We can drive the tubes harder. We can do this without compromising the tone of the low gain settings.

Points to note about the SIAB:

1: there is only one tube, you can only get so much tube distortion out of the circuit. You can't exceed 40V output on the tubes, but heavily distorting 12AX7's often exceed this (not a problem on a guitar amp with about 200V on the preamp tubes).

2: the output of the first tube stage is reduced by the line in / line out configuration, making it more difficult to heavily overdrive the second tube stage.

New Design and schematics!

Sound Samples at last! About 110k each. Direct to soundcard, no speaker sim.

C:\WINDOWS\Desktop\siabatyd.mp3

C:\WINDOWS\Desktop\siabash.mp3

Proposed enhancements:

1. Add a diode clipper inbetween the 2 tube stages. Maybe make it switchable. This makes it sound more like a "Stack".

2. You can increase R22 to 1M to drive the first tube stage a little harder.

3: If you never use the FX in/out, reduce the value of the FX Out resistor (R9) from 680k to 100k (you can change R11 to 100k too). Otherwise, double the value of the 250k "Drive" control. This lets you drive the second tube stage harder. You can increase the R4 plate resistor to 470K for more gain on the first tube (metal film resistors are less noisy here).

4. I increased B+ from 40V to 57V by adding to extra voltage multiplication stages, and increased the 50V filter cap to 100V. This my be overstressing the design, but I like the extra headroom.