plate ccs

[Sheldon]

I've got a low power 801 SE amp that is resistor loaded and cap coupled to the OPT. Thinking of playing around with a CCS plate load. This will double or so the power (going from 0.5W to a massive 1W). I have some DN2540's but they are only rated to 400V and my supply is at about 625V. Can I use two FET's for the top of the cascode to divide the voltage load, something like this: Don't much mind the resistor values, I haven't done a calculation yet.

[croccodillo]

What about to drive a high voltage N Mosfet with your fet?
You can place the mosfet over the fet, and thus cascode them.
It you can provide a fixed voltage (10V) to the Mosfet gate you can easily drive it with the DN2540, preserving the good fet characteristics while handling up to thousand volts.
I've used IRFBC40 mosfet, but you can use for example IRFBG20 or IRFBG30 (1000V).
This is a schematic that should work fine.
With this schematic the voltage across your jfet would never be more than 6-7 Volts.
Better again would be to use an external supply for the fixed voltage on the Mosfet gate; you can use a battery or, as I done, a well regulated low power supply (I used a spare 6V heater winding from y power trafo).
I never tried jfets (I always used Mosfet over Mosfet), but I think I would give them a try in the next future.

Best regards,
Giovanni

[Eli Duttman]

The DN2540 is a depletion MOSFET. Biasing for depletion MOSFETs is similar to biasing for tubes and JFETs. AFAIK, JFETs are strictly small signal devices.

While genuine experts will have the last word, I believe max. performance from Constant Current Sources for plate loading is best achieved with "P" type semiconductors. OTOH, "N" type semiconductors are "best" in Constant Current Sinks for use with cathode followers and differential circuit blocks.

[SY]

Have to disagree. I've gotten terrific performance from the DN2540 cascode as plate loads. I don't have Gary Pimm's measurement capability, but in theory, the sel-biased DN2540 cascode has better performance than a p-mode plate load since the p-mode will have a parasitic capacitance to ground via the collector-base (or drain-gate) capacitance of the device closest to the tube. Since that's usually the "big" device of the pair, that capacitance could be significant in some situations.

Channeling Brian Beck, I can also point out that this collector-base capacitance is modulated by the signal voltage. Now admittedly, I've never seen any evidence that this is a problem in my real-world measurements, but with high rp tubes and low currents, the effect could certainly show up.

[Sheldon]

Good, no one gagged on the concept, so I take it as a sign that it is doable. Croccodillo's suggestion, as I understand it, is to replace the top two 2540's with a single mosfet, and bias that with a zener. Would there be any performance benefit/loss as compared to the two 2540's? Looking at parts I have in the box, though I could probably pick up some appropriate Mosfet's at the local surplus.

I also have some Lovaltech LU1014D. Suppose I could use those on the bottom?

Sheldon

[croccodillo]

The only benefit in using a Mosfet in the upper side is the increased voltage range.
I never "heard" a depletion mosfet in audio circuit, so I cannot say if there are differences in sound.
What I can say is that if well designed a cascode mosfet CCS will sound very well.
My impression is that joining a dpletion mosfet togheter with a power mosfet one can realise a really good and powerful circuit.

Ciao,
Giovanni

[Eli Duttman]

Originally posted by SY
Have to disagree. I've gotten terrific performance from the DN2540 cascode as plate loads. I don't have Gary Pimm's measurement capability, but in theory, the sel-biased DN2540 cascode has better performance than a p-mode plate load since the p-mode will have a parasitic capacitance to ground via the collector-base (or drain-gate) capacitance of the device closest to the tube. Since that's usually the "big" device of the pair, that capacitance could be significant in some situations.

Channeling Brian Beck, I can also point out that this collector-base capacitance is modulated by the signal voltage. Now admittedly, I've never seen any evidence that this is a problem in my real-world measurements, but with high rp tubes and low currents, the effect could certainly show up.

Hmmm. This seems to be yet another win some/lose some situation. Phooey! Using "P" type on plates and "N" type on cathodes tends to maximize the dynamic AC impedance.

Compromises will always be with us.

[SY]

I'm not even sure that's the case. Gary's site is down at the moment, but he showed dynamic impedance versus frequency for several different CCS configurations connected as plate loads. Best was his IRF820-based unit. Second best was the FAR simpler self-biased DN2540 cascode, and it was some ridiculously high number.

[Tweeker]

Gary's site is down for the count, IIRC.

[SY]

Sic transit gloria.