After a more careful perusal of the schematic, I see the volume control is in the antenna. My bad. Okay, short the grid of the output tube to ground and let's see what that does to the hum.
Lewis
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::: ::: ::: ::: ::: ::: ::: ::: ::: ::: ::: :::Sir John . . . . ::: ::: ::: In consulting the sets schematic and putting theory into real time practice, initially, one sees that the purpose of that R36 resistor is to hold the return to ground path of the B+ ABOVE ground level potential. ::: ::: :::That is being done in order to obtain a "free" negative "C" supply bias voltage for the 1st grid of the #42 AF output tube across that R36 resistor. ::: ::: :::You can see that bias insertion path being via the R26- - - 490,000 ohm grid resistor, and with that high value of resistor, the max possible current being able to pas thru that loop will be down in the microampere range. ::: ::: :::That is CONSIDERING . . no faulty 42 tube pulling grid emission(rare) being gassy OR any dc leakage occurring in the item 23 coupling capacitor. ::: ::: :::THEREFORE, any significant current passage through your R36 will be related to the main B+ path additionally getting to ground by its passage thru it. ::: ::: :::Looking at the finite series resistance actually in the B+ path, one sees the 364ohms of the AF output transformer primary, the 1250 ohms of the speaker field coil winding, and your R36's 325 ohms for a total of ~1940ohms. ::: ::: :::Now if one hypothesizes a perfect TOTAL conduction of the 42, that would permit up to ~125 ma closed loop current. ::: ::: :::NEVER to be . . . . but by using those WORST case numbers, the total voltage would then divide ratiometrically, with your R36 :::having 48 volts across it at 125 ma . . . .for 6 watts dissipated by it. ::: ::: :::However, in actual REAL operating circuit analysis: ::: ::: :::If one looks at the voltage of the PRIMARY power consumer of the radio, the 42 tube, one will see a voltage on the plate of 240 volts with a higher B+ voltage of 255 on its screen. ::: ::: :::That confirms having 15 volts across the 365 ohms primary of the AF output transformer, therefore expect 41 ma to be the averaged current dissipation being in the TOTAL current loop. ::: ::: :::When that R36's resistance is ratiometrically computed against the two other series resistance values, that will be computing as having - 13V for your bias voltage. ::: ::: :::With the figure of 41 ma current passage and the 13 V across the resistor, figure on a wattage need of .533 watt across it. :::Double that value for a safety margin. ::: ::: :::I personally, would have a 3-5 Watt wirewound or metal film being used. ::: ::: :::Soooooooo with your 10 watts you should be good thru 3012AD . (if a rat doesn't sneak in and urinate upon it.) ::: ::: ::: ::: ::: :::73's de Edd ::: ::: ::: ::: ::: :::Take care to get what you like or you will be forced to like what you get. ::: ::: ::: ::: ::: ::: ::: ::: :::

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::::Check that the primary of the audio output transformer is not open. The 300 ohm, 10 watt resistor is fine.
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:::::Sorry, I was wrong. Measured again and only 10 volts at the plate of the 42 even with the tube out. Got to checking around and found R36 open. Looks like 325 ohms but what wattage? I have a 300 at 10W. Would that be enough wattage?
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Make sure the electrolytic (37?)across R36 is connected properly, i.e., it's plus goes to chassis/ground. Also the minus of the other power supply electrolytics go to the centertap and not chassis/ground.

Clifton

::After replacing R36 I have 270V on the plate of the 42 and 17v AC at 60cycle there also. Loud hum. I have double checked the filters that I changed and compared wiring with one other Model 80 that works and don't see anything wrong with my replacements. Changed the 42 and the 80 with known working tubes and still loud hum exists. Not sure why!
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:: :: :: :: :: :: :: :: :: :: :: ::Sir John . . . . :: :: :: In consulting the sets schematic and putting theory into real time practice, initially, one sees that the purpose of that R36 resistor is to hold the return to ground path of the B+ ABOVE ground level potential. :: :: ::That is being done in order to obtain a "free" negative "C" supply bias voltage for the 1st grid of the #42 AF output tube across that R36 resistor. :: :: ::You can see that bias insertion path being via the R26- - - 490,000 ohm grid resistor, and with that high value of resistor, the max possible current being able to pas thru that loop will be down in the microampere range. :: :: ::That is CONSIDERING . . no faulty 42 tube pulling grid emission(rare) being gassy OR any dc leakage occurring in the item 23 coupling capacitor. :: :: ::THEREFORE, any significant current passage through your R36 will be related to the main B+ path additionally getting to ground by its passage thru it. :: :: ::Looking at the finite series resistance actually in the B+ path, one sees the 364ohms of the AF output transformer primary, the 1250 ohms of the speaker field coil winding, and your R36's 325 ohms for a total of ~1940ohms. :: :: ::Now if one hypothesizes a perfect TOTAL conduction of the 42, that would permit up to ~125 ma closed loop current. :: :: ::NEVER to be . . . . but by using those WORST case numbers, the total voltage would then divide ratiometrically, with your R36 ::having 48 volts across it at 125 ma . . . .for 6 watts dissipated by it. :: :: ::However, in actual REAL operating circuit analysis: :: :: ::If one looks at the voltage of the PRIMARY power consumer of the radio, the 42 tube, one will see a voltage on the plate of 240 volts with a higher B+ voltage of 255 on its screen. :: :: ::That confirms having 15 volts across the 365 ohms primary of the AF output transformer, therefore expect 41 ma to be the averaged current dissipation being in the TOTAL current loop. :: :: ::When that R36's resistance is ratiometrically computed against the two other series resistance values, that will be computing as having - 13V for your bias voltage. :: :: ::With the figure of 41 ma current passage and the 13 V across the resistor, figure on a wattage need of .533 watt across it. ::Double that value for a safety margin. :: :: ::I personally, would have a 3-5 Watt wirewound or metal film being used. :: :: ::Soooooooo with your 10 watts you should be good thru 3012AD . (if a rat doesn't sneak in and urinate upon it.) :: :: :: :: :: ::73's de Edd :: :: :: :: :: ::Take care to get what you like or you will be forced to like what you get. :: :: :: :: :: :: :: :: ::

