Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sdp1234 on July 12, 2017, 01:16:03 pm

Title: MOSFET CF using a hoffman style layout
Post by: sdp1234 on July 12, 2017, 01:16:03 pm

I'm looking for an example MOSFET CF when using a Hoffman layout.  I've seen them with the TOS and some other designs.  I'm building a Rocket\AC30 type circuit but using a mosfet CF.  I've done it before but it was cobbled together on tag strips.

Title: Re: MOSFET CF using a hoffman style layout
Post by: kagliostro on July 12, 2017, 02:27:38 pm

The TOS is the better example to which I can think

Franco

Title: Re: MOSFET CF using a hoffman style layout
Post by: tubeswell on July 12, 2017, 02:56:17 pm

See tubenit's thread on MOSFET source followers (Note: he calls it a 'MOSFET cathode follower')


http://el34world.com/Forum/index.php?topic=21873.0 (http://el34world.com/Forum/index.php?topic=21873.0)


For the pin payout, refer to the correct data sheet.


An IRF820 data sheet is attached. IRF820 make an ideal source follower in a tube amp. They are rated for 500V and 1W dissipation (in the standard TO220 package), so they are practically a drop-in replacement for a 12AX7 triode, with a few tweaks. They are always for sale on eBay. Just remember to use a 100R-1k gate stopper resistor (to discourage oscillation), and to run a 12V zener between the source and the gate (banded end pointing to the gate) to protect the quartz insulation layer between the source and the gate from static discharges. A 1/2W zener will do. Don't touch the drain when the amp is running - it is at HT potential.