Can I use the GMX kit as a power amp? If I drive it from a 12AX7 will it sound like a real tube amp?
Yes, you can use the GMX (Transconductance Multiplier) kit as the complete output stage, or merely to augment the tone and/or power of a conventional output stage. There are a few things to consider. Assuming you wire the 12AX7 as if it were two triodes driving the output transformer (OT), the sound will be that of a triode amplifier: clean, transparent, and very linear. The GMX circuit will just make this sound even larger and more transparent. In this arrangement, the plate supply must be kept within what a 12AX7 can handle, so something less than 270V – more like 200V to 250V, in turn requiring the use of a custom OT. This will accommodate the OT flyback voltage. Note that the current sample and sense resistors would have to be significantly altered in proportion to keep the current through the tube within its limits. If you were looking at a more “modern” application, the 12AX7 becomes a gain stage and a concertina splitter, or is wired as a Schmitt splitter, with the split outputs
