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Philco 20 voltages
#30

So I played with the bias on the detector by dropping the cathode resistor to under 10K ohms. This brought the plate voltage down to about 75 volts, but I started to pick up distortion. I presume the tube reached very close to full saturation. No way I can figure how they got the 35 volts unless their 1000ohms/volt meter drags it down that far... It doesn't! I dug out an old VOM and it dropped the plate voltage only about 6 volts.

Chris, the grid is at ground potential. Coil is very low resistance. About 4 ohms. There is some current as the drop across the two plate resistors is about 20 volts. Not fully at cutoff.

For grins and giggles I hooked up the scope to the detector grid and plate. On the grid, with a strong AM signal, I got about 800mv modulated RF with the probe set to 10/1. Even this reduced the volume quite a bit. On the plate with the probe sill at 10/1, I get about 200 mv of RF, and almost 4 volts of AF. No reduction in audio volume with the probe connected to the plate, as would be expected.

Patrick, make sure your coils are good. I know mine measured ok, but there was a lot of green corrosion due to the dreaded Philco coil disease. I just decided to rewind them to be sure. Only the outside winding needs to be replaced.

Rob


Messages In This Thread
Philco 20 voltages - by Patrick - 02-26-2021, 11:41 AM
RE: Philco 20 voltages - by Radioroslyn - 02-26-2021, 01:22 PM
RE: Philco 20 voltages - by Patrick - 02-27-2021, 08:29 AM
RE: Philco 20 voltages - by Radioroslyn - 02-27-2021, 09:58 AM
RE: Philco 20 voltages - by RodB - 02-27-2021, 11:20 AM
RE: Philco 20 voltages - by 462ron - 02-27-2021, 11:28 AM
RE: Philco 20 voltages - by rfeenstra - 02-27-2021, 01:20 PM
RE: Philco 20 voltages - by Radioroslyn - 02-27-2021, 01:34 PM
RE: Philco 20 voltages - by 462ron - 02-27-2021, 01:45 PM
RE: Philco 20 voltages - by Patrick - 02-27-2021, 03:38 PM
RE: Philco 20 voltages - by RodB - 02-27-2021, 08:25 PM
RE: Philco 20 voltages - by RodB - 02-27-2021, 09:14 PM
RE: Philco 20 voltages - by Patrick - 02-27-2021, 09:57 PM
RE: Philco 20 voltages - by radio1 - 02-27-2021, 10:11 PM
RE: Philco 20 voltages - by rfeenstra - 02-28-2021, 01:16 AM
RE: Philco 20 voltages - by Patrick - 02-28-2021, 06:46 AM
RE: Philco 20 voltages - by Radioroslyn - 02-28-2021, 07:41 AM
RE: Philco 20 voltages - by Radioroslyn - 02-28-2021, 09:46 AM
RE: Philco 20 voltages - by Bill Bacco - 02-28-2021, 09:56 AM
RE: Philco 20 voltages - by Patrick - 02-28-2021, 11:25 AM
RE: Philco 20 voltages - by Patrick - 02-28-2021, 11:32 AM
RE: Philco 20 voltages - by Patrick - 02-28-2021, 12:03 PM
RE: Philco 20 voltages - by OZ4 - 02-28-2021, 03:06 PM
RE: Philco 20 voltages - by radiohenry - 02-28-2021, 05:11 PM
RE: Philco 20 voltages - by Patrick - 02-28-2021, 05:17 PM
RE: Philco 20 voltages - by radiohenry - 02-28-2021, 05:23 PM
RE: Philco 20 voltages - by OZ4 - 02-28-2021, 10:11 PM
RE: Philco 20 voltages - by rfeenstra - 02-28-2021, 10:33 PM
RE: Philco 20 voltages - by OZ4 - 02-28-2021, 10:42 PM
RE: Philco 20 voltages - by rfeenstra - 02-28-2021, 11:49 PM
RE: Philco 20 voltages - by Patrick - 03-01-2021, 06:19 AM
RE: Philco 20 voltages - by Patrick - 03-14-2021, 09:22 PM
RE: Philco 20 voltages - by morzh - 03-14-2021, 10:47 PM
RE: Philco 20 voltages - by Patrick - 03-15-2021, 06:15 AM



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