IRF710 as Ampeg style verb driver?

[imaradiostar]

I'm looking at using an IRF710 as a transformerless reverb driver for high impedance tanks ala Ampeg. I had toyed with the idea of a cascode and other various hybrids of mosfets and jfets but I really like the idea of creating a simple bias network and having the mosfet soldered to the turret board with a little heatsink on it- less parts, less space consumed, hopefully less to break.

Of course...input capacitance is a problem. The IRF710 is one of the lowest Ciss mosfets I could find with a suitable power rating. How low a driving impedance will I have to use to get a usable frequency range out of it?

Would adding a source resistor (say 330 ohms?) diminish the hi frequency rolloff enough to make a usable verb driver without killing all the gain? I bet RG will have something to say!

I prefer to use a 1m pot instead of the grid resistor on fender circuits for an adjustable verb drive control. I'd like to achieve something similar here without increasing the parts count too much but I understand that input capacitance could be a problem. I'm trying to use up some old high impedance tanks I've collected over the years and the Ampeg circuit really seemed like the way to go...but I don't want to waste valuable chassis space or heater current.

jamie

[Steve Conner]

MOSFETs have about 100 times more gm than any preamp tube (the datasheet quotes it in amps per volt, not mA/V!) So you'll end up needing that source resistor anyway, to get it behaving more like a tube, in any application except source follower. If you use a configuration with voltage gain, you also need to worry about Crss and Miller effect.

Another option might be a Darlington pair made of high voltage BJTs. Say a MPSA42 followed by a MJE340. It should draw so little base current that you can bias it like a MOSFET, and have lower capacitances than a MOSFET, and again you use an emitter resistor to get the gm down to what you want. I believe Peavey used tricks like this in their Transtube technology.

Or the MJE340 might even work on its own. I remember seeing a hybrid tube/SS radio receiver in a ham radio projects book that used a MJE340 as the audio power amp, running off the same B+ as the tubes and driving the speaker through an output transformer... The designer mentioned that he didn't have enough current on the low voltage rails to drive an ordinary transistor amp!

[imaradiostar]

Thanks for the reply. I was thinking more people would have input on this idea but I guess that just shows that I don't know much!

I was thinking the mosfet would have low enough capacitances that it would work as a verb driver but I haven't had the time or patience to simulate it and see where the high frequency response rolls off. If it won't work relatively simply and I have to have multiple transistors then I'd rather just make room for another tube. I think a magnatone style drive/recovery using a dual (dissimilar) triode would use very little space and current while solving my "simple reverb" problem.

jamie