I have a post war radio that I restored about 3 months ago that has recently developed a low hum. The restoration included the usual stuff. It was recapped (including the electrolytics), replaced out-of-tolerance resistors, and was re-aligned. The other day I noticed a low hum that wasn't there before. The hum isn't really intrusive at normal listening levels, but I can tell it is there and with the volume turned completely down, it is very apparent.

It is not affected by the volume control so I guess that eliminates the RF and IF sections.

Could this be caused by a rectifier tube starting to go bad? Or an audio output tube maybe? I'd answer my own question if I had a replacement tube on hand, but since I don't I'm asking here.

I tend to think that it is not anything inductive right now (like placement of a wire or part under the chassis) because nothing under the chassis has been moved since I got it working and the problem is new in the last few days.

Any ideas?

If it is the normal DC type of hum, if you used new electrolytics, then they are likely still good. If you used old electrolytics that were good at the time, one may be starting to fail. A short in the output tube cathode condenser (if any is present) will increase hum. A failing tone condenser will cause hum sometimes. If the set uses a solid state rectifier, the rectifier can start to leak reverse current, which will cause both hum and eventually destroyed electrolytics. Since you say that there is a tube rectifier, though, this problem is probably not possible.

Check your electrolytics anyway, even if they are new. Be sure that the needle on your resistance meter goes all the way up to zero. On any condenser above 1 MFD, it should. Be sure that the needle goes all the way back down. Be sure that you didn't install the electrolytics backwards. Sometimes you can do this and nothing bad will happen initially, though usually a lot happens (blown rectifier and resistors). Eventually, however, trouble will result even if it didn't initially. Slowly the electrolytic will start to short out. Also, if you installed electrolytics rated at too low of a voltage (less than 150 volts), they may work initially, but then break down and cause hum and eventually havvoc. Usually they overheat right away and blow up, but not always. If you're not new to repairing tube type radios, then you probably already know about all of this, but if you are, I just thought I'd mention it. It's a good idea to check everything out anyway, as even the experienced make simple mistakes.

If it isn't a DC hum, or perhaps even if it is, the 1st amplifier tube or the output tube may have developed a heater to cathode leak. The rectifier may have even developed such a leak. If the rectifier develops such a leak, it can cause both hum and electrolytic distruction if severe. It will ruin the electrolytics eventually even if they aren't ruined initially. This problem is the same as having a solid state rectifier that leaks reverse current.

Another thing that can cause DC hum as well as AC hum is how the low end of the volume control is connected to B-. This usually is a problem from the start, however, and does not develop with time. Certain radios can be picky when the low end of the volume control is connected too close to the line cord. The line cord is B-, but too much fluctuation exists here. Connecting the low end of the control to a part of B- somewhere else in the radio reduces this problem (using a resistor capacitor connection to B- helps sometimes, too). I would count this out for now, though. It's a freak issue, and like I said, it's usually a problem from the start.

Thomas

Thomas,
What is this, eh DC, ha ha hum you noted in your reply?
marv

:If it is the normal DC type of hum, if you used new electrolytics, then they are likely still good. If you used old electrolytics that were good at the time, one may be starting to fail. A short in the output tube cathode condenser (if any is present) will increase hum. A failing tone condenser will cause hum sometimes. If the set uses a solid state rectifier, the rectifier can start to leak reverse current, which will cause both hum and eventually destroyed electrolytics. Since you say that there is a tube rectifier, though, this problem is probably not possible.
:
:Check your electrolytics anyway, even if they are new. Be sure that the needle on your resistance meter goes all the way up to zero. On any condenser above 1 MFD, it should. Be sure that the needle goes all the way back down. Be sure that you didn't install the electrolytics backwards. Sometimes you can do this and nothing bad will happen initially, though usually a lot happens (blown rectifier and resistors). Eventually, however, trouble will result even if it didn't initially. Slowly the electrolytic will start to short out. Also, if you installed electrolytics rated at too low of a voltage (less than 150 volts), they may work initially, but then break down and cause hum and eventually havvoc. Usually they overheat right away and blow up, but not always. If you're not new to repairing tube type radios, then you probably already know about all of this, but if you are, I just thought I'd mention it. It's a good idea to check everything out anyway, as even the experienced make simple mistakes.
:
:If it isn't a DC hum, or perhaps even if it is, the 1st amplifier tube or the output tube may have developed a heater to cathode leak. The rectifier may have even developed such a leak. If the rectifier develops such a leak, it can cause both hum and electrolytic distruction if severe. It will ruin the electrolytics eventually even if they aren't ruined initially. This problem is the same as having a solid state rectifier that leaks reverse current.
:
:Another thing that can cause DC hum as well as AC hum is how the low end of the volume control is connected to B-. This usually is a problem from the start, however, and does not develop with time. Certain radios can be picky when the low end of the volume control is connected too close to the line cord. The line cord is B-, but too much fluctuation exists here. Connecting the low end of the control to a part of B- somewhere else in the radio reduces this problem (using a resistor capacitor connection to B- helps sometimes, too). I would count this out for now, though. It's a freak issue, and like I said, it's usually a problem from the start.
:
:Thomas

A DC hum buzzes like the buzzer on an old Telechron alarm clock. It sounds like when the electrolytics go bad. An AC hum, usually caused by stray signals getting into the audio grids, sounds different, and I can't explain it other than it sounding kind of of "nare"-ish. The hum when you touch the grid of an audio tube may be a loud blaaat, which is a DC hum, and is caused by the tube rectifying the AC in your finger (your body picks up stray AC signals in your home from the wiring), or may be a "nare" sound if the signal is not strong enough for the tube to rectify.

