BRINGING A GIBSON DUO-MEDALIST BACK TO LIFE, PT. 2 BY JEFF BOBER
Last month we began renovating and modding a
late-’60s Gibson Duo-Medalist
combo belonging to Tommy
Kirst. (You’ll find the first
installment of this series, along
with the amp’s schematic and
a photo of its unusual control
panel, at premierguitar.com.)
Okay, Tommy: We left off
after working on your amp’s
Normal channel. Now, let’s continue with this Duo-Medalist
makeover by focusing on the
Reverb channel, as well as a mod
to improve the amp’s top end.
With the Reverb channel,
you have two options. If you
wish to make the same mod on
the Reverb channel that we did
on the Normal channel—which
improves the frequency response
and increase its gain—you’ll likely
lose the reverb capability, because
this mod essentially cancels the
reverb signal by tying the reverb
input and output signals together.
If that’s okay with you,
proceed as follows: Locate the
tag board terminal where the
wire from connector S2 Pin
13 attaches. Now locate Pin 5
of V2 (which is next to V1),
and run a short wire between
the two. Locate the .0068 µF
capacitor in the circuit and disconnect the lead connected to
the junction of the two 220 kΩ
resistors. You’ve now executed
this mod on both channels.
You asked about rigging a
footswitch for the tremolo function. If you can find a way to
connect a standard 1-button
footswitch between pins 1 and
4 of the footswitch jack (S5) on
the preamp chassis, that should
work. If the footswitch cable
you use is shielded, connect the
shield to Pin 1 and the center
conductor to Pin 4.
If giving up reverb isn’t an
option, a couple of alternative
mods could improve Reverb-channel performance while leaving the reverb function intact.
The first would be to perform
part of the modification we did
to the Normal channel. Instead
of completely bypassing the RC
network on the input of V2 Pin
5, remove the lead of the .0068
µF capacitor that’s connected
to the junction of the two 220
kΩ resistors in the RC network.
This will reduce the network
load on the signal, but because
the rest of the network is intact,
it should provide enough isolation between the reverb circuit’s
input and output signals to
allow normal operation.
In this state, you may want
to run your guitar signal to both
channels. Unlike a Fender amp,
where the Normal and Reverb
channels are out of phase, both
channels in the Gibson Duo-Medalist are in phase with each
other. This lets you simultaneously use them without the
phase cancellation associated
with the Fender circuit.
If this alternative mod leaves
you less than happy with the
Reverb channel, complete the
first part of the modification
that we already performed on
the Normal channel. (To review:
Locate the tag board terminal
where the wire from connector
S2 Pin 13 attaches. Now locate
Pin 5 of V2 and run a short
wire between the two.)
Let’s look at a different mod
that should keep the reverb
functioning. For this, you’ll
need to access the control chassis of this multi-chassis amp.
This is the chassis that houses
nothing but the controls.
Locate the wire connected
between the CW (clockwise)
terminal of the Reverb control
and the Volume-control wiper.
Remove this wire, which is too
short for our plans. Install a
new, longer wire connected on
one end to the CW terminal of
the Reverb control. Connect the
other end to the Reverb-channel
Volume control’s CW terminal.
Left: A fine example of a vintage Gibson Duo-Medalist combo.
Top Right: Peeking at the late-’60s Duo-Medalist’s wiring.
Bottom Right: The amp chassis is stamped May 27, 1969.
Photos by David Hilts
This should isolate reverb-circuit’s input and output signals enough for the reverb to
function. It should also produce
another cool byproduct: reverb
without dry signal. This enables
you to turn up the Reverb control without having the Reverb-channel Volume control turned
up. Total reverb wash!
Okay, with these improvements to both channels, let’s go
back into the preamp chassis for
additional improvements. In its
stock configuration, according
to the schematic, the Normal
channel should sound more full
than the Reverb channel. This
is due to the coupling capacitor
coming from the first gain stage.
To increase the Reverb channel’s
fullness, locate the .0068 µF
capacitor connected between V2
Pin 7 and S2 Pin 10. You may
either replace it with a .01 µF
capacitor or—to make it easier
to return to stock configuration—simply parallel another
.0068 µF capacitor across the
existing cap. This will yield a
total of .0136 µF and should
perform fine in this situation.
In amps of this era, you may
find the infamous “chocolate
drop” capacitors. These caps are
radial lead (that is, both leads
come out the bottom) and have
a dull, dark brown finish. If you
find any in your amp, consider
replacing them. Most of the ones
I’ve come across are electrically
leaky, which compromises the sig-
nal path. Replacing them usually
improves performance, reduces
noise, and puts an additional
spark of life back into an amp.
Don’t expect miracles, but you’ll
definitely hear an improvement.
Special thanks to amp fanatic
David Hilts for sharing photos
of his Gibson Duo-Medalist.
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 firstname.lastname@example.org.