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Ask Amp Man: Bringing A Gibson Duo-Medalist Back to Life, Pt. 1

Raising the cool factor and tone of a lackluster Duo-Medalist

Hi Jeff:
I recently traded my Vox Valvetronix VT50 for an old Gibson Duo-Medalist. Although the online comments I initially found about the Duo-Medalist were lackluster, I’ve come across articles that tout the untapped potential of this old tube amp—they even cite [legendary Trainwreck amp builder] Ken Fischer to support this claim. Though I’m a novice, I have a friend who is capable of performing electronic surgery. I was wondering if you could shine additional light on this amp and weed out fact from fiction concerning its mod potential. For your reference, I’ve attached information I’ve found online about the Duo-Medalist, including its schematic. By the way, the amp’s footswitch is MIA, so I can’t engage the tremolo. I’m curious whether we can perform a miracle on that, as well.

Sincerely, Tommy Kirst

Hi Tommy:
I’m glad to see you’ve found a cool old tube amp! But simply having tubes doesn’t guarantee an amp will sound great. Some amps need a bit of help before they can ascend to the “pretty cool” category. Let’s see if we can raise the cool factor of your Gibson Duo-Medalist.

The 2-channel Duo-Medalist in all its ’70s glory. Photo by David Hilts

In your email, you supplied me with some posts and links that mention information supposedly supplied in The Trainwreck Pages by the late, great Ken Fischer. I knew Ken and was familiar with the original Trainwreck Pages published in the Angela Instruments catalog of many years ago (this fantastic parts catalog was obviously a true labor of love for Steve Melkisethian at Angela Instruments). There’s no mention of Gibson amps in that publication, but that’s not to say that Ken didn’t supply this information somewhere at some point in time.

The bottom line is that the modification mentions removing a few resistors and capacitors to improve the amp’s performance. I’ve taken a look at the schematic, and it’s my opinion that this modification should definitely improve the frequency response and increase the gain of your amp. Let me explain the modification and then give you a better way to execute it. We’ll then also look at a few other potential modifications that can make your Duo-Medalist even better.

This schematic for a vintage Gibson Duo-Medalist can also be downloaded at Image courtesy of Gibson Guitar Corp.

In the initial modification, we will be removing an RC (resistor/capacitor) network in the preamp stages of the Normal and Reverb channels. It appears that this was incorporated to set the frequency response of the amp. Removing this will definitely change its tonal characteristics, but since you aren’t happy with the performance of the amp anyway, we’ve got nowhere to go but up.

In looking at the schematic, we can see a network of three resistors (470k, 220k, and 220k) and three capacitors (330 pF, 470 pF, and .0068 μF) attached to pin 5 on both V1 (the Normal channel) and V2 (the Reverb channel). Since this is a multi-chassis amp, your friend will need to remove the Duo- Medalist’s preamp chassis—the chassis containing all the preamp tubes—to locate these parts.

Now, although I said we would be removing these components from the circuit, that doesn’t mean we need to completely remove them. I’m going to give you a way to execute this that’s both substantially easier and reversible, should you ever wish to return the amp to its stock circuitry.

Locate the tag board terminal where the wire from connector S2 Pin 1 attaches. This should be the start of the RC network. Now locate Pin 5 of V1 (the 6EU7 closest to the side of the chassis). This is where the RC circuit connects to the preamp tube. Run a short wire from the S2 Pin 1 connection to V1 Pin 5. Now locate the .0068 μF capacitor that is part of this network. One side of it should be connected to a grounded terminal. Leave that lead connected and disconnect the lead that’s attached to the junction of the two 220k resistors. Lifting this lead will eliminate any additional noise from being injected into the signal path.

That’s it for the Normal-channel mod. By today’s standards, this should really lead to an improvement in the amp’s sonic performance, but if you ever wish to return the amp to stock, simply remove the wire and reconnect the .0068 μF capacitor.

But we’re not done yet. Next month, we’ll continue with our Duo-Medalist makeover by focusing on the Reverb channel, as well as a mod to improve the amp’s top end. We’ll take a look at your tremolo-pedal question, too, so stay tuned.

Special thanks to amp fanatic David Hilts for sharing a photo of his Gibson Duo-Medalist. Look for additional cool photos from Hilts in the April 2011 issue.

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