Thomas

Thomas,
A buzz is not a hum, could be any frequency depending on cap failure/insulation breakdown/applied voltage, and totally unrelated to the standard definition of hum. NARE, NARISH are unfamilar adjectives to me. Rectified AC is still AC hum @60 or 120 Hz. DC (current) is non-cyclic/non-redundant and still noted for its quietness in audio reproduction circuitry, but should not be confused w/AC hum, rectified or otherwise.
marv

:A DC hum buzzes like the buzzer on an old Telechron alarm clock. It sounds like when the electrolytics go bad. An AC hum, usually caused by stray signals getting into the audio grids, sounds different, and I can't explain it other than it sounding kind of of "nare"-ish. The hum when you touch the grid of an audio tube may be a loud blaaat, which is a DC hum, and is caused by the tube rectifying the AC in your finger (your body picks up stray AC signals in your home from the wiring), or may be a "nare" sound if the signal is not strong enough for the tube to rectify.
:
:Thomas

If I can beg/borrow/buy a scope, wouldn't it be possible to measure the frequency of the hum on the output to the speaker to determine if its 60 or 120?
Larry

:A DC hum buzzes like the buzzer on an old Telechron alarm clock. It sounds like when the electrolytics go bad. An AC hum, usually caused by stray signals getting into the audio grids, sounds different, and I can't explain it other than it sounding kind of of "nare"-ish. The hum when you touch the grid of an audio tube may be a loud blaaat, which is a DC hum, and is caused by the tube rectifying the AC in your finger (your body picks up stray AC signals in your home from the wiring), or may be a "nare" sound if the signal is not strong enough for the tube to rectify.
:
:Thomas

Larry,
Yes, if you use an o'scope to observe the offending waveform, simple math on a scientific or Windows calculator can pinpoint the frequency. You can do this at the speaker without program audio present. I suspect that anything exceeding 10% ripple would be objectionable, but others of the learned Cadre at Nostalgia Air may disagree. If a cyclic/redundant waveform is observed, evaluate in the time domain (o'scope) the time from 0 crossing to 0 crossing of the next cycle. Some newer o'scopes (if you're lucky) will give a direct readout of the frequency with a simple button push. With a scientific calculator enter the time as it is measured in milliseconds (number and -3 exponent) or microseconds (number and -6 exponent) Then press the 1/X function on the calculator to derive the offending frequency. 60 Hz (1/2 wave rectifier) will measure 16.66 milliseconds (ms). 120 Hz (full wave rectifier) will measure 8.33 milliseconds.
marv

:If I can beg/borrow/buy a scope, wouldn't it be possible to measure the frequency of the hum on the output to the speaker to determine if its 60 or 120?
:Larry
:
::A DC hum buzzes like the buzzer on an old Telechron alarm clock. It sounds like when the electrolytics go bad. An AC hum, usually caused by stray signals getting into the audio grids, sounds different, and I can't explain it other than it sounding kind of of "nare"-ish. The hum when you touch the grid of an audio tube may be a loud blaaat, which is a DC hum, and is caused by the tube rectifying the AC in your finger (your body picks up stray AC signals in your home from the wiring), or may be a "nare" sound if the signal is not strong enough for the tube to rectify.
::
::Thomas

You know Marv, wouldn't it be awesome if someone made a signal generator program? Then you could always be sure that the frequency was right on. Anyone could align a radio without having to resort to an old drifty generator or a very expensive new generator (of course the program could be expensive, too). The only problem would be if someone's computer didn't match the sample rate or whatever you call it. My computer currently has this problem, and so all music sounds like chipmunks. Haven't figured out how to fix it, yet. I've looked everywhere.

Thomas

Thomas,
They do!! Its called ATE. Automatic or Automated Test Equipment. Its all over the electronics industry. Its all controlled by special software, strictly for this purpose. A plethora of test equip is needed, but all under the control of a computer. You simply power up and hit the "GO" button, then come back later for a complete print-out of results. As for your computer, have you cleaned the filters lately. May be a couple chipmunks trapped in there somewhere. Assume your playbck media is a CD, and somewhere in the playback application software "PREFERENCES" is an "EDIT function. Make sure its playback mode IS NOT set to "ALVIN" or "CHIPMUNKS"
marv

:You know Marv, wouldn't it be awesome if someone made a signal generator program? Then you could always be sure that the frequency was right on. Anyone could align a radio without having to resort to an old drifty generator or a very expensive new generator (of course the program could be expensive, too). The only problem would be if someone's computer didn't match the sample rate or whatever you call it. My computer currently has this problem, and so all music sounds like chipmunks. Haven't figured out how to fix it, yet. I've looked everywhere.
:
:
:Thomas

larry,
Am not an expert - any chance it is something simple e.g have you moved the radio to a noise source. or fluorescent light, etc?

:I have a post war radio that I restored about 3 months ago that has recently developed a low hum. The restoration included the usual stuff. It was recapped (including the electrolytics), replaced out-of-tolerance resistors, and was re-aligned. The other day I noticed a low hum that wasn't there before. The hum isn't really intrusive at normal listening levels, but I can tell it is there and with the volume turned completely down, it is very apparent.
:
:It is not affected by the volume control so I guess that eliminates the RF and IF sections.
:
:Could this be caused by a rectifier tube starting to go bad? Or an audio output tube maybe? I'd answer my own question if I had a replacement tube on hand, but since I don't I'm asking here.
:
:I tend to think that it is not anything inductive right now (like placement of a wire or part under the chassis) because nothing under the chassis has been moved since I got it working and the problem is new in the last few days.
:
:Any ideas?
:
:

Fluorescents and other radio frequency interferance sources usually only cause trouble only with the volume control up.

T.

Too many onlys!