02-20-2021, 10:25 PM
This is what I've noticed. I agree w/Greg filament/cathode/grid leakage can cause a hum issue that normal troubleshooting (short of swapping tubes) will detect. Grounding the control grid or adding more mf to the filter section in the p/s will have little to no effect. Generally the bad ones are the higher dissipation tubes (higher heater voltage) or the 1st audio/det.
As time marched on and 1950's came a long ac/dc set's designs got cheaper. The speakers got smaller as did the output transformers. And there was a reason for this. The obvious is that it cost less, true but by having a sm opt and spkr it cuts the low frequency response of the set. So hum was less of an issue.
The sets that have the least hum are going to be the ones that uses an electrodynamic spkr. Inductors make great filters @60cy conversely resistors suck unless you use 3 or 4 in series w/large caps. The issue there is that you have so much series resistance there is a significant voltage drop. Chokes for the most part have a fairly low resistance vs inductance so there's little voltage drop.
I'll mention another issue that may not have noticed. Some sets that use the 12AT/V6 have a leakage problem from the diode the grid. When you turn the volume all the way down you can still hear it playing. It's not loud but if it's on your night stand it's kinda annoying.
Generally what I do is replace the filter caps w/47mf for both. Some have a very high value grid resistor on the 1st audio stage like 6-10M. You can lower that value a little some as low 2 or 3M This will lower the gain a bit and may lower the hum too. Make sure the volume control is in good order. Short of adding choke to the p/s that's about it.
As time marched on and 1950's came a long ac/dc set's designs got cheaper. The speakers got smaller as did the output transformers. And there was a reason for this. The obvious is that it cost less, true but by having a sm opt and spkr it cuts the low frequency response of the set. So hum was less of an issue.
The sets that have the least hum are going to be the ones that uses an electrodynamic spkr. Inductors make great filters @60cy conversely resistors suck unless you use 3 or 4 in series w/large caps. The issue there is that you have so much series resistance there is a significant voltage drop. Chokes for the most part have a fairly low resistance vs inductance so there's little voltage drop.
I'll mention another issue that may not have noticed. Some sets that use the 12AT/V6 have a leakage problem from the diode the grid. When you turn the volume all the way down you can still hear it playing. It's not loud but if it's on your night stand it's kinda annoying.
Generally what I do is replace the filter caps w/47mf for both. Some have a very high value grid resistor on the 1st audio stage like 6-10M. You can lower that value a little some as low 2 or 3M This will lower the gain a bit and may lower the hum too. Make sure the volume control is in good order. Short of adding choke to the p/s that's about it.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got it!
Terry
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