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MOSFET source follower driver - help!

1/14/2000 1:10 PM
lion	MOSFET source follower driver - help! I’m determined to try to implement a driver to the output tubes on my latest amp project – to eliminate grid blocking and optimize power, as per Randall Aiken and RG’s suggestions. I’ve been considering the pros and cons of a tube cathode follower versus a MOSFET source follower – and I’ve finally ruled out the tube CF (for the time being) for two reasons 1: I’m not sure the PT can handle the extra 300mA load on the filament supply 2: The B+ has to be dropped quite a lot – from 430V to 250V not to exceed a 12ax7’s max plate to cathode voltage rating. Just to make sure that I’ve got it right – simple as it may be - I’ve drawn up a schematics of the circuit: http://www.dizz.dk/mosfet.gif Please take a look - and correct me if I’m wrong! I also need some advise on the following: 1: How much negative voltage at the MOSFET source should I go for - with 430V at the drain? 2: How do I calculate the size of the source resistor? 3: How about the output tube bias adjustment – any changes here? Any advise or comments will be appreciated. Thanks in advance. If anyone’s interested I’ll be glad to post my results. Regards Lion

1/14/2000 1:10 PM

lion

MOSFET source follower driver - help!

I’m determined to try to implement a driver to the output tubes on my latest amp project – to eliminate grid blocking and optimize power, as per Randall Aiken and RG’s suggestions.

I’ve been considering the pros and cons of a tube cathode follower versus a MOSFET source follower – and I’ve finally ruled out the tube CF (for the time being) for two reasons
1: I’m not sure the PT can handle the extra 300mA load on the filament supply
2: The B+ has to be dropped quite a lot – from 430V to 250V not to exceed a 12ax7’s max plate to cathode voltage rating.

Just to make sure that I’ve got it right – simple as it may be - I’ve drawn up a schematics of the circuit:

http://www.dizz.dk/mosfet.gif

Please take a look - and correct me if I’m wrong!

I also need some advise on the following:
1: How much negative voltage at the MOSFET source should I go for - with 430V at the drain?
2: How do I calculate the size of the source resistor?
3: How about the output tube bias adjustment – any changes here?

Any advise or comments will be appreciated. Thanks in advance.

If anyone’s interested I’ll be glad to post my results.

Regards
Lion

Replies:

Aaron V. Hey, I'll be doing the same thing, ... -- 1/14/2000 3:33 PM
R.G. I'll take a swing, since I'm the on... -- 1/14/2000 4:09 PM
anonymous I'll take a wild swing at answering... -- 1/14/2000 4:14 PM