problem with GA35-RVT


<rick@xxxxxxxxxx> wrote in message
news:1153789640.418891.16140@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Phil - thanks for the reply and the offer to help. First I need to
correct myself - the Amp model is the GA-35RVT (not 55). Sorry - I was
looking at a 55 too and had it in my brain. I did as you suggested -
pulled the finals and took voltage measurements. Both V6 and V7 (the
two 7591's) are at about 400 v (+/- 1 or 2) on the plates and screens.
The grid on one is -14.9v and the other is -14.4v. The cathodes are
0. According to the schematic I downloaded the Plates should be 410,
screens 409 and grids -20.5 and cathodes 0. So it sounds to me like
they are relatively in-spec other than maybe the grids are a little
off. Do you agree?

So next I decided to do the same voltage measurements on the three
12AU7s and the two 6EU7s. I compared the readings to the schematic and
all seem pretty close with the exception of V4 - a 12AU7 which is
reading low on the plates and high on the cathodes. The one plate is
supposed to be reading 175 and it is 93, the other is supposed to be
152 and reading 108. The one cathode is supposed to be 1.56 and reads
4, the other is supposed to be 1.1 and reads 5.4. I don't know if this
variance is enough to cause the kinds of problems I described or not?

Any suggestions on next steps?

Thanks again,


Hi there, Rick. First some ordinary matters...

Please bottom post. It makes it much easier to read the thread. That is
the reason for this standard in usenet ettiquete. It took me a while to get
used to it.

Next., I'm an amateur, so please understand that I don't have the technical
chops some of the other guys here have, the ones who do this for a living.
Don't take everything I say at face value. I own two old Gibsons, and did a
great deal of trouble shooting on the GA20-RVT, which is similar in many
respects. I have built 5 amps, 1 kit and four scratch, and they all work,
some better than others ;~}. I won't intentionally mislead you, but there
is stuff I don't know. Also, I assume you are familiar with amp safety.
You managed the readings and lived to tell about it. If you have any
doubts, a decent read on this topic can be found at www.aga.rru.com .

V4 is a problem on both sides. In particular, I'm not liking the 93v
reading. I would try to swap a known good tube. It may be even better to
swap with one that is already in the amp (V2) to see if the plate voltage
problem follows the tube. If it does, you know it needs to be replaced.
(While this might be a problem, I am not convinced it is *the problem* with
your finals and OT.) Anyhow, if a tube swap fixes this particular problem,
find a good replacement, stop here, take new voltage measurements and post
results. If not go on. If you don't have a spare and can't find one, use
the one in V2 and leave that hole open. It will disable the reverb, but I
don't think that has anything to do with your problem and shouldn't impair
diagnosis.

If the problem stays with the socket, you will need to divide and conquer.
This is a technique that involves lifting some parts out of the circuit so
that you can isolate the problem. If I understand the schematic properly,
you can lift one side of C9 (hard to read the #) -- located after R14. See
what happens to V4 plate voltage and T3 secondary voltage when you do this
and let us know.

Have you taken reading on the primary and secondary of the interstage
(driver) transformer (T3)? Check all five legs and post what you get.
Well, I see you already posted the grid reading for the finals, so that
takes care of two legs on the secondary. It is 5V too low (well actually
high because the reading is neg). I'm not liking the -14 on T3.

Also, disconnect the output transformer and check for continuity on both
primary and secondary windings. Might as well get this out of the way. If
you need new one, you'll need to get to work on that. To disconnect, you
don't need to do much. With the finals pulled, the primary is already
disconnected and you can measure ohms from pin3-pin3 (amp off!!!) and also
check pin 3 to ground on both sockets to check the outer legs to the CT; and
on the secondary, disconnect the speaker and read the ohms between the two
wires.

Take a good look around and see if any grounds are looking suspect. You
might reflow solder if questionable.

Stop here and post results. This is a pile of stuff to do. Don't get
overextended, and post questions (below!) if you aren't clear or don't feel
comfortable. We'd like both you and the amp to survive diagnosis.

BTW, there are a few *Gibson guys* who hang out here. Maybe they will see
the change in title and pick up the thread.

Phil


.

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