This vintage GA-79RTV has a non-original
I’m doing a cap job on a Gibson
GA-79RTV. I can’t see the symbols next
to the respective connections on the
bottom of the cap can to verify which
capacitance values and voltages I need to
use as a replacement. The values listed
on the can are 80/450, 40/450, 30/450,
and 40/150. The amp schematic shows
two 20 μF, so I believe all I need are two
20 μF/200-250V electrolytics. Is that
correct? I’ve heard you should replace
old caps with a higher-voltage cap if
you can. Any recommended brands?
I recently replaced the caps and
installed a 3-prong AC cable in a ’63
Gibson Skylark combo. Not a huge job,
but it came out well and the amp sounds
much better. However, this GA-79
is another animal! I’m planning on
replacing all of the electrolytics, including
the 30 μF/450V yellow Sprague.
Would any electrolytic of that value be
a proper replacement, or should I opt
for a more expensive metal-foil type?
Finally, the fiber washers between
the tube socket screws and washers
have disintegrated after 40-some years.
The sockets are getting really loose
and I’m afraid the connections will be
jeopardized. Could I use small rubber
washers as a replacement? If so, any
suggestions for finding such items?
Thanks in advance.
Thanks for your question. For those who
don’t know, the Gibson GA-79RVT is pretty
cool—a true stereo amp with two discrete
output stages, each feeding a 10" speaker.
Each output stage uses one half of a shared
12AU7 as the phase inverter, driving a pair of
6BQ5/EL84 output tubes. This would generate
approximately 15 watts per side, making
this a 30-watt combo. The GA-79RVT also
included reverb and tremolo.
A mono/stereo switch allows you to use
the amp in several configurations. Plugged
into channel 1 with the switch in the mono
position, the signal goes to both output
stages and sound emanates from both speakers.
However, because the reverb and tremolo
are exclusive to channel 1, these effects
are only present in the channel 1 speaker.
With the switch in the stereo position,
the amp accepts dual signals from stereo guitars,
such as Gibson’s ES-345 and ES-355. In
this mode, the neck and bridge pickups feed
separate channels. Because only channel 1
has effects, only one pickup can have effects.
Alternatively, you can plug a separate
instrument into each channel, which might
be cool for a duo in a small coffeehouse jazz
gig. Perhaps the coolest way to use the amp
today would be to feed each channel with the
output of a stereo delay. One amp, true stereo.
Anyway, back to your question about
capacitors. But first, the disclaimer: Tube
amps can contain lethal voltages—even when
shut off. If you don’t know what you’re doing,
please engage the services of someone who does!
Click for larger
Looking at the schematic, I see the
power supply section (which is a separate
chassis mounted to the bottom of the cabinet)
is shown as having two 20 μF capacitors.
In the photo you’ve provided, the can
cap shown is obviously a replacement for
the original cap(s) and probably installed
many years ago by someone trying to get
the amp up and running with whatever
parts were immediately available. Since
you stated that the values of the can were
80-40-30/450 and 40/150, I’m assuming
that to remain close to the original values,
the sections used here would have been the
40 μF and 30 μF 450V.
You also mentioned you couldn’t see the
symbols next to each terminal that correspond
to the capacitance values indicated on
the side of the can. Sometimes these symbols
are indicated next to each terminal, but
other times they are actually punched as part
of the terminal cutout itself. Look closely at
the area where the terminal exits the bottom
of the can, and it’s likely you’ll see part of the
cutout itself in the shape of a triangle, half
moon, or square, as indicated on the side
of the can cap. You should then be able to
tell which sections are connected. Not that
it really matters, as your intention was to
replace all the electrolytic caps anyway.
Go ahead and replace both 20 μF caps in
this section with 20 μF/500V and you should
be fine. Since this part of the power supply is
a “pi” type, increasing the value of the filter
in the first stage for potentially quieter operation
would probably not make a significant
difference in this case. The output transformers
are fed by the second stage of this power
supply, which is after the choke and not
before, as in most guitar amp designs. Just
remember to ground the caps to the same
place that the original cap was grounded, as
this can sometimes make a difference.
Regarding the filter caps in the audio
section of the amp: The schematic again
shows the use of two 20 μF capacitors. It’s
not quite clear from your photo if the large
yellow Sprague in this section is a dual cap,
but feel free to use two discrete 20 μF/500V
caps here as well. But again, be sure to attach
their negative connections to the same place
where the existing cap is grounded.
As far as brands, most makes available from
the distributors that supply the repair industry
would work fine. And the more expensive
metalized polypropylene-style caps may have a
place in audio, but personally I don’t feel that
it’s inside a guitar amp. Plus, they are physically
larger, so you may run into a space issue.
With regards to the re-mounting of the
“shock mounted” preamp tube sockets,
I’d recommend checking a local hardware
store that has a good selection of individual
screws, nuts, and related hardware, as they
may have a selection of small rubber grommets.
If you cannot source any locally,
check the Mouser Electronics website, as
they should have what you’re looking for
and I believe they sell to retail customers as
well as businesses. I hope this helps improve
the sound of your amp from both sides!
is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at