MOSFET Phase Inverter?

[Ripthorn]

I read about this possibility over on GeoFEX and it is something that I would really like to use.  So I was wondering if anyone knows if I could just take a traditional cathodyne or LTP phase inverter topology and drop in something like an n channel mosfet.  Anyone?

[brett]

Hi
MOSFETs are used for digital inverters all of the time.  There's no reason why it won't work.
However, my choice is always JFET over MOSFET, except for high current apps or where -ve gate-source is difficult to obtain.
cheers

[snap]

scroll! http://www.muzique.com/schem/mosfet.htm

[R.G.]

I read about this possibility over on GeoFEX and it is something that I would really like to use.  So I was wondering if anyone knows if I could just take a traditional cathodyne or LTP phase inverter topology and drop in something like an n channel mosfet.  Anyone?

In general, no, you can't just drop one of these in. The problem is that the stock PI circuits for tube amps are set up to bias the depletion mode tubes. If you just drop in a typical MOSFET, it will be biased completely off by the circuit and nothing at all will happen. The reason you can drop an N-channel MOSFET into a source follower is that the grid on a source follower is biased many volts positive, so the fact that the MOSFET source sags to maybe five volts less than the tube's cathode would have been is of no consequence. This is not true in self-biased circuits like 99+ % of PIs.

scroll! http://www.muzique.com/schem/mosfet.htm

"Cathodyne" of a kind. That's the out-of-the-textbook phase inverter circuit applied to a MOSFET. However, it must be scaled up for tube amp application, and it won't work for those amps which use a long tailed pair for a PI, as these not only have gain, but allow things like presence controls by having feedback from the output.

The long tailed pair with MOSFETs works fine if biased correctly, but suffers from too much gain to just drop into existing amp stages. The MOSFETs you'd use have a much higher transconductance than the tubes they replace. This upsets the feedback networks and can push you into oscillation. It's possible to pad this back down with source resistor degeneration, but you have to then tailor it to each MOSFET.