[Tubesforme]

I recently purchased a Philco Model 66. It was pretty well hacked up by someone. I now have the chassis restored to original and it pulls in station really great. However I am having a problem with two resistors. One is #47, which is a 37K resistor, which looked to be about 3W. It was way up in value. I replaced it with a pair of 75K 2W flame proof resistors in parallel. Then Resistor # 15 32K, I replaced with a 33K 1W carbon type. I have no idea of what the original wattage was. Someone had installed a 39K 1W resistor in its place and it had gone up in value. After the radio plays for between 5 and 10 minutes, you can't put your fingers on the 37K # 47 and the 32K #15 is also to hot to want to keep your fingers on it. Everything except the 6A7 tube has been replaced and I used my VTVM in the high meg scale and checked for high resistance to ground, which would indicate a short. None show up. Naturally capacitor #49 takes a charge when doing this so I disconnected it to make sure. All capacitors, except the micas have been replaced by new capacitors. All off value resistors have been replaced. I did have to install a replacement choke because it was missing. I used the exact part that Philco called for. I can tell by the discoloration on the small board where resistor #47, the 37K is mounted, that the original ran hot. When I pull the 6A7 tube, the 32K resistor #15, remains cold, resistor 47, the 37K is barely warm after 5-10 minutes. The only thing these two resistors have in common is the 6A7, however even after leaving it heat up at a higher filament voltage, in the tube tester it still tested good. I do not have a new one to try. Does anyone have any ideas? Would anyone know the correct wattage for the 32K resistor #15? Thank you guys.

[morzh]

#47 will dissipate at least 1.2W so even if it is a 3W resistor it will get uncomfortably hot to touch. It is normal.
#15 resistor will dissipate at least 0.5W and so I would use at least 2W in its place, and it will also get hot.

[dacsman]

Hi tubeforme.. I recently restored a model 66 myself.  Based on my calculations for the voltage drop across both resistors which I believe is about 175V... both the 33K and 37K resistors may need to dissipate about one watt of power during normal operation.  Using a 1 watt resistor is right at it's maximum rated power and does not allow for a safety factor.  You minimally want a resistor to be twice the size it is expected to dissipate or in this case 2 watt.  So for an ample safety factor I ordered a 3 watt resistor for my radio.  

I recommend you check your screen voltage on the 6A7.  According to the schematic chart it should read about 85v if the radio is operating correctly.  In fact you should always check all voltages even if the radio seems to be working okay.  Mine were all very close to the chart.

[Tubesforme]

Thank you. I checked on the Philco Service hints page, when it was still up and resistor 47 was rated at 2W. I have a pair of 2W in parallel, which should give ample wattage. Resistor 15 maybe should be a 2W however it's part number wasn't listed. I think I will order a 2W flame proof, just to be sure. I did check the voltages on the 6A7, with my VTVM and they measured right at what they are indicated in Riders. In the past I have seen some of the older oscillator/detector tubes do some strange things and still test correctly. The only true tube test is to substitute it.