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:::Check that the primary of the audio output transformer is not open. The 300 ohm, 10 watt resistor is fine.
:::-----------------
::::Sorry, I was wrong. Measured again and only 10 volts at the plate of the 42 even with the tube out. Got to checking around and found R36 open. Looks like 325 ohms but what wattage? I have a 300 at 10W. Would that be enough wattage?
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:John,
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:Make sure the electrolytic (37?)across R36 is connected properly, i.e., it's plus goes to chassis/ground. Also the minus of the other power supply electrolytics go to the centertap and not chassis/ground.
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:Clifton
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:::After replacing R36 I have 270V on the plate of the 42 and 17v AC at 60cycle there also. Loud hum. I have double checked the filters that I changed and compared wiring with one other Model 80 that works and don't see anything wrong with my replacements. Changed the 42 and the 80 with known working tubes and still loud hum exists. Not sure why!
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:::
:::

::: ::: ::: ::: ::: ::: ::: ::: ::: ::: ::: :::Sir John . . . . ::: ::: ::: In consulting the sets schematic and putting theory into real time practice, initially, one sees that the purpose of that R36 resistor is to hold the return to ground path of the B+ ABOVE ground level potential. ::: ::: :::That is being done in order to obtain a "free" negative "C" supply bias voltage for the 1st grid of the #42 AF output tube across that R36 resistor. ::: ::: :::You can see that bias insertion path being via the R26- - - 490,000 ohm grid resistor, and with that high value of resistor, the max possible current being able to pas thru that loop will be down in the microampere range. ::: ::: :::That is CONSIDERING . . no faulty 42 tube pulling grid emission(rare) being gassy OR any dc leakage occurring in the item 23 coupling capacitor. ::: ::: :::THEREFORE, any significant current passage through your R36 will be related to the main B+ path additionally getting to ground by its passage thru it. ::: ::: :::Looking at the finite series resistance actually in the B+ path, one sees the 364ohms of the AF output transformer primary, the 1250 ohms of the speaker field coil winding, and your R36's 325 ohms for a total of ~1940ohms. ::: ::: :::Now if one hypothesizes a perfect TOTAL conduction of the 42, that would permit up to ~125 ma closed loop current. ::: ::: :::NEVER to be . . . . but by using those WORST case numbers, the total voltage would then divide ratiometrically, with your R36 :::having 48 volts across it at 125 ma . . . .for 6 watts dissipated by it. ::: ::: :::However, in actual REAL operating circuit analysis: ::: ::: :::If one looks at the voltage of the PRIMARY power consumer of the radio, the 42 tube, one will see a voltage on the plate of 240 volts with a higher B+ voltage of 255 on its screen. ::: ::: :::That confirms having 15 volts across the 365 ohms primary of the AF output transformer, therefore expect 41 ma to be the averaged current dissipation being in the TOTAL current loop. ::: ::: :::When that R36's resistance is ratiometrically computed against the two other series resistance values, that will be computing as having - 13V for your bias voltage. ::: ::: :::With the figure of 41 ma current passage and the 13 V across the resistor, figure on a wattage need of .533 watt across it. :::Double that value for a safety margin. ::: ::: :::I personally, would have a 3-5 Watt wirewound or metal film being used. ::: ::: :::Soooooooo with your 10 watts you should be good thru 3012AD . (if a rat doesn't sneak in and urinate upon it.) ::: ::: ::: ::: ::: :::73's de Edd ::: ::: ::: ::: ::: :::Take care to get what you like or you will be forced to like what you get. ::: ::: ::: ::: ::: ::: ::: ::: :::

:::
:::
:::
:::
:::
::::Check that the primary of the audio output transformer is not open. The 300 ohm, 10 watt resistor is fine.
::::-----------------
:::::Sorry, I was wrong. Measured again and only 10 volts at the plate of the 42 even with the tube out. Got to checking around and found R36 open. Looks like 325 ohms but what wattage? I have a 300 at 10W. Would that be enough wattage?
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