02-28-2021, 10:49 PM
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
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
