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
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 firstname.lastname@example.org.