Hi all,
I've worked on a lot of radios over the years, mostly AA5 & 6, '40's table radios. I'm just a hobbyist, not an engineer, so I struggle with theory at times. Every once in a while I'll have a radio receiving and playing beautiful except for a very low hum. You don't have to add much volume to drown it out, but at very low volume it's there and makes use of the radio not what it should be. So I go about looking for a cause and I get nowhere. This is after re-capping, checking resistors, re-routing grid wires, checking tubes, etc. My question is, am I expecting too much? Should I expect a humless sound at whisper volume or am I driving myself nuts trying to make an 80 year old radio sound like something it's not. Maybe I need to relax and just turn up the volume?
If the set is a AA 5-6 most will have a little hum the filter circuit well is lacking. 35W4, 35Z5 are half wave so lots of ripple. David
It's sort of like those who just get into vinyl records, they cannot accept the background noise. With a good system though, it will make the goose bumps rise. I read in an old radio publication that all radios hum to a certain extent, but if too much when installed in the cab it could indicate a problem somewhere. Most good tube radios I have listened to have very little hum, it's there, not that bad. Might try lead dress and movement to minimize. Some tubes can hum, and they test good. It could be in your house wiring or other interference, many things can cause it. The AA5's I have had listened to had virtually no hum. Alignment can dial in hum, been there, seen it first hand. If everything you have is unacceptable, something is wrong somewhere, not normal.
Thanks for the reply, just so happens that the radio bugging me right now does use a 35Z5.
Alan
Do you have a good tester? You might have some heater/cathode leakage and that will cause hum for sure. Try changing tubes with known new or for sure good ones, start with the audio output as that one one has the most stress on it. They all seem to have a slight hum but it should be minimal.
Gregb
Yeah, I have a Hickok, tested em and swapped them. I did just notice that I have another radio sitting here that also uses a 35Z5 and has no hum at all (a Philco of course). I think I'll go swap those rectifiers.
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.
Hello lasurveyor,
One the big issue I find nowadays with my Am radios is noise coming from other sources like phone chargers and cable boxes and Led light bulbs .
Sincerely Rich
P.S. The radios with power transformers and electromagnetic speakers seem to do a great job filtering out noise and some sets are over all designed better .
Two more things to think about. One, the connection of the volume control center terminal to the first AF amp can pick up a lot of hum and is sometimes a shielded wire or the coupling cap has a shield around it. Two, I have seen filaments in pre-amp tubes in guitar amps running on full wave, filtered DC to keep cathode to filament coupling at minimum.
Thanks all for the thoughts and tips. With Covid these days new old projects are harder to come by with flea markets closed and all, so I'm going back to past irritants.
I swapped out the 35Z5 with one from a quiet Philco Wedge, and noticed a subtle difference in that quiet hum, and it does have a PM speaker which isn't helping. I'll give a few things a try, like increasing the electrolytics, decreasing the grid resistor and sheilding the grid to volume wire. This'll keep me busy for a while, I'll let you know how it goes.
Thanks!!