01-07-2022, 09:20 PM
Also keep in mind the reference point for the measurements.
If you measure the 71A plate voltage from plate to chassis, with the chassis as ground reference, the voltage will read high. This is because the filaments of the 71A's are +40V with respect to the chassis because of the voltage drop across the bias resistor. So if you measure 220V DC from plate to chassis, the actual voltage across the 71A from plate to filament is only 180V because the filament is at +40 to create the negative bias at the grid. Subtracting, actual plate voltage is 220V - 40V = 180V.
It is also confusing because Philco listed their voltage measurements for the early sets using their set tester. This test box was used by removing a tube, plugging the tester adapter into the tube socket, and then plugging the tube back into the adapter. Because the socket had no ground pin, voltage measurements were taken from filament to plate, not to chassis. This resulted in the situation above where voltage readings differ from those taken from chassis ground.
So in other words, you need to measure the 71A plate voltage from plate to filament, NOT to chassis to get the published voltage readings. This also applies to the 26 tube readings, but difference is much lower due to the lower voltage drop (10V to 15V) across their bias resistor.
If you measure the 71A plate voltage from plate to chassis, with the chassis as ground reference, the voltage will read high. This is because the filaments of the 71A's are +40V with respect to the chassis because of the voltage drop across the bias resistor. So if you measure 220V DC from plate to chassis, the actual voltage across the 71A from plate to filament is only 180V because the filament is at +40 to create the negative bias at the grid. Subtracting, actual plate voltage is 220V - 40V = 180V.
It is also confusing because Philco listed their voltage measurements for the early sets using their set tester. This test box was used by removing a tube, plugging the tester adapter into the tube socket, and then plugging the tube back into the adapter. Because the socket had no ground pin, voltage measurements were taken from filament to plate, not to chassis. This resulted in the situation above where voltage readings differ from those taken from chassis ground.
So in other words, you need to measure the 71A plate voltage from plate to filament, NOT to chassis to get the published voltage readings. This also applies to the 26 tube readings, but difference is much lower due to the lower voltage drop (10V to 15V) across their bias resistor.
