04-11-2016, 10:35 AM
David, keep in mind that the Philco published voltage readings were taken with the Philco set tester. There are two important factors to consider.
First, the set tester measured voltages with respect to the filament pins of the tube, not the chassis. This is important because the main 2.5V filament winding is connected to the center tap of the HV winding, which is at a negative voltage compared to the chassis. The 45 output stage has a separate filament winding and a cathode bias resistor so it is also not at ground potential. To get comparable readings to the published voltages, you must connect the negative probe of your DVM to a filament pin of the tube you are testing.
The second factor to consider is that the Philco voltmeter had a very low input resistance, so it tended to load any high resistance circuit under test. A modern DVM has a 10 megohm input resistance so it loads the circuit much less.Therefore the high readings for some of the screen and plate circuits are most likely due to this effect.
First, the set tester measured voltages with respect to the filament pins of the tube, not the chassis. This is important because the main 2.5V filament winding is connected to the center tap of the HV winding, which is at a negative voltage compared to the chassis. The 45 output stage has a separate filament winding and a cathode bias resistor so it is also not at ground potential. To get comparable readings to the published voltages, you must connect the negative probe of your DVM to a filament pin of the tube you are testing.
The second factor to consider is that the Philco voltmeter had a very low input resistance, so it tended to load any high resistance circuit under test. A modern DVM has a 10 megohm input resistance so it loads the circuit much less.Therefore the high readings for some of the screen and plate circuits are most likely due to this effect.
