Now that we have a rough schematic, it is possible to get an overview of the signal path through the amp:
A few things I think are of interest:
- The transistor stage, of course. Why use a transistor when the rest of the amp uses tubes? Why not all tubes, or all solid state? There’s space on the chassis for tubes where the transistor circuit is. Nine tubes use a lot of heater current, but would one more (which could replace the transistors) be enough more current to influence the design? The answer to that is yes, if there was a large existing stock of power transformers that would only power the nine tubes and not one more. Or perhaps the designer was taking advantage of a stock of transistors on hand? Or maybe they got a good deal on the power transformer or the transistors. Or maybe it was just cool to throw in a solid state front end because they could. This was built near the dawn of the sold state era, after all, and almost nobody, if anybody, was talking about “tube distortion” as a desirable design feature.
- The volume controls are placed at the end of the preamplifier, just before the splitter. This suggests that the preamplifier was intended to produce an undistorted signal at maximum volume, which was always the goal back in the day! This is because the volume control can only attenuate the signal from the preamp, since it is at the end of the preamp, i.e., if the preamp distorted the signal, it would be as distorted at the lowest volume level as it would be at the highest. Of course, from there the signal then goes on to the power amp, which may provide its own distortion.
- Two 12AX7 tubes (four triodes) are devoted to tremolo. This is a lot. I’ll discuss the tremolo circuit (and all other circuits as well) in more detail below. I’m not sure I totally understand what’s going on there yet.