Actually, I have one more observation that I'd appreciate an opinion about: this "Hum", which disappears when you ground the grid of the 6f6, is extremely bad. I turn the set off within seconds when it builds up because it seems like it could do damage. For that reason I believe the problem is internal, but it's a guess.

By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.

Grounding the grid of the 6F6 grounds the grid and it's bias. I would then disconnect the coupling capacitor to the 6F6 grid. If the loud hum was still there then I would check out the negative bias voltage from the grid to the centertap of the power transformer HV secondary. Make sure C12 is installed correctly. It's minus goes to the centertap and not chassis.

Clifton

:By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.
:
:

:Grounding the grid of the 6F6 grounds the grid and it's bias. I would then disconnect the coupling capacitor to the 6F6 grid. If the loud hum was still there then I would check out the negative bias voltage from the grid to the centertap of the power transformer HV secondary. Make sure C12 is installed correctly. It's minus goes to the centertap and not chassis.
:
:Clifton
:
::By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.
::
::Thanks, I'll follow up on these ideas.
:
:

::Grounding the grid of the 6F6 grounds the grid and it's bias. I would then disconnect the coupling capacitor to the 6F6 grid. If the loud hum was still there then I would check out the negative bias voltage from the grid to the centertap of the power transformer HV secondary. Make sure C12 is installed correctly. It's minus goes to the centertap and not chassis.
::
::Clifton
::
:::By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.
:::
:::Thanks, I'll follow up on these ideas.
::
::Disconnecting the coupling capacitor definitely does stop the hum. Voltages on the 6F6 seem a little high: -17 on the grid, +250 on the plate and +271 on the screen. I should point out, too, that the radio does not work, though it did before the loud hum appeared. I don't think there is a cathode-heater short in the 6F6, based on continuity check. All of the tubes light up and show no signs of gassy glow. I wonder if a bad tube elsewhere could cause the loud hum. I don't have a tester for these older tubes: 6A8G oscillator, 6K7G i.f. amp, 6Q7G second detector, 5Y3G rectifier.
:
:

If your tester has a listing for a 6A8, 6K7, 6Q7 you can test the tubes or 6A8GT, 6K7GT, 6Q7GT.

Clifton

:::Grounding the grid of the 6F6 grounds the grid and it's bias. I would then disconnect the coupling capacitor to the 6F6 grid. If the loud hum was still there then I would check out the negative bias voltage from the grid to the centertap of the power transformer HV secondary. Make sure C12 is installed correctly. It's minus goes to the centertap and not chassis.
:::
:::Clifton
:::
::::By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.
::::
::::Thanks, I'll follow up on these ideas.
:::
:::Disconnecting the coupling capacitor definitely does stop the hum. Voltages on the 6F6 seem a little high: -17 on the grid, +250 on the plate and +271 on the screen. I should point out, too, that the radio does not work, though it did before the loud hum appeared. I don't think there is a cathode-heater short in the 6F6, based on continuity check. All of the tubes light up and show no signs of gassy glow. I wonder if a bad tube elsewhere could cause the loud hum. I don't have a tester for these older tubes: 6A8G oscillator, 6K7G i.f. amp, 6Q7G second detector, 5Y3G rectifier.
::
::
:
:

:If your tester has a listing for a 6A8, 6K7, 6Q7 you can test the tubes or 6A8GT, 6K7GT, 6Q7GT.
:
:Clifton
:
::::Grounding the grid of the 6F6 grounds the grid and it's bias. I would then disconnect the coupling capacitor to the 6F6 grid. If the loud hum was still there then I would check out the negative bias voltage from the grid to the centertap of the power transformer HV secondary. Make sure C12 is installed correctly. It's minus goes to the centertap and not chassis.
::::
::::Clifton
::::
:::::By grounding the input grid of the 6F6 and the hum stops. This would indicate the hum is not from the power supply. With no hum at a cold start, but builds up after warm up. This may indicate a heater to cathode short. The 6F6 tube runs very hot, and a heater to cathode short in this tube is common to find.
:::::
:::::Thanks, I'll follow up on these ideas.
::::
::::Disconnecting the coupling capacitor definitely does stop the hum. Voltages on the 6F6 seem a little high: -17 on the grid, +250 on the plate and +271 on the screen. I should point out, too, that the radio does not work, though it did before the loud hum appeared. I don't think there is a cathode-heater short in the 6F6, based on continuity check. All of the tubes light up and show no signs of gassy glow. I wonder if a bad tube elsewhere could cause the loud hum. I don't have a tester for these older tubes: 6A8G oscillator, 6K7G i.f. amp, 6Q7G second detector, 5Y3G rectifier.
:::
:::Unfortunately it doesn't accommodate any of those larger, older tubes.
::
::
:
:

Hi Mike,

If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.

If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.

Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.

Let me know how you do.

All the Best,

Bill Grimm

:Hi Mike,
:
:If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
:
:If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
:
:Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
:
:Let me know how you do.
:
:All the Best,
:
:Bill Grimm
:
Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks

:Hi Mike,
:
:If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
:
:If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
:
:Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
:
:Let me know how you do.
:
:All the Best,
:
:Bill Grimm
:
Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks

Hi Mike,
H-K shorts in tubes are often not present when the heater is cold, so putting an ohm meter on it usually isn't conclusive.
You can get a 6Q7. They are cheap, but first there are some things to try. First is checking the dressing of the wiring in that stage. A misplaced wire can pick up hum easily because this circuit is high impedance.
Also an problem in the volume control can be a problem. Make sure the volume control still has the stock value.
The wire for grid cap of the 6Q7G needs to have a shield on it. It may have been replaced at some time with an unshielded wire and never worked again.
After that, double check all the connections and capacitors you have changed in that circuit. We all make mistakes.

