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.
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 Reverbchannel
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 signal
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.
One more tweak you could
perform involves the top end. If
you feel either or both channels
could use more sparkle, locate
the 100 kΩ resistor and 300 pF
capacitor connected to Pin 7 of
the first gain stage. That would
be V1 Pin 7 for the Normal
channel and V2 Pin 7 for the
Reverb channel. The 300 pF
capacitor is shunting some of the
signal’s high frequencies across
the plate load resistor to the
power supply. To brighten either
channel, simply disconnect the
side of the 300 pF capacitor that
is connected to Pin 7.
There you have it. A much
more dynamic Duo-Medalist!
Special thanks to amp fanatic
David Hilts for sharing photos
of his Gibson Duo-Medalist.
Jeff Bober 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
pgampman@gmail.com.