04-29-2017, 05:10 PM
I would agree with Terry, normal DC current draw should be about 100 mA. You have:
2 6K6 @ 35 mA each = 70 mA
2 7H7 @ 12 mA = 24 mA
1 7F8 @10 mA = 10 mA
Total = 104 mA
The dissipation in the 300 ohm resistor is not the straightforward DC V x I, because the current through it is not pure DC. Rather, it is a high peak to average ratio pulsating current, since the rectifier only conducts for a portion of the AC cycle at the peak of the waveform. So the DC voltage drop across the 300 ohms does not accurately predict the power dissipated.
The 500 ohm resistor now being used is getting extra hot since it is dissipating 66% more power than a 300 ohm resistor would under the same conditions. If you had a 300 ohm of the same size and power rating as the present 500 ohm it would not get nearly as hot. Using this series resistor was never really a great idea to replace the field coil speaker. It was just something Philco did to avoid using the right power transformer and get sets out the door. The original and replacement resistors did not last long in service as was mentioned.
If you want to measure the actual DC current draw, just measure the DC voltage across R30, the 170 ohm Candohm bias resistor. Current = Vmeasured/170 ohms.
In any case, no matter what type 300 ohm resistor you use it has to dissipate appreciable power, so why not just use the 50 watt chassis mount and be done with it?
2 6K6 @ 35 mA each = 70 mA
2 7H7 @ 12 mA = 24 mA
1 7F8 @10 mA = 10 mA
Total = 104 mA
The dissipation in the 300 ohm resistor is not the straightforward DC V x I, because the current through it is not pure DC. Rather, it is a high peak to average ratio pulsating current, since the rectifier only conducts for a portion of the AC cycle at the peak of the waveform. So the DC voltage drop across the 300 ohms does not accurately predict the power dissipated.
The 500 ohm resistor now being used is getting extra hot since it is dissipating 66% more power than a 300 ohm resistor would under the same conditions. If you had a 300 ohm of the same size and power rating as the present 500 ohm it would not get nearly as hot. Using this series resistor was never really a great idea to replace the field coil speaker. It was just something Philco did to avoid using the right power transformer and get sets out the door. The original and replacement resistors did not last long in service as was mentioned.
If you want to measure the actual DC current draw, just measure the DC voltage across R30, the 170 ohm Candohm bias resistor. Current = Vmeasured/170 ohms.
In any case, no matter what type 300 ohm resistor you use it has to dissipate appreciable power, so why not just use the 50 watt chassis mount and be done with it?
