Re: Please help ID old Supro instrument amp.

On Sat, 7 Nov 2009 23:06:03 -0800 (PST), RD Jones <annonn@xxxxxxxx>
wrote:

On Nov 7, 2:46 am, spud <g...@xxxxxxxx> wrote:
Hi, I'm working on an old Supro head and cab amp that has a hard to
fix hum. It would be helpful to find a schematic.
I put 2 small pictures up here:
Group: alt.binaries.atest
Subject Line: suprotest
Upload/Download took about 10 secs on DSL. (finally moved into the 21
century,..)
The model# is worn off but I think it says, "62-9018". It might be
9058. I've searched for hours with no result. These may have been sold
under several different names: Supro, Valco, Airline, Montgomery Ward,
Gretch and so on.
Normal channel: 2 inputs.
Reverb/Trem channel: 2 inputs.
4x total 12ax7: 1 for reverb recovery, one phase, one each for the
inputs.
One 6v6 for the reverb driver, 2x 6V6 output stage.
4 diode rectifier section. Heater string is CT grounded. The speaker
cab is enclosed and houses 2x 12" speakers.
It sounds wonderful except for a loud hum that doesn't change with
volume. No crackle or scratchy pots. Reverb and Trem work perfectly.
I've reworked all the PS caps and a couple of interstage caps.
Replaced the diode section. I replaced the mains cord with a 3 prong
and removed the hot-to-ground cap. The cab sounds fine with another
amp. Is there an actual method for finding a hum? You know, a system,
a routine? I normally just shotgun the cap section and blocking caps
and that's it. If anyone remembers these amps it would be great to
hear from you. It really sounds cool. Thanks as always, s.
p.s. I have already heard the one about how the amp is humming because
it doesn't know the words,..

Is it possible either the power or output transformer was replaced ?

not likely.

If so, improper orientation can cause AC to directly couple from
the power tranny to the output if they are close enough to each other.
You might test for this with a sheet of metal between them.

How hot is the bias ? The tubes aren't red are they ?

Not sure. It's cathode biased. Tubes aren't red.

That amp is old enough that it should know the words by now ;->

yup,..
rd

.

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