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7/18/2002 8:04 PM
Help needed with MOSFET source followers
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:
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)
Questions (for now):
Which B+ supply node do I use for the MOSFET drain (plate), and why?
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.
I adjust the -ve supply so I get -36V on the cathodes, right?
How do I bias the grid (gate) of the MOSFET?
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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Stan Bailey (email@example.com)
7/19/2002 6:36 PM
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.