How about an IXCP10M90 as a FETRON - diyAudio
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Old 27th October 2013, 01:52 PM   #1
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Default How about an IXCP10M90 as a FETRON

Silly idea - but why not use a IXCP10M90 as a direct drop in replacement for a pentode in a Schade valve amp. Would have to drive an OT of course since the current limit is 100mA.

Any thoughts ?

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Old 27th October 2013, 02:16 PM   #2
Bigun is offline Bigun  Canada
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How do the inter-electrode capacitances compare ?
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Old 27th October 2013, 02:26 PM   #3
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Higher no doubt - but impossibly so, I doubt.
I would use a driver transistor of some type which would be run fairly hard, say 30mA, to push the input capacitance into submission. The input transistor is a necessity anyway to correctly implement the schade feedback.

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Old 27th October 2013, 02:38 PM   #4
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Stack a pair of them as done in most of the CCSs and your input capacitance problems go away... Also, when testing the idea on IFR820b MOSFETs the high order distortion drops significantly.

Test data can be found in the SS Tabor page on my website.
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Old 27th October 2013, 02:44 PM   #5
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Thanks, will take a look.
Is the SS tabor as good as the Tube version.
It looked a bit complex last time I looked - but that because its not using depletion mode FET's ?

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Old 27th October 2013, 03:05 PM   #6
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The SS version is a bit more open and spacious compared to the tube versions. I suspect that this is the lack of microphonics with the MOSFETS.

The SS version has no hint of any "Solid state" sound. Pretty much sounds like the tube versions.

The biasing needs of each pair of enhancement mode MOSFETs makes them more complicated that a pair of depletion mode MOSFETs. The upside is biasing the lower MOSFET at a higher DC voltage that gets the part into a much better operating point with respect to inter-element capacitances. Also, there is a larger selection of HV enhancement mode MOSFETs.

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Last edited by Gary P; 27th October 2013 at 03:06 PM. Reason: More thoughts...
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Old 27th October 2013, 03:16 PM   #7
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I love the sound of my 807 based Tabor clone, so same but better sounds good.

My wattage needs are less than 2watts for my vintage German speakers so running some IXCP10M90's at say 200V 50mA would provide me with more than enough output. Cascading would seem the sensible solution to crushing the gate capacitance. Basically i would build out each sudo pentode as a cascade CCS.

Unfortunately i have a habit of frying these chips and they are not easy or cheap to get on this side of the pond.

This whole exercise is both a though experiment and a pursuit of increased efficiency without sacrificing the performance of my valve amps.

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Old 27th October 2013, 03:38 PM   #8
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Sounds like a fun project. The expense and rarity of the depletion mode parts is one of the reasons I've stuck with the enhancement mode parts. That and the fact that they work better with the higher voltage available from the local bias circuit. The depletion mode parts also work better with more bias voltage available but that kind of defeats the charm of depletion mode parts.

The enhancement mode parts take a bit more to get them going but they are dirt cheap when you let the smoke out...

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Old 27th October 2013, 03:55 PM   #9
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Here's a thought.
I have just successfully built a SE spud E55L with an input choke (1:2 step up) and the plate to grid feedback applied to the input choke. It worked very well.
Why not go for a spud FETRON using the same principle. It seems to me that one of these babies should bias up to about -1.5V at 200V and 50mA with a gain in excess of 50. This puts it in the same ball park as a direct drop in replacement for the E55L but in a PP implementation.

I don't see why it wouldn't work as an experiment in extreme simplicity.

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Old 27th October 2013, 04:59 PM   #10
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Sounds like a good experiment.

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