The Preamp circuit:

A fairly standard circuit, with a shelving treble boost located at 3k8 Hz for added "cut", "sparkle", and "jangle". I included a gain cut switch at the input to cut down the signal and input impedance for use with line-level devices. This switch also disables the treble boost, for a more "hi-fi" response appropriate for amplifying the processor I use (a Vox ToneLab SE) cleanly with little coloration.

With a 12AX7 here, the gain and tone are not too far removed from the old 18W Marshall combo. With a 12AY7, the tone is nice and even for amping my processor. In either case, it cuts nicely through the wash of cymbals when the "Cut" control is turned up....

The voltages shown here are for a 12AX7.

The Phase-Inverter circuit:

A variation on the standard HIWATT phase inverter circuit, with DC bias for the grids supplied by a voltage divider in the power supply circuit. This allows the use of any 9A-based preamp tube as phase inverter with no biasing issues due to the different tube types.

Since the amplifier uses no "negative feedback"* around the output section, I removed the presence circuit from the HIWATT circuit and replaced it with a Vox-style "Cut" circuit, which applies a variable amount of high-frequency shunt across the output of the phase inverter to reduce treble response.

* Negative Feedback - a small portion of the amplifier's output fed back into the circuit out-of-phase to correct distortion in the output section and output transformer

The Output circuit:

After various online chats with Ken Gilbert on ampage and having heard so many good things about Dr. Z's Route 66 amp, I decided to use an "Ultra-Linear" layout, wherein the screens are hooked to taps in the output transformers' primary winding. The 7591A outputs live happily even thought they're above the normal screen voltage limit, thanks to the UL circuit keeping the screens from being too far above the plate and burning up with excess current....

The biasing arrangement may look strange, but the preamp filaments make a fine cathode resistor (as long as they're bypassed!), and that way I get free DC filaments for my preamp! I put a string of Zener diodes to prevent the filaments from going much over their 12.6V rating. I bias the grids positive to achieve the correct bias, because the voltage developed across the three filaments is way more than is needed and would put the outputs nearly in Class-B. Once the bias is set to put the cathodes at 30-33VDC, it sounds great, and the side-effect of the Zeners limiting the cathode voltage rise is that the amp acts like a cathode-biased amp when clean, but tightens up like a Marshall Plexi when cranked!

Oh well, it sounds good to me, and that's what counts, right? }:^)>

The Power Supply circuit:

Not much to say about the power supply - it's a fairly standard affair with the only unusual twist being the 175VAC high-voltage winding, the voltage-doubler circuit layout that it requires.... I did use an old trick I found in an old Knight KM15 hi-fi kit amp, though. I made the artificial centerap for the 6.3V filament line from a pair of 22nF caps instead of the usual 100 ohm resistors. This breaks the DC path between the filament string and the rest of the circuit, and helps reduce hum.

So, how does it sound?:

Overall, with 12AX7/ECC83 tubes loaded in the phase inverter slot, a 12AY7 in the front end, and the input pad engaged, the amp responds really well to my stereo effects processor (a Vox ToneLab SE), doing a great job amplifying the tone generated therein and giving it a bit of light compression on the way out to the speakers (WeberVST Alnico Blue Pups and F150Ts RULE! :^D), which gives me a bit of extra sustain and helps the guitar sit just right in the mix with no compression necessary at the board. With a 12AX7/ECC83 tube loaded in the front end and the pad off, the amp has a character not too far removed from the Marshall 18W, but with a tad more beef and low-end control, and less squish, thanks to the Zener-limited cathode bias setup (which also protects the preamp filaments from overvoltage :^D). The nice thing about this biasing arrangement is that the amp compresses the sound a bit when clean, but beyond a threshold, the cathode voltage can't rise any more due to the Zener clamping it. This makes the amp tighten up considerably when driven hard, at which point it takes on a more aggressive, less squishy character similar to a fixed-bias amp.

P i c t u r e s

Stereo Amp Front View

Stereo Amp Oblique Left View

Stereo Amp Side View

Note that the picture is from 2001 and shows the amp's former configuration with Sovtek 5881 output tubes....

Stereo Amp Rear View

Stereo Amp Top View

Stereo Amp Innards - Front View

Stereo Amp Innards - Rear View

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