Hey there, I'm playing with some new ideas lately, and was needing a lower voltage mosfet suitable for use as a lower power source follower at low to moderate rail voltages. Think ±20 volt or so rails, for flea-power, EL84, or similar bias ranges. I would normally use a big TO220 type device for most of my experiments, but I would rather use something much smaller so that I can make some very compact PCBs later on, and would prefer a physically smaller device.
Assuming that I'm using +12 and -24 volt rails, would something as simple as a 2N7000 work? Using an IRF820 as comparison, it seems like the 2N7000 would work, but since it's sort of a dinosaur, I was curious as to any other suggestions. It does seem to have much less capacitance than the IRF820. Would a logic level device be a good idea? It seems that it would give me a tiny increase in usable voltage, and may be a little more consistent, and since I don't need much in the way of power handling, I think a TO92 device should be fine for this.
Any thoughts?
Assuming that I'm using +12 and -24 volt rails, would something as simple as a 2N7000 work? Using an IRF820 as comparison, it seems like the 2N7000 would work, but since it's sort of a dinosaur, I was curious as to any other suggestions. It does seem to have much less capacitance than the IRF820. Would a logic level device be a good idea? It seems that it would give me a tiny increase in usable voltage, and may be a little more consistent, and since I don't need much in the way of power handling, I think a TO92 device should be fine for this.
Any thoughts?
My 2 favorite enhancement MOSFETs are the ZVN0545A and the IRFBC20. Both devices exhibit a low and stable reverse transfer capacitance. The little ZVN0545A is so benign that even the very wimpy 12AX7 drives it with complete aplomb. 
Don't forget the protective Zener diode from gate to source, when DC coupling these items to a tube's plate.
In depletion mode MOSFETs, the DN2540 comes in both TO92 and TO220 packages.
Don't forget the protective Zener diode from gate to source, when DC coupling these items to a tube's plate.In depletion mode MOSFETs, the DN2540 comes in both TO92 and TO220 packages.
The advantage of the enhancement mode type is that DC coupling to a tube's plate is a "no brainer". You certainly can cap. couple a depletion mode FET to a tube's plate and configure things just like you would with a cap. coupled triode cathode follower.
The critical parameter for FETs in voltage follower service is reverse transfer capacitance. Low and stable is "what the doctor ordered". "Bandersnatch" likes to cascode FETs, to take reverse transfer capacitance out of the picture.
The tweaked RCA phono setup I'm associated with shows how to DC couple a ZVN0545A to a 'X7 section plate. If you decide to build a tweaked RCA, make certain to include a protective 12 V. Zener diode between the FET's source and gate. Turn on transients have been known to damage the FET.
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^ neat little guy, haven't seen grid leak bias in a while.
My primary need for these is for small source followers in lower voltage bias supplies (I want to be able to do fixed bias with the capability of providing grid current for A2 or AB2 operation for tubes like EL84, 6V6, or a flea-power 6SN7 AB2 I've got in the works) or for trying out a mosfet in place of the triode in a cathodyne circuit, so the ZVN0545A looks like exactly what I need. One of my circuits in particular is a 6SL7 grounded cathode gain stage directly coupled to a mosfet cathodyne, so I think the ZVN0545A should work well.
My primary need for these is for small source followers in lower voltage bias supplies (I want to be able to do fixed bias with the capability of providing grid current for A2 or AB2 operation for tubes like EL84, 6V6, or a flea-power 6SN7 AB2 I've got in the works) or for trying out a mosfet in place of the triode in a cathodyne circuit, so the ZVN0545A looks like exactly what I need. One of my circuits in particular is a 6SL7 grounded cathode gain stage directly coupled to a mosfet cathodyne, so I think the ZVN0545A should work well.
A FET makes an excellent "concertina" phase splitter. It has an advantage over a triode in that it can swing closer to the B+ rail. That ability was crucial to ultimate success in a 6Y6 near B1 project, where B+ volts were at a high premium. Even then, it was touch and go tweaking 6922 "idle" current to stop the splitter from clipping.
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