Ampeg Gemini II G-15

Ampeg Gemini II G-15
1966 Ampeg Gemini II G-15. Amp and photos courtesy of Greg Mayo.
Preamp tubes: three 12AX7, one 6CG7, one 7199.
Output tubes: two 7868.
Rectifier: solidstate
Controls: volume, treble and bass on each channel; echo (reverb) and speed and intensity (tremolo) on channel two.
Speaker: 15″ Jensen Concert.
Output: approximately 30 watts RMS.

Catch but a glimpse of an Ampeg combo like this 1966 Gemini II G-15 and you’re thinking “jazz club”: a blue-tinged smoke haze lacing the air of a low-ceiling/walk-down venue, gently-melting ice clinking in a glass with two fingers of Scotch, seductive sounds oozing from the stage nudged by a syncopated brushed-snare rhythm. You can almost hear that warm, bold, clean guitar tone, right? Think again.

Though the Gemini II was intended for jazz (like the majority of the Ampegs of its day), this one somehow slipped past the “Gatekeeper of Clean,” head honcho Everett Hull. There’s something grimy, rich, and raw going on when you crank it up, and that opens a world of sonic possibilities beyond the archetypal smooth. To be fair, it’ll do smooth if you keep it down to 9 or 10 o’clock on the dial, lulling you back into that mid-’60s vibe. But, fast-forward nearly 50 years and these things make great avenues to original tones in the arenas of alternative, indie, blues-rock, Americana, and roots-rock. Get it cooking, and it also lets the listener know you’re plugged into something other than the usual Vox, Marshall, or Fender-derived amp design. That, and it’s a vintage amp you’re likely to pick up for well south of a grand. Try to claim that of your blackface Super Reverb, JMI AC30, or JTM45 Bluesbreaker combo!

What’s the catch? Well, kind of a big one; the 7868 output tubes required to run this rendition are difficult to come by. Even the recent-issue replacements made by Electro-Harmonix are pricey. But forget all that for now. What makes these amps so much fun is the way they surprise your jam-night buddies with tone, tone, and more tone, and convert that jazz-looking box to a grinding, raw, edgy rock machine that lathers on the good stuff while doing a surprisingly good job preserving your guitar’s note clarity. How did they do it? Who knows. But it’s fun poking around for the answer.

Like that of so many other Ampegs, the circuit of this one looks little or nothing like those of the other popular amps of the day. Each channel has two stages of gain from its own 12AX7 preamp tube (so at least that’s a conventional replacement part!) with the Volume control sandwiched between. Bass and Treble controls follow the second of those gain stages in a configuration like nothing you’ll find in the classic Fender/Marshall/Vox topologies, but one that’s quite functional in the way that everything done by Ampeg seems to be. Also different from the common denominator is the lack of any cathode-bypass caps on the first gain stages of each channel, a component usually used to voice the tube, often fattening up the signal in many amps. Omitting this cap helps to keep things tight up front, an effort that in itself kind of hints at jazz intentions, but the added half of a 12AX7 between the Volume and Tone controls ramps it all up pretty good, and there’s a .1µF bypass cap on the cathode of the second gain stage of Channel 1, for a crispy boost of sorts. Channel 1 also gets the famously lush Ampeg reverb (labeled “Echo”), as well as an opto-cell tremolo that acts directly on the circuit between the second gain stage and the tone controls. In addition, each channel has an Ultra High switch on the treble pot that really lives up to its name; turn the knob fully clockwise until the switch clicks to engage a super-bright mode, which is definitely not jazz-certified.

Ampeg Gemini II G-15
The Gemini II’s 15″ Jensen.
The phase-inverter tube used in this amp, a 7199, is even harder to find than the output tubes, since none are in production anywhere today, and supplies of NOS tubes are drying up fast. But don’t panic – there’s an easy fix. Moving three wires on this tube’s socket lets it use a more-plentiful 6U8A tube in place of the 7199, for comparable – and possibly improved – performance. As for the phase-inverter circuit, Ampeg used a rendition of the split-phase (a.k.a. cathodyne) inverter with a driver stage in front of it, as employed in Fender’s 15- to 30-watt tweed amps in the late ’50s like the Super, Pro, and Deluxe, and the Princeton throughout the blackface and silverface years. This topology is prone to some clipping, and might be partly responsible for the Gemini II’s fat crunch when cranked up, so it’s a wonder Ampeg didn’t use a cleaner, bolder long-tailed pair inverter… go figure.

Ampeg purportedly moved to the “new” 7868 output tubes because they were the latest upgrade from Sylvania to the 7591s they had previously used, and were said to be similar performance-wise. What might have seemed a propensity to predict which tubes would be difficult for future owners to acquire – and then go with those – was more likely just an effort to keep abreast of the latest tube technology, along with a drive to be different from the pack. If you want to get your own vintage Gemini II pumping on something more standard, and more readily available, a good tech should be able to convert it to use 6L6s, though that’s likely to alter its original voice somewhat (in ways we can’t entirely predict).

A final major factor in the Gemini II’s mojo is its 15″ Jensen speaker, which is mounted in a bountiful combo cab with a back panel that closes off more of the interior than most. The entire configuration serves to really fatten up a Stratocaster, and it positively roars with a Les Paul. This example, long enjoyed by VG reader Greg Mayo, is in sweetly clean condition, with an interior that’s mostly original other than a few necessary bits of maintenance. Hip, retro, and appealingly alternative, it’s a vintage piece worth hanging onto.

Ampeg Gemini II G-15


This article originally appeared in VG July 2015 issue. All copyrights are by the author and Vintage Guitar magazine. Unauthorized replication or use is strictly prohibited.