Using HV Hexfets in tube-style p-p o/p stage with o/p tranny. - diyAudio
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Old 4th July 2002, 02:32 PM   #1
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Default Using HV Hexfets in tube-style p-p o/p stage with o/p tranny.

I can hear everybody asking "why would you want to do that? Has the strain of getting the Circlotron Doomsday Amplifier going proved too much so that his brain broke?" Well it's like this' I'm up to my armpits in various big second-hand 500v Hexfets and suitable large electro's and I thought why not use a pair of them in push-pull with a conventional output tranny, probably a mains toroid seeing that I am such a cheapskate!

The way I see it, a Hexfet has a similar drain (plate) characteristic to a pentode, so it would have a relatively high output impedance and need a bit of feedback to get it under control. Perhaps that higher impedance would contribute to a "tube sound". I'm sure some has done this before though probably not commercially. Where is there some good stuff I can have a look at?

GP.
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Old 8th July 2002, 06:41 PM   #2
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I suggest that you take the Son of Zen and instead of
8 ohm Drain resistors you drive the primary of the
transformer center-tapped to +200 volts.

The MOSFETs will want to be biased at something like
100-200 mA, and you can do this with the appropriate
resistances off the Source pins going to some nominal
negative rail voltage. Experimentation should give you
the appropriate values.
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Old 8th July 2002, 06:50 PM   #3
hifi is offline hifi  Sweden
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Look upp Jean Hiragas old Nemesis amps...
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Old 12th July 2002, 08:18 AM   #4
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Actually, here's another idea that should work quite well though I have not tried it, is to run a pair of N-channel Hexfets as push-pull source followers driving an output transformer, the centre tap going to earth. Use a transformer 120-0-120 vac to whatever voltage you like depending on the power output you want. e.g if you use a 24vac secondary you will get (24x24) / 8 ohms=72 watts. Actually, if you use rectified 120vac (dangerous though) the resulting 170vdc supply rail will be just right for the transformer.

The interesting thing about this arrangement is that any nonlinearities that show up as the change in source voltage not equalling the change in gate voltage exactly will be reduced by the same amount as the transformer turns ratio. e.g. say we want to swing 24 vrms into an 8 ohm load *without* using a transformer and the resulting distortion might be 5%. Now we use a transformer and so we have to swing 120vac instead of 24vac. The nonlinearity between gate and source is still the same in outright volts, but is now a much smaller fraction of the total voltage swing. With the transformer suggested it would be reduced by a factor of 5, bringing things down to 1% distortion.

This is probably a bit of an oversimplifcation, but I'm sure there is something valid there. A "one day" project perhaps.

GP.
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