Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cygnus_X1 on August 15, 2010, 03:04:33 pm
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Price was right, and I wanted to get this amp working prior to modding it into something usable.
Tubes light up, B+ looks healthy, but zero sound.
So I looked up troubleshooting techniques on the output transformer and found I don't own a filament transformer and decided to shoot the easiest AC signal I could imagine into it.
Portable radio out the headphone jack to the primary, speaker hooked up normally.
Sound!
So I guess the OT isn't shorted out.
(Is that even a valid test???)
Snooped around for anything fishy.
6L6 cathodes are tied to ground through a resistor that tests 150meg.
:rolleyes:
Why?
Schem has a 10 ohm two watt resistor there.
Most amps I see are tied directly to ground...
Also...green wire and white wire are both soldered to the positive on the speaker jack but the NFB wire is disconnected and wrapped.
Think they got the two green wires mixed up?
Looks like the green is 8 ohm output, white is 16 ohm.
Schem is here:
http://www.freeinfosociety.com/electronics/schematics/audio/univox1236.pdf
Thanks for any responses.
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6L6 cathodes are tied to ground through a resistor that tests 150meg.
That will kill your sound. Put a correct value there.
You need to separate that green and white wire if they both come from the OT.
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Thank you sluckey.
That cathode resistor had simply gone awry.
Stuck a 10 ohm in there, looked at those green wires again.
The meter told me I had "Lyin' Eyes", the NFB was properly connected.
It works!