Photo by Bruce S. Gates

Ampeg V4

Details and Specs

Manufacture date: Circa 1975
Output: 100 watts
Power section: Four 7027s
Bias: Fixed
Preamp: Two 12AX7s, one 6K11, one 12DW7, and one 6CG7 reverb driver
Phase inverter: One 12AU7
Rectifier: Solid-state
Controls: Volume knob and 3-position input-sensitivity switches for each of the two 1/4" inputs, treble, midrange, bass, reverb, ultra-hi switch, midrange-selection switch, cab-impedance slider (2, 4, or 8 ohms), reverb-spring retainer for minimizing rattle during transport, hum-balance trim pot for curtailing tubes’ AC-filament noise
Played through: ’70s vertical Kustom 2x15 cab with Eminence Delta speakers

Despite its striking resemblance to vintage Ampeg SVT and current-production V-4B reissue bass heads, the original V4 was actually intended for guitar. When I first serviced this venerable amp, I realized it’s anything but typical. It offers a lot of unusual ways to tweak operation, and as I reviewed the schematic during subsequent repairs I became more and more intrigued.

How’s the V4 different? For starters, it has an extremely unique tube complement in both the preamp and the power stage. Two 12AX7s drive the front end, which has a passive James tone stack—an interesting circuit with a pair of filters that have an almost notching effect at lower settings, but that instill a bit of character when set flat, and offer a surprising boost when cranked. It’s a subtractive filter, meaning that it takes away some guitar signal when used, but its tweakability is worth the tradeoff. There are also two switches that set either input’s sensitivity at 0 dB or pads them at -9 or -6 dBs. A 6K11 tube—a nifty bottle containing two 12AX7 sections and one lower-gain triode section (that’s right, three triodes in one valve!)—boosts and changes the input-signal impedance (via the cathode follower) in order to better drive the inductor-based midrange-selection switch, which sets the midrange knob’s midpoint at 300, 1,000, or 3,000 Hz. Each of the switch’s settings effectively reinvents the way the bass and treble controls react.

But wait, there’s more! A 12DW7 valve (which is similar to a 12AU7) drives the preamp-out and power-amp-in connections—two flexible options that, even today, are pretty specialized and forward thinking. Meanwhile, the 12DW7’s second triode stage is used as a paraphase inverter, and a 6CG7 drives the reverb tank. The V4 also uses a solid-state rectifier and offers separate taps for screen voltages on the power transformer.

The unusual circuit would mean nothing if the V4’s full-bodied tone didn’t utterly dominate

As with the Beamish, the unusual circuit would mean nothing if the V4’s full-bodied tone didn’t utterly dominate. If you want to brutalize your audience with thunderous lows, this amp has you covered—especially through something like an old Kustom 2x15 speaker cabinet. The 7027-driven power section stays pretty clean throughout the travel of the volume controls, although higher settings do elicit a pleasing distortion with glassy top end but no hint of harshness. Even aggressive tones remain smooth and creamy. The input-sensitivity options are fantastic for making sure you get the amp’s best tones from guitars with varying output levels—but they’re also great for getting the drive characteristics you prefer at different amp volumes. The tone controls offer incredible flexibility, and at higher settings they can have a big impact on the amount of drive available.

Though V4s have gained some momentum in the used market recently, you can still find them for around $450 to $600—which is truly a steal!

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