Re: Need help with vintage amp

In between...

"Rbn" <rick@xxxxxxxxxx> wrote in message
news:1154049293.464850.98860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Ok - I did get some time to fiddle with this amp this evening. Here is
what I've found. First of all, I had misread the schematic originally
- I thought V4 was a 12AU7 like V2 and V5, but it isn't - it is a
12AX7. Walter had pointed this out - thanks! Anyway, when I put a
12AX7 in V4 socket - the reading came much more into range of what
would be expected. Plate on pin 1 is reading 178 (should be 175),
cathode on pin 3 is 1.7 v (should be 1.56 I think), Plate on pin 6 is
reading 137 (should be 152) and cathode on 8 is reading 1.9 v (should
be 1.1).

Sorry that I didn't pick this up, but the commonly available schematic is
hard to read. I've got a magnifier out now ;~}. Yes, this would account
for the apparent problem and I think it is safe to say it is fixed. So,
what is plate voltage on V5A? That drives the inverter transformer. Just
curious, as I'm not sure it is related to the problem.


So I got to thinking - could the fact that I put a 12AU7 into V4 have
caused the overheat of the OT, so I went ahead and put the power tubes
(V6 and V7) back in there and with the 12AX7 in V4, it doesn't have the
loud hum and no evidence of the OT overheating.

So I plugged in a guitar and still find the original symptoms - not
very loud and a very tinny, shallow sound - like it is distorting even
when the volume is low.


I think with all the good readings on the preamp tubes, we need to be
looking at the power amp section. The good news is that there are less
parts there!

Now I have a Gibson GA 20RVT which I had bought new (believe it or not)
when I was a 15 year old in a garage band in the mid sixties. This GA
20RVT still sounds great (I did have to replace the caps but other than
that is all original). When I drive the speaker of my GA 20 RVT with
the AMP of the GA 35 RVT, I get a much better sound, although the sound
in the normal channel is much better than the sound in the reverb
channel. I would say the sound coming out of the normal channel is
almost as good as the GA 20 RVT. So this leads me to conclude that the
speaker on the GA 35 must be shot. However, there is still something
amiss on the reverb side of the amplifier, since it still sounds very
distorted, even through the GA 20 speaker.

This was a good thing to do. I agree that it suggests you need a new
speaker. If you want to do this on the cheap, you can probably do OK with a
Weber signature series speaker. Call Ted and talk to him. BTW, the new
Jensens aren't real Jensens; just something made in Italy by someone who
owns the name. Not suggesting you steer clear, but lower your expectations
if you were contempating this route.

Let's leave the reverb channel out of the discussion for now. Perhaps you
do have a problem, but I it shouldn't affect the normal channel. In fact, I
might lift a few select components from the board to remove the reverb
channel from the circuit temporarily. I'm not so good at this, but I think
you'd lift R25, R35, and R36 from the 220V B+ rail. Anyway, I think this is
optional for now.

I think it is better to concentrate on the output section.


I did double check my new capacitors and I'm sure they are all in there
correctly. The orginals were 20 mfd and I replaced with 22 mfd. The
originals were either 450 V or 300 V. I replaced the 450's with 450's
and the 300 with a 350.

So, no problems here.

Here are the voltage measurements I read:

Line (AC) voltage is 111v. Voltages coming off the secondary of the HT
are 313v AC and 308v AC at the two sends. (should be 320 I think).
Voltages on either side of the choke are 404 v DC on the transformer
side of the choke and 396 on the other side. (should be 410 at that
point according to the schematic). The voltage on the other side of
R39 from the choke is reading 253 (should be 252 so very close). The
voltages that bias the power tube cathodes seem to be all high (less
negative). At the diode side of R43 (the 330K resistor) I'm reading
173 v AC, then on the other side of the diode (D2) I'm reading -14v DC.
After it passes through the 7.5K resiseter R41 I read -12 V DC. At
this point I think it is supposed to be -20.5 V DC. I figured that
maybe the resistors R43 (330K) or R51 (7.5K) may be bad, but they read
good on an ohmmeter. There is a difference in the circuit from the
schematic - I find two diodes inserted on either side of the R41 7.5K
resistor, with other side to ground. I guess that shouldn't effect the
voltage levels, but it is a difference from the schematic.


These are all cheap parts. I would replace R43 330K and R41 7.5K. (Typo
above "R51"?). I would also replace the diode shown on the schematic with
an N4007. I'd clip the ones that aren't shown on the schematic. You can
always add them back later if you feel the need. If you've got ~310vac on
the HT secondary, what accounts for the drop to 173? You are taking the
same raw AC that feeds the power section. I'm fuzzy on the math here and
too lazy to get out a book, but my gut tells me 173 is way low and that is
affecting the whole bias supply. I think I'd change R40 47K just for good
measure as it is also in that network.

All readings are a bit low. AC line voltage is 5% below 117V, so everything
else is likely to be 5% below, also.

Next thing I did was measure the voltages on V6 and V7 with the tubes
inserted and powered up, since it no longer is melting the OT. When I
do this I find that the readings are even further away from spec than
what they read with no tubes inserted. I am reading roughly 340 v on
the plates and screens (goes up to maybe 350 or so when I turn the
volume up) and reading -8 on the grids (schematic says should be
-20.5). The plates and screens according to the schematic are supposed
to be about 410.


I think the bias supply problem is affecting plate voltage on the power
tubes. When you correct the bias supply, closer to -20, you will see the
plate and screen voltage rise. Your B+ after the choke is 396, about right.
Focus on bias supply.

At this point, I am thinking next step may be to get another speaker
since that clearly seems to be part of the problem. I am also thinking
perhaps I ought to replace the 330K R43 with a smaller value to try to
bring the cathodes more negative - closer to -20. I am thinking maybe
that is why there seems to be more distortion at low voumes, becuase
the cathodes are not negative enough. Does that make sense?

See above. I would deal with the bias supply before replacing the speaker.


Sorry for the long post, and for the red herring I led you all down
with the V4 readings caused by my misreading the schematic and thinking
it was a 12Au7 instead of 12ax7.

It doesn't matter...it was an honest error. Can't speak for anyone else,
but I never thought that was the "real" problem anyway.


Thanks in advance for any ideas,

Rick


Regards,
Phil


.

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