Hey guys,
I don't like silicon in my tube amps. However, I'm about to go off to college, and I want to make my ST-70 as roommate-proof as possible. Not to mention the fact that this would just be really cool and would eliminate any need to rebias an amp.
My initial idea was to make a self-biasing amplifier. This isn't exactly fixed bias, per se. I was thinking about making a voltage source using a zener diode and a voltage divider to get between 400mV and 450mV. Then using a differential amplifier, a PNP transistor, and a resistor, one could effectively make a simple feedback circuit to make sure there is always the correct DC voltage on the grid. One input of the differential amp is taken off of the cathode resistor (10Ω in my case) and the other is the voltage source (400mV or so). The output is connected to a PNP transistor, and the emitter is grounded. The collector is tied to a resistor (which goes to the negative DC bias voltage) and the grid.
Could this work? I think it'd be a super simple op-amp based circuit. Has anyone done anything like this? Is there a better way?
Kyle