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Old 23rd February 2007, 05:17 AM   #1361
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They might have been, Nixie. I bought a hundred 5 years ago--still have them, not to fond of the sound.
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Old 23rd February 2007, 10:30 AM   #1362
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Default testing

I have read that the power type Mosfets are more "hearty" than thier
small signal counterparts. on to the test jig they go
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Old 23rd February 2007, 11:42 PM   #1363
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Quote:
Originally posted by Nixie
I'm curious, is it possible to replace the front end with triodes and retain DC coupling?
Yes, but since the triodes are "N" channel, you must invert the
entire circuit polarity and use P channel outputs.
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Old 24th February 2007, 12:32 AM   #1364
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So what are suitable P-FETs for output? I'm thinking of trying this project as I have some 6DJ8s floating around and they should be enough to drive the MOSFET gates.
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Old 24th February 2007, 01:03 AM   #1365
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Now that sounds like a cool project!
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Old 24th February 2007, 02:15 AM   #1366
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Reasonable rail voltages for an Aleph-X do not readily lend themselves to getting decent performance out of tubes. Yes, I've seen 6922s run with 30V or so (e.g. Sunfire preamp), but the transfer curve will look like a snake and distortion will be much higher than if you had used a more reasonable rail voltage...say 100 to 150V.
Obviously, that's getting a little high for the MOSFETs in the output stage.
A much better way to approach this would be to run the front end tubes properly, then cap couple to the outputs. The only problem to be solved would be setting the bias in the output, but that's not an insurmountable difficulty.
This idea comes up about once or twice a year. A number of threads have been started for either Alephs or Aleph-Xs with tube front ends. I don't remember ever seeing one completed, but maybe I just missed that particular thread.
Note that an Aleph--with its proportionately higher rail voltages--would be a better candidate if you intend to directly connect to the outputs. If you decide to go with DC blocking caps, you can use a cathode follower. Even a 6922, a fairly robust tube in terms of current, is marginal for driving a gang of MOSFETs. You could do something like use one tube as the front end differential, then use another with both sections in parallel as a follower. The differential can easily drive two followers and you'd be able to get good current and a lower Zout.

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Old 24th February 2007, 02:25 AM   #1367
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Obviously I was intending a separate HV supply for the tubes. Can't the positive and negative supply for the tubes be set at such a point that capacitor coupling will not be necessary?
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Old 24th February 2007, 03:28 AM   #1368
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Given that the grids will be at ground potential and the plates some 100V (or more) north of there, I'm not sure I see how you intend to make that come together without DC blocking caps.
Even assuming that you can, it'll be a dicey proposition. You absolutely must regulate both sets of rails and tie them together with a resistive divider. Without regulation, if your line voltage drifts, the tube rail will drift much, much further than the solid state rail. The bias for the output will be thrown off with cataclysmic results. The only reason the Aleph style amps bias so nicely is that the front end and output share a common reference at the negative rail. Anything that upsets that balance is going to make life difficult.
The problems can be solved, but you need to think each step through carefully before even trying to breadboard a test circuit.

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Old 24th February 2007, 03:32 AM   #1369
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I was thinking to use a BJT in common base as voltage shifter. Drift can be handled by a servo. Sure, a bit extra complexity, but that's not really my main concern.
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Old 24th February 2007, 03:45 AM   #1370
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If you throw enough parts at it, you can solve the problems...but you run the risk of losing the very tube qualities that you wanted in the beginning.

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