TECH TIPS > ASK AMP MAN
RECAPPING A VINTAGE GIBSON GA-79RTV STEREO COMBO BY JEFF BOBER
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.
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!
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
to view the Gibson GA-79RTV schematic at premierguitar.com/jun2012
This vintage GA-79RTV has a non-original
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!
JEFF BOBER, one of the godfathers
of the low-wattage amp revolution, co-founded and was the principal designer
for Budda Amplification. Jeff launched
EAST Amplification in 2010, and he can be
reached at email@example.com.
50 PREMIER GUITAR JUNE 2012