I had been wanting to try an octal preamp for a while, so I figured 2011 would be a good time to build one. This amp has gone through about a dozen revisions with tons of experimentation along the way. After 2 years without major modification, I ditched the cascode and moonlight tone stack on Channel 2 in favor of a two-stage design.
After a ton of trial and error, attempts at all sorts of interesting preamps, the current “final” version squeezes roughly 50 watts into a chassis smaller than your typical 18-watter. It was quite a challenge, and it’s not entirely immune to microphonics or oscillation, but it’s a beast quite unlike anything else I’ve heard.
The Preamp
Channel 1 is a single high-mu gain stage with a tweed tone control. It really shows off the big, thick sound of the 6SL7GT.
Channel 2 is a dual medium-mu stage design using a 6SN7GT. The first stage shares a cathode with Channel 1, while the second is biased for a touch of asymmetry. It runs into a version of the Big Muff tone stack modified for a high input impedance. The end result is slightly more gain than Channel 1 but with a slight scoop in the frequency response that brightens up the sound and responds better to overdrive.
Intermediate Stage
The two channels are then mixed using 270K resistors and go into a common gain stage (6SL7GT). Like the intermediate stage on most high-gain amps, it is biased cold to avoid driving into grid conduction and blocking distortion.
The signal then goes into a long-tail phase inverter (6SL7GT) with a 47K tail for added stability (like many Vox amps). There is no negative feedback loop. A pair of fixed-bias output tubes finish off, pushing an Edcor CXPP30-MS-3.8K output transformer.
Although I haven’t tried it yet, it might do well as a bass amp with some 6550s and a solid-state rectifier. The output transformer is built for continuous 20hz operation and the circuit isn’t particularly high-gain, but the frequency response might be set a bit too high.
The Look
I ended up doing the faceplate with aluminized duct tape, 1/8″ black pinstripe from the auto parts store, ultra clear shipping tape and an ultra fine point black sharpie. It straddles the line perfectly between homebrew and professional-looking custom amp.
The Result
Did I mention it is HEAVY? Those Edcor transformers pack a ridiculous amount of weight into that tiny little head cab.
So, how does it sound? HUGE. Between the octal preamp, EL34s with no NFB and 6 pounds of output transformer, the punch from this thing hits you right in the chest. I’m mating this amp up with a 2×12 closed back cabinet with a Weber 1230 Hemp and a Weber Silver Bell.
Tube Lineup
- V1: GE 6SL7WGT
- V2: GE 6SN7GT
- V3: GE 6SL7WGT
- V4: Groove Tubes EL34M
- V5: Groove Tubes EL34M
- V6: JJ GZ34
