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41-255 power dissipation reality check
#2

I think that you are about right. The entire current of the B+ supply passes through the resistor. It is essentially the cathode resistor for the pair of '41 output tubes. At 180V (The output voltage of the power supply), the current draw across that resistor is about 44 mA. Throw in the current from the other tubes and you approach the max current allowed for the 84 rectifier.

If you believe the RCA Receiving Tube Manual (and there are disparities, such as the cathode resistor being 330 Ohm for 2 '41s in push- pull as opposed to the 192 Ohm total resistance of the Candohm), the voltage drop should be about 14V. Multiplied by 0.06A (max allowed current for '84, RCA says 0.050A, but hey, Sylvania developed the tube, not RCA so we go with your #s). The resistor dissipates 0.8W. However, considering the pic of your Candohm, 3W would be prudent, especially for the 146 Ohm section.

This is a rather interesting radio (I have one). It performs better than the specs. They could have gotten a lot more Audio power out of this model if they used an 80 / 5Y3, but since Philco was a major car radio manufacturer and the '84 was developed for car radios (6V heater, separate cathode sleeve), they likely saved money using this circuit (possibly shows up in a car radio too). Also of interest is how they get the inverted phase for the 2nd '41. It comes from the screen of the 1st '41 instead of from an Inverter Stage. Verify the accuracy of all resistors in that circuit, especially the 3900 Ohm resistor, R56 between the 2 screens and the 2 470 KOhm Grid Resistors, R53 and R54 for minimum distortion.

Hope this helps.

Best Regards,

John, MrFixr55

"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis

Best Regards, 

MrFixr55


Messages In This Thread
RE: 41-255 power dissipation reality check - by MrFixr55 - 08-20-2022, 12:12 AM



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