then as the tube gets hot enough the grid emits and bleeds off some bias - but I think it usually takes several minutes to heat up enough to show that.
:Eddie
:
I pulled the 42 out of another Philco working radio. It works fine with the 42 from the Grunow. Of course I'm happy both are working fine now.