Re: Need help with vintage amp


"Rbn" <rick@xxxxxxxxxx> wrote in message
news:1154200649.388789.169860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Phil S. wrote:

This is excellent news! You don't say what the plate voltage is on the
finals, now that bias voltage is corrected.

Hum on these Gibsons can be difficult to isolate. On my Minuteman, I
found
that the whole floating ground scheme wasn't working properly, and I
worked
through it by trial and error. Careful, though, I made a mistake and
blew
the optoisolator for the trem. (Z1). I got very lucky and found a
replacement, but they are not made any more, so if you f---up, you will
need
to build one with heatshrink, a bulb, and a LDR. I would start by
pulling
preamp tubes one at a time on the reverb channel to see which one makes
the
hum disappear. It's only 2 tubes, V2 and V3. Well, there is also the
0A2.
I wonder (out of my depth here) if the 0A2 can be replaced by a ss diode?
If not hard wired, you might swap the send and return cables.

You also have the regular preamp section of the reverb/trem ckt, so also
try
pulling V1 and V4.

Remember, amp off when pulling. Only pull one tube. Power back on.
Listen. Write down observation.

If the output tranny is grounded on the eyelet board, I would move that
ground to the chassis. I'm guessing you have a terminal strip running
across (the short dimension) the chassis where the PT B+ ckt is
terminated
and the rectifier diodes are located. There will probably be an unused
lug
that is integral to a rivet or screw mount to the chassis. Use this lug.
Just guessing here, but based on experience with my Minuteman. Memory is
faded, so don't trust me on this one; check things carefully. Also, one
big
difference is that the Minuteman is cathode biased and your amp is not.

Phil

Phil,

I think I solved the hum issue with reverb on. It came to my while I
was mowing my grass just now. I have the amp chassis on my work bench
and when I tested the reverb, the tank was immediately below the tubes
(within a few inches). I just now tested again and when I lift the
chassis up and separate the tank from the chassis by 6 or 8 inches,
which is the way it would be when in the cabinet, the hum subsides
dramatically. So I am thinking at this point, I am going to just get
the new speaker, clean it up, put it all back together and I should be
good to go.

By the way - that strange little OA2 tube V8 visibly strobes at the
same frequency as the tremolo.

You have been a great help through this exercise, and I learned a lot
from your advice.

Thanks again,

Rick


Rick,
You're welcome for the advice I don't know much, but was happy to share
what I know. Glad it's fixed!

The 0A2 is a voltage regulator. It probably turns on when Z1 is off? I'm
not exactly sure what this thing does. I don't have any of these in my
amps.

Regards,
Phil


.

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