Univox 1236 Bass Head-DOA

[Cygnus X1]

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 image 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.

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.

Thanks for any responses.

[Cygnus X1]

OK, that 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!

[Alexo]

Is that thing really running 660 volts on a pair of 6L6's??

[Structo]

And a 10K OT?

[Cygnus X1]

I measured 335V but did not check that CT.

I'll have to check...weirdness.

[Cygnus X1]

Is that thing really running 660 volts on a pair of 6L6's??

Looks like it's a voltage doubler running into the primary center tap of the output transformer.
I measured 335V at B+2 to the 6L6's.

10K on that output transformer with 4 and 8 ohm tap.
Think it would be safe to use the 4 ohm tap as an 8, and build assuming a 5K primary?
(Bad phrasing...sorry).

Or would that cut the power rating in half as well?

Univox claimed 100 watts output for the amp but who knows what they based that on.
Looks like a weak 50 watt output transformer to me.

[Alexo]

Welp, if they were 6550's running at 600 volts with 300 volt screens, then you could expect 100 watts. Cost cutting, maybe? Can't imagine a pair of 6L6's lasting too long in a 100W power amp.

Seems like you've got a couple issues to sort out - that scheme does show a voltage doubler, why is it not working?

Personally, yeah, I would just rewire it for 330 volts and use the OT at 5K. But it's troubling if the PSU is only giving you half power...

..I might also clip those .01 caps between the pi output and ground (??), cut that 1K5 cathode resistor on the pi down to 470 or 820, maybe ditch that weird 250p cap on the tail of the ltp, but then I guess it would lose some of its unique character..

[Cygnus X1]

Thanks Alexo.
I gutted the amp, I don't think there is anything wrong with it.

Other than being a Univox.



It was working prior to my chainsaw surgery...

[Cygnus X1]

Dubble Post!

Arg.