Tubehead Preamplifier circuit diagram
Channel 2 is identicle to Channel 1, and uses IC4b for the clipping meter and IC3 for the input/output driver. The input impedance of the TubeHead is about 20k ohms, which is consistant with most gear like Synths, Effects Processors, Mixers, EQs, and so on. 20k is too low for a proper match with high impedance sources like guitar pickups, but a few minor changes take care of this. To use the Tube Head as a instrument pickup preamp, remove the 47k* resistor and the 20pf capacitor from the feedback loop of the driving Op-Amp. Then change the 47k” resistor to 680k and the 22k resistor in the feedback loop of the driving OpAmp to 100k. Now the Tube Head can be used to warm up a cold sounding guitar amp or just provide a great preamp tone.
This is just what I’ve been looking for… a push-pull plate voltage starved pre-amp stage to add to my other guitar gear. Do you happen to sell PCBoards or parts kits for this amplifier?