If all that looks good, then order the new tube.

All the Best,

Bill Grimm

::Hi Mike,
::
::If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
::
::If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
::
::Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
::
::Let me know how you do.
::
::All the Best,
::
::Bill Grimm
::
:Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks
:

:Hi Mike,
: H-K shorts in tubes are often not present when the heater is cold, so putting an ohm meter on it usually isn't conclusive.
: You can get a 6Q7. They are cheap, but first there are some things to try. First is checking the dressing of the wiring in that stage. A misplaced wire can pick up hum easily because this circuit is high impedance.
: Also an problem in the volume control can be a problem. Make sure the volume control still has the stock value.
: The wire for grid cap of the 6Q7G needs to have a shield on it. It may have been replaced at some time with an unshielded wire and never worked again.
: After that, double check all the connections and capacitors you have changed in that circuit. We all make mistakes.
:
: If all that looks good, then order the new tube.
:
:All the Best,
:
:Bill Grimm
:
:::Hi Mike,
:::
:::If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
:::
:::If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
:::
:::Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
:::
:::Let me know how you do.
:::
:::All the Best,
:::
:::Bill Grimm
:::
::Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks
::
Bill, Thanks for all your help with this. I'm pleased to report that I finally have been able to solve this hum problem. It turned out to be a poor ground through the a 6q7 socket rivet! I stumbled on it while poking around looking for bad solder joints. I wired the socket to a firm screw in the chassis and the hum disappeared. Always learning something new.
:

::Hi Mike,
:: H-K shorts in tubes are often not present when the heater is cold, so putting an ohm meter on it usually isn't conclusive.
:: You can get a 6Q7. They are cheap, but first there are some things to try. First is checking the dressing of the wiring in that stage. A misplaced wire can pick up hum easily because this circuit is high impedance.
:: Also an problem in the volume control can be a problem. Make sure the volume control still has the stock value.
:: The wire for grid cap of the 6Q7G needs to have a shield on it. It may have been replaced at some time with an unshielded wire and never worked again.
:: After that, double check all the connections and capacitors you have changed in that circuit. We all make mistakes.
::
:: If all that looks good, then order the new tube.
::
::All the Best,
::
::Bill Grimm
::
::::Hi Mike,
::::
::::If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
::::
::::If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
::::
::::Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
::::
::::Let me know how you do.
::::
::::All the Best,
::::
::::Bill Grimm
::::
:::Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks
:::
:Bill, Thanks for all your help with this. I'm pleased to report that I finally have been able to solve this hum problem. It turned out to be a poor ground through the a 6q7 socket rivet! I stumbled on it while poking around looking for bad solder joints. I wired the socket to a firm screw in the chassis and the hum disappeared. Always learning something new.

Mike:
As I have said many times before, I had rather deal with a completely dead unit than try to figure out some seemingly slight problem on one that works almost OK.
Lewis
::
:
:

Classic!
My favorite was a tube socket with a socket pin that had been distorted to too big. That was a bear!

Corroded socket rivet is one I have to remember. It is just a matter of time.

All the Best,

Bill Grimm

::Hi Mike,
:: H-K shorts in tubes are often not present when the heater is cold, so putting an ohm meter on it usually isn't conclusive.
:: You can get a 6Q7. They are cheap, but first there are some things to try. First is checking the dressing of the wiring in that stage. A misplaced wire can pick up hum easily because this circuit is high impedance.
:: Also an problem in the volume control can be a problem. Make sure the volume control still has the stock value.
:: The wire for grid cap of the 6Q7G needs to have a shield on it. It may have been replaced at some time with an unshielded wire and never worked again.
:: After that, double check all the connections and capacitors you have changed in that circuit. We all make mistakes.
::
:: If all that looks good, then order the new tube.
::
::All the Best,
::
::Bill Grimm
::
::::Hi Mike,
::::
::::If it is an H-K short causing the problem it is most likely the 6Q7G. Ground the tube cap as a test. If the hum goes away, then the 6Q7G is the one with the short.
::::
::::If not and while you are at it, ground the cap of the 6K7G as a test. If the hum goes away, it is that tube.
::::
::::Sometimes tube testers don't catch the H-K shorts. I have a 35C5 that does not test short but put it in a radio and you get plenty of hum.
::::
::::Let me know how you do.
::::
::::All the Best,
::::
::::Bill Grimm
::::
:::Bill, grounding the q7's grid cap cuts the hum significantly but not to the pont that I would consider normal. It's not as quiet as grounding the grid to the output tube, for example. Meanwhile, grounding the grid cap of th k7 has no effect on the hum. I'm wondering if a heater/k short happens only when the tube is operating, or should I be able to hear the short with a continuity meter? Thanks
:::
:Bill, Thanks for all your help with this. I'm pleased to report that I finally have been able to solve this hum problem. It turned out to be a poor ground through the a 6q7 socket rivet! I stumbled on it while poking around looking for bad solder joints. I wired the socket to a firm screw in the chassis and the hum disappeared. Always learning something new.
::
:
: