First a note of thanks to Norman L and the rest of the folks that guided me to the bad 12SA7 tube filament which I recently replaced to finally get my 1st radio project to 'light up' - thanks! I replaced it w/ a 12SA7GT which was a few bucks cheaper - yeah I know, I'm cheap.
Well I recapped the '48 Airline AA5 and along w/ the new tube it now functions. The volume, tuning plates and such were cleaned up a little w/ some color TV tuner spray from rad shack. There's absolutely no static from operating the on/off/volume knob. My inexperienced ears don't detect any hum problem at all. I also turned each of the tuner adjusting screws just a tad back and forth before putting them in their exact original positions (all the time listening to a good strong source to ensure their correct return). The speaker is old, brittle, cracked and has a few holes where pests ate away at it. But I hooked it up to a receiver and it sounded OK.
But I have a couple 'follow-up' questions:
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1) The sound on most radio stations sounds a little, well, 'gurgley' for the lack of a better term. It's particularly noticable in talk vs music (easily found on AM radio!). The speakers sound like they have a frog in their throat or maybe one of those electronic voice boxes or something.
Q: Any thoughts on a common ailment that might cause this.
2) After a little while of operation the light bulb went very bright and the sound disappeared from the speaker. It sat there for a little while and refused to do anything but remain mute with an intense light. Uh oh. I left it off and came back a few hours later. Now it seems to be working fine again however at warmup the light slowly comes up to a very bright intensity then fades back to a warm glow at which point the radio cackles to life. Every once in a while there is also a brief 1 or 2 second spike of silence.
Q: Anything to be concerned with?
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I may just chalk it up to aging electronics and could probably live w/ it the way it is. I thought maybe the speaker was causing the sound quality problem but the same speaker sounded fine w/ a modern receiver. These are not end-all problems for me, just some find tuning issues. Thanks for any feedback.
-Todd
No problem replacing a 12SA7 with GT version. In some stages a shield may be needed but seldom on this tube.
The bright lamp indicated higher current than normal was being drawn. Can't remember if you replaced electrolytic filter caps. If not they should be replaced. They can cause this problem.
A bad 50L6 or positive grid #1 voltage can cause excessive current and distortion. Pin #5 should be negative by 5-7 volts when measured from pin #8.
Norm
: Just a thought but could it have been a mistake to have substituted the 12SA7GT for the 12SA7 (metal) tube
: No problem replacing a 12SA7 with GT version.
Thanks for the sanity check!
: The bright lamp indicated higher current than normal was being drawn. Can't remember if you replaced electrolytic filter caps.
Yes, All brand new caps. I've tried the radio off and on all day long today and the problem (bright light, mute sound) has NOT repeated at all. I don't think there is a problem here per se - probably just an old radio that hasn't been run in 40 years starting to get broken in again if that makes any sense...
: A bad 50L6 or positive grid #1 voltage can cause excessive current and distortion. Pin #5 should be negative by 5-7 volts when measured from pin #8.
Well I sort of fixed the problem by simply reversing the polarity of the recepticle plug just now. I originally had the hot wire going to the on/volume knob so that the chassis (opened on my bench) was only hot when it was on. Now I reversed it and the distortion is gone. I switched it a couple more times to be sure but the poor mans fix (inexperienced in my case) is to simply wire the radio up with the nuetral wire going to the on/volume switch. Not sure I like this 'fix' but does that clue give you any ideas?
Newbie question here but to check the voltage between 5 and 8 on 50L6 do I simply touch two VAC multimeter probes to the pins under the chassis? But in AC it wouldn't be Pos or Neg would it?
Thanks,
-Todd
Reversing the plug will make a difference in hum. Some people use a polarized plug so the radio is always plugged in right.
You can measure between pin #5 and #8 on 50L6 with a DC volt meter. As long as pin #5 is negative by 5 to 7 volts the tube is biased right.
Norm
: Norman,
: : No problem replacing a 12SA7 with GT version.
: Thanks for the sanity check!
:
: : The bright lamp indicated higher current than normal was being drawn. Can't remember if you replaced electrolytic filter caps.
: Yes, All brand new caps. I've tried the radio off and on all day long today and the problem (bright light, mute sound) has NOT repeated at all. I don't think there is a problem here per se - probably just an old radio that hasn't been run in 40 years starting to get broken in again if that makes any sense...
: : A bad 50L6 or positive grid #1 voltage can cause excessive current and distortion. Pin #5 should be negative by 5-7 volts when measured from pin #8.
: Well I sort of fixed the problem by simply reversing the polarity of the recepticle plug just now. I originally had the hot wire going to the on/volume knob so that the chassis (opened on my bench) was only hot when it was on. Now I reversed it and the distortion is gone. I switched it a couple more times to be sure but the poor mans fix (inexperienced in my case) is to simply wire the radio up with the nuetral wire going to the on/volume switch. Not sure I like this 'fix' but does that clue give you any ideas?
:
: Newbie question here but to check the voltage between 5 and 8 on 50L6 do I simply touch two VAC multimeter probes to the pins under the chassis? But in AC it wouldn't be Pos or Neg would it?
: Thanks,
: -Todd
It actually sounds just fine by switching the polarity!
: You can measure between pin #5 and #8 on 50L6 with a DC volt meter. As long as pin #5 is negative by 5 to 7 volts the tube is biased right.
OK with the polarity set to the 'good sound' I get -3.93VDC. WHen I swap it to the 'bad sound' I get -3.82VDC.
Like I said it really sounds pretty darn good keeping the polarity the 'good' way.
I would leave it if you are satisfied. Voltage could a little more negative but your meter could be loading it down? May be a little leakage within the output tube but better than a lot of them.
Norm
: : Reversing the plug will make a difference in hum. Some people use a polarized plug so the radio is always plugged in right.
: It actually sounds just fine by switching the polarity!
: : You can measure between pin #5 and #8 on 50L6 with a DC volt meter. As long as pin #5 is negative by 5 to 7 volts the tube is biased right.
: OK with the polarity set to the 'good sound' I get -3.93VDC. WHen I swap it to the 'bad sound' I get -3.82VDC.
: Like I said it really sounds pretty darn good keeping the polarity the 'good' way.