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For the source V- you could use a real small 5-6 volt transformer with it's secondarys hooked up to your 5V filament windings. Make sure your V+ and V- to the Drain and Source doesn't exceed the MOSFETs D-S Voltage.

3. I adjust the -ve supply so I get -36V on the cathodes, right?

I think you leave your existing bias intact, and hook up one leg to it's own Mosfet gate through a 100R-220R to prevent oscillations. After you connect everything as shown on schematic, just adjust bias as you did before. The source follower will "follow" the bias input voltage.

4. How do I bias the grid (gate) of the MOSFET?

I believe it will self bias to about 3 volts?

5. I seem to recall some previous posts here about blowing MOSFETs in this application if not done properly. Is there anything else I need to add that's not mentioned in the R.G.'s article?

I always have problems with Mosfets if I don't put a 12V zener connected from gate to source for protection.

I have done the direct coupling thing with both tube and fets.As others have stated, I really can't hear the difference between the two topologys. Anyway, I have a few threads saved to disk. Just e-mail me. Good luck, Stan

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From:

Shea (stmoxon@earthlink.net)

Date:

7/20/2002 10:07 PM

Subject:

Re: Help needed with MOSFET source followers

I was the one who blew MOSFETS, but I didn't use any zeners to limit the gate-source voltage. That probably was the problem, or part of it.

I rectified the negative voltage from the HT winding. I'm thinking about using a different method that will give a lower voltage. Maybe I'll use a 1:1 isolation tranny that I haven't been using.

Shea

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From:

R.G.

Date:

7/21/2002 3:38 PM

Subject:

Re: Help needed with MOSFET source followers

Lemme take a shot. I've never done an AB763, so this will reflect my approach to it, not experience with that amp.

1. Which B+ supply node do I use for the MOSFET drain (plate), and why?

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