Could anyone tell me what value power resistor would be needed to replace the field coil of a Philco PT-93. The tube set is 50L6, 35Z3, 7A8, 7B7, 7C6? Also what would be a safe wattage for the resistor. Thanks for any info!
Mike.

You could calculate it with circuit analysis. Or...

You could try 1K ohms. Then check the B+ voltage at the load end of the field replacement resistor - should be 100V dc as shown on the schematic. Adjust resistance if necessary.

Calculate power disipation: add up the plate voltages using a tube manual, use I^2R for power. Double for resistor rating for safety. 20W should be plenty fine, probably overkill.

I came up with 20W this way with his horseback approach: maximum plate current of the 50L6 is 50 mA. Double for allowance for the other tubes = 100 mA. 0.1^2 x 1000 = 10W. Double for safety = 20W.

Hi

Also need to increase filter caps from 20 mf to 47 mf to eliminate hum? Dual 20 mf works well with a field coil as it has inductance but resistor replacement needs more capacity.

Norm

:You could calculate it with circuit analysis. Or...
:
:You could try 1K ohms. Then check the B+ voltage at the load end of the field replacement resistor - should be 100V dc as shown on the schematic. Adjust resistance if necessary.
:
:Calculate power disipation: add up the plate voltages using a tube manual, use I^2R for power. Double for resistor rating for safety. 20W should be plenty fine, probably overkill.
:
:I came up with 20W this way with his horseback approach: maximum plate current of the 50L6 is 50 mA. Double for allowance for the other tubes = 100 mA. 0.1^2 x 1000 = 10W. Double for safety = 20W.
:

I checked the RCA tube manual about the largest allowable filter cap next to a 35Z3 rectifier - it says 40-uF; well within, tolerance-wise, of the 47-uF cap suggested by Norm.

If you want to be ultra-conservative, use 33-uF right after the rectifier and 47-uF after the resistor. And maybe even a 100-uF after the resistor.

But, Norm is certainly correct - jacking up the filter caps is a good idea. Even then, you might hear a bit of hum without the choke.

After the resistor is better than before it if you want to beef up the smoothing capacitor. So long as you don't have motorboating, the half wave rectifier will only have significant A.C. at 60 HZ, which will not be reproduced to any significant degree by the oiginal speaker.

:I checked the RCA tube manual about the largest allowable filter cap next to a 35Z3 rectifier - it says 40-uF; well within, tolerance-wise, of the 47-uF cap suggested by Norm.
:
:If you want to be ultra-conservative, use 33-uF right after the rectifier and 47-uF after the resistor. And maybe even a 100-uF after the resistor.
:
:But, Norm is certainly correct - jacking up the filter caps is a good idea. Even then, you might hear a bit of hum without the choke.
:

Thanks everyone for your advice and ideas. It's a BIG help!
Mike.

:After the resistor is better than before it if you want to beef up the smoothing capacitor. So long as you don't have motorboating, the half wave rectifier will only have significant A.C. at 60 HZ, which will not be reproduced to any significant degree by the oiginal speaker.
:
::I checked the RCA tube manual about the largest allowable filter cap next to a 35Z3 rectifier - it says 40-uF; well within, tolerance-wise, of the 47-uF cap suggested by Norm.
::
::If you want to be ultra-conservative, use 33-uF right after the rectifier and 47-uF after the resistor. And maybe even a 100-uF after the resistor.
::
::But, Norm is certainly correct - jacking up the filter caps is a good idea. Even then, you might hear a bit of hum without the choke.
::
: