heathkit a9c power resistor getting hot
I have 2 heathkit a9c amps and the main 10 watt 360 ohm power resistors get quite hot. According to my cheap infrared thermometer around 350 degrees. I tried paralleling two 720 ohm resistors and they both still get about that hot. Is that normal? Both amps have been recapped and had out of spec resistors replaced. They are in my garage so I just cut holes in the shelf and put two computer fans under them for ventilation but that still seems like a lot of heat.
I have 2 heathkit a9c amps and the main 10 watt 360 ohm power resistors get quite hot. According to my cheap infrared thermometer around 350 degrees. I tried paralleling two 720 ohm resistors and they both still get about that hot. Is that normal? Both amps have been recapped and had out of spec resistors replaced. They are in my garage so I just cut holes in the shelf and put two computer fans under them for ventilation but that still seems like a lot of heat.
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Any specific reason for your concern? That 360 Ohm resistor (R48) is designed to dissipate about 7 watts to drop the supply voltage to 360 Vdc (after the resistor).
What wattage of the parallel 760 Ohms did you replace the original R48 with, at least 5 watts and preferably higher? If yes, you should be fine. I suggest that you measure the voltage before R48 (schematic states 410v) and after (360V), and if close to these specs, be done with it and enjoy the music.
What wattage of the parallel 760 Ohms did you replace the original R48 with, at least 5 watts and preferably higher? If yes, you should be fine. I suggest that you measure the voltage before R48 (schematic states 410v) and after (360V), and if close to these specs, be done with it and enjoy the music.
I just hadn't seen a sand cast resistor discolor from heat before.
The parallel resistors are both 10 watt
One amp had the proper voltages and the other was about 20 volts high. I switched rectifier tubes and the voltages followed.
The 5V4G tubes seem to be a little rare and expensive
The parallel resistors are both 10 watt
One amp had the proper voltages and the other was about 20 volts high. I switched rectifier tubes and the voltages followed.
The 5V4G tubes seem to be a little rare and expensive
Yes, a 10W resistor dissipating 7W will live a tough life. A 10W sand cast resistor running at 7W will have a temp rise of around 170C, which puts total temp around 200C (a little higher than what you're seeing).
If you parallel two 10W 720 ohm resistors, you should be in far, far better shape. That should give about an 80C temp rise and part temp will be a lot closer to 100C.
If you parallel two 10W 720 ohm resistors, you should be in far, far better shape. That should give about an 80C temp rise and part temp will be a lot closer to 100C.
Good, with two 10 watt resistors dissipating 3.5 watts each they will live happily ever after.
Regarding the one rectifier resulting in 20 Vdc higher than expected, did you compare the voltages listed 410 Vdc before and 360 Vdc after R48? A worn-out rectifier could produce lower than the bogey value, but 20V higher than bogey is unexpected. For both amps, please measure the transformer output Vac, and the Vdc before and after R48 and report here - perhaps someone could help diagnose. Also, measure both amps cathode voltages and compare to specs, i.e. one is not running much higher currents in the output tube pairs.
5V4 rectifiers are fortunately not really expensive and scarce. For example, check here for $12 each:
Tube Supplier - Tube List A-Z
Although you've recapped the amps, cap leakage can also draw down B+. You'd smell that soon enough though.
thanks, I have another 5V4G on the way to compare. Currently:
Amp A with rectifier A is:
426 VDC before R48 (360 ohm)
368 VDC after R48
22.8 VDC on pin 8
415 and 415 on transformer output
Amp B with rectifier A is:
434 VDC before R48 (parallel 750 ohm)
386 VDC after R48
22.9 VDC on pin 8
57 VAC before R48
8 VAC after R48
Amp A with rectifier B is:
405 VDC before R48 (360 ohm)
350 VDC after R48
21.8 VDC on pin 8
56 VAC before R48
8 VAC after R48
Amp B with rectifier B is:
417 VDC before R48 (parallel 750 ohm)
372 VDC after R48
21.8 VDC on pin 8
411 and 413 on transformer output
I uploaded the Heathkit A9C schematic for reference.
Hope others will weigh in, but here are my observations:
1. Your output tubes are biased acceptably as the spec wants to see 23V on pin 8. You are just a bit below that in spite of slightly higher b+. That makes me think perhaps your cathode resistors drifted above the specified 195 ohms. I suggest you measure the 7watt cathode resistors to ensure they are equal. The bias at ~22 is probably acceptable, running just a bit lower currents. Your call to get the bias closer to spec by replacing cathode resistors, bit I probably wouldn’t.
2. Your power transformers secondaries are respectively 10Vac, and ~7Vac above the design Vac of 405. This is likely because your power company is proving 125Vac rather that the 115Vac expected 60 years ago when these amps were young. Check your wall power and if necessary you could take some measures to reduce your wall output voltage (Bucking transformer, variac, etc), or live with slightly more dissipation in R48, which you fortuitously increased from 360 to 375 Ohms (750//750), and reducing your B+ slightly for amp B, which is good. Do the same for amp A for this reason as well as higher dissipation ratings. Keep an eye out that the transformers are not overheating in this case.
3. Regarding the difference between rectifiers, my guess is B is more used-up than A. Ironically, with B and the 375Ohm R48, you are very close to specs with B+. IIRC 5V4 has a 2 amp heater, so perhaps you could consider another 2 amp rectifier with higher voltage drop to deal with the higher B+
4. I’m puzzled by the higher B+ from Amp B with either rectifier, in spite of having lower transformer secondary voltage outs, and R48 higher that Amp A. Could be the unreplaced R48 on A has drifted up. Hope others have some input.
Hope this helps. Good luck.
Hope others will weigh in, but here are my observations:
1. Your output tubes are biased acceptably as the spec wants to see 23V on pin 8. You are just a bit below that in spite of slightly higher b+. That makes me think perhaps your cathode resistors drifted above the specified 195 ohms. I suggest you measure the 7watt cathode resistors to ensure they are equal. The bias at ~22 is probably acceptable, running just a bit lower currents. Your call to get the bias closer to spec by replacing cathode resistors, bit I probably wouldn’t.
2. Your power transformers secondaries are respectively 10Vac, and ~7Vac above the design Vac of 405. This is likely because your power company is proving 125Vac rather that the 115Vac expected 60 years ago when these amps were young. Check your wall power and if necessary you could take some measures to reduce your wall output voltage (Bucking transformer, variac, etc), or live with slightly more dissipation in R48, which you fortuitously increased from 360 to 375 Ohms (750//750), and reducing your B+ slightly for amp B, which is good. Do the same for amp A for this reason as well as higher dissipation ratings. Keep an eye out that the transformers are not overheating in this case.
3. Regarding the difference between rectifiers, my guess is B is more used-up than A. Ironically, with B and the 375Ohm R48, you are very close to specs with B+. IIRC 5V4 has a 2 amp heater, so perhaps you could consider another 2 amp rectifier with higher voltage drop to deal with the higher B+
4. I’m puzzled by the higher B+ from Amp B with either rectifier, in spite of having lower transformer secondary voltage outs, and R48 higher that Amp A. Could be the unreplaced R48 on A has drifted up. Hope others have some input.
Hope this helps. Good luck.
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Thank you for the detailed response. Those points make a lot of sense.
I'll check the one resistor this evening. I have been wanting to do the parallel resistors on that one too but I keep waiting to need something else to order with them. And although I need a couple inductors and resistors for a speaker build the places I've found that have both of those don't have resistors with these values too.