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From:

Man o'Blues

Date:

7/18/2002 8:04 PM

Subject:

Help needed with MOSFET source followers

Hi all,

I think I'm ready to try using a pair of IRF 820's as source followers to drive the outputs in my BF clone, as per R.G. Keen's article at:

http://www.geofex.com/tektips.htm

The article seems straightforward, but I could really use some implementation advice specific to my amp, whose power section is set up as follows:

  • *

    Standard AB763 12AT7 LTPI but with 47K plate loads.

    • *

      B+ nodes are: 410V/screens, reverb, and trem, 330V/PI and 260V/preamp section

    • *

      100K/100K bias resistors

    • *

      -36V bias voltage (derived from PT's 50V bias supply)

    • *

      6V6GT outputs

Questions (for now):

  • 1.

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

  • 2.

    What is the best way to derive the high negative voltage for the cathodes? Note: I get 425V B+ from the rectifier plate. Also, the PT's 2A/5V

filament winding is unused, if this can be used at all.

  • 3.

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

  • 4.

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

  • 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've never handled any ss devices, so mucho TIA for any tips on the above!

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From:

Stan Bailey (whipzippy@aol.com)

Date:

7/19/2002 6:36 PM

Subject:

Re: Help needed with MOSFET source followers

Since this passed by the more knowing individuals on this forum, I'll take a crack at it.

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

I just tapped off one of the b+ nodes (PI) and decoupled it with a 10K and cap. Why? Because it's quick and easy :O)

2. What is the best way to derive the high negative voltage for the cathodes? Note: I get 425V B+ from the rectifier plate. Also, the PT's 2A/5V filament winding is unused, if this can be used at all.

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