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I did some work with some Zetex high voltage MOSFETs. These are a bit unique in that they are TO-92/E-line packages, same size as your garden variety small plastic signal transistor. They are 0.7W, 450V devices, and their input capacitance (Ciss) is only 55pF, unusually low for a high voltage MOSFET.
That's low enough in fact that a 69K rp 12AX7 plate will drive it flat to over 100kHz.
So I messed with this thing as a LTP. I put 100K plate loads on each side, used a 22K common source resistor and 1M bias resistors to a 68V zener bias point.
The result? It's beautifully balanced, produces high drive, and more importantly, has enough output that you could shape the "clipping" nature of the thing to be as tube like as you want to mess with. This thing will swing 200V peak to peak on each side of the LTP with a 350V supply.
And did I mention it had high gain?
It was so high that I had to put a monster divider in front of it to get it down to a signal level on the output tube plates that was not just all square waves all the time.
I modelled it up and the sim says that it has an open loop gain of a bit over 80db... That's enough to just plug your guitar right into the phase inverter and still overdrive the output tubes.
So I put some source degeneration into the LTP, 1K on each source before the 22K source resistor. That got it back down to only 46db.
For anyone wanting to play with the MOSFET Heresies, these devices are good fodder. The Ciss is low enough to let you get full audio band response from them in an environment which 12AX7 tubes work in. They're too linear to actually be tubes, of course, and they need enhancement biasing instead of depletion biasing, but other than that, they're a really useful adjunct to tube stages.
I've messed with the cascode setup a bit - depletion mode JFET running into the source of the MOSFET - and that's interesting as well, but I don't have any firm circuits yet.
Another thing I have to go mess with is current buffering a 12AX7. I have a couple of ideas on how to make a ZVN0540A multiply the current out of a 12AX7 by a fixed amount. That would let us use a "12AX7" as an output tube.
Sigh. So many circuits and so little time...
That's low enough in fact that a 69K rp 12AX7 plate will drive it flat to over 100kHz.
So I messed with this thing as a LTP. I put 100K plate loads on each side, used a 22K common source resistor and 1M bias resistors to a 68V zener bias point.
The result? It's beautifully balanced, produces high drive, and more importantly, has enough output that you could shape the "clipping" nature of the thing to be as tube like as you want to mess with. This thing will swing 200V peak to peak on each side of the LTP with a 350V supply.
And did I mention it had high gain?
It was so high that I had to put a monster divider in front of it to get it down to a signal level on the output tube plates that was not just all square waves all the time.
I modelled it up and the sim says that it has an open loop gain of a bit over 80db... That's enough to just plug your guitar right into the phase inverter and still overdrive the output tubes.
So I put some source degeneration into the LTP, 1K on each source before the 22K source resistor. That got it back down to only 46db.
For anyone wanting to play with the MOSFET Heresies, these devices are good fodder. The Ciss is low enough to let you get full audio band response from them in an environment which 12AX7 tubes work in. They're too linear to actually be tubes, of course, and they need enhancement biasing instead of depletion biasing, but other than that, they're a really useful adjunct to tube stages.
I've messed with the cascode setup a bit - depletion mode JFET running into the source of the MOSFET - and that's interesting as well, but I don't have any firm circuits yet.
Another thing I have to go mess with is current buffering a 12AX7. I have a couple of ideas on how to make a ZVN0540A multiply the current out of a 12AX7 by a fixed amount. That would let us use a "12AX7" as an output tube.
Sigh. So many circuits and so little time...
and also a 12AX7 output stage!
What's the part number of the device you used?
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