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Philco 77 resistor wattage - chalupa13 - 10-02-2020

Hello, would anyone know what wattage are the resistors I highlighted in yellow? Those are wirewound ones on a brown tube. I replaced them with 5W but they get extremely hot, all of them, and especially 800 Ohm one. Is it supposed to be that way? Thank you.

RE: Philco 77 resistor wattage - morzh - 10-02-2020

This resistor (800 ohm) dissipates 3W, (ok, 2.88w if the currents of the 45 tubes are 30mA each) so it will get hot at 5W rating.
Make it 10W.

RE: Philco 77 resistor wattage - Bill Bacco - 10-03-2020

You should be able to compute. Power = Voltage squared divided by resistance. Measure the voltage across each resister and calculate the normal running wattage. Then maybe take that x2 and find the standard wattage that is close to that. The larger the wattage, the more cooler they will be and the longer they will last.

RE: Philco 77 resistor wattage - chalupa13 - 10-03-2020

Thanks will do that. My biggest problem is with 250 Ohm resistor it gets to 140 C. Rest are running around 80C. All 5W. The radio power draw is normal around 120w, it rated at 110w.

RE: Philco 77 resistor wattage - morzh - 10-03-2020

Measure the drop across the resistor: calculate the current as V/R.
Them we will know if someone draws the excess current.

RE: Philco 77 resistor wattage - Bill Bacco - 10-04-2020

The 250 ohm resister is carrying the load for most of the system, since it is basically providing the ground path for the power supply. Once you get the voltage measurements, you will know whether there is something else going on or whether a larger wattage resister is just needed.

RE: Philco 77 resistor wattage - morzh - 10-04-2020

Even if it is 100mA of total current (and this is a lot) the dissipation would be 2.5W. Nothing to sneeze at but 7.5W resistor will take it well.
My rule of a thumb: Rating is 3x dissipation.

RE: Philco 77 resistor wattage - chalupa13 - 10-05-2020

Thanks. The radio works fine, I've measured all of the voltages and they seem to be in line with the schematic. So I've ordered 10w resistors to replace the 5w ones.