02-28-2021, 12:16 AM
I took the time to fire up my 20 and take some measurements. All voltages are referenced to the center tap of the A filament supply, Volume on full with the antenna disconnected. Line Voltage is at about 121 volts. It seems obvious that the voltages on the 71A's have to be listed incorrectly. If nothing else, they forgot a - sign. My 24 voltages also have the plate much higher than the listed voltage. The radio works great for a TRF with no AVC. I did have to rewind 2 of the RF coils, one of which was the detector coil.
Looking at the given detector voltages, the factory gives a net grid voltage of -7 volts. My 20 voltages give a relative grid voltage of -2.6 volts. The tube in my radio should be closer to saturation than the factory numbers so the factory plate voltage should be higher than mine. Obviously they are not. Perhaps if I tried a NOS tube it would change a bit. But, again I think the factory numbers are off. FYI, all resistors are spot on.
Oh I forgot! I have a bucking transformer in this thing that brings the line voltage down to about 107 volts.
Looking at the given detector voltages, the factory gives a net grid voltage of -7 volts. My 20 voltages give a relative grid voltage of -2.6 volts. The tube in my radio should be closer to saturation than the factory numbers so the factory plate voltage should be higher than mine. Obviously they are not. Perhaps if I tried a NOS tube it would change a bit. But, again I think the factory numbers are off. FYI, all resistors are spot on.
Oh I forgot! I have a bucking transformer in this thing that brings the line voltage down to about 107 volts.
