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previous: lion Thanks for the fast replies! -- 1/10/2000 7:18 PM

Re: Cathode follower drive to output tubes

1/10/2000 7:58 PM
R.G.	Re: Cathode follower drive to output tubes Sure. Wherever you have a cathode follower, pull out the tube, connect in the MOSFET with drain=plate, gate=grid, source=cathode. Turn it on. The MOSFET biases with its gate about 3-4V above the source, not 1V less than the cathode like the tube. Almost all cathode follower circuits work with a MOSFET subbed in one for one. An IRF820 for example will do 500V, 3A and 75 W dissipation. It's fast enough and has a sufficientlly wide voltage range that the FET will still be linear when the tube driving it and the tube it's driving are distorting, so the other tubes hide any nonlinearity that the FET might do - the tube distortions limit circuit action before you hit the limits of the MOSFET. You may have to heat sink it if you use it with lots of power dissipation. The TO-220 package will only dissipate maybe 1W in still air. No heater current, of course The effective gate-source capacitance is reduced by the circuit feedback of the device in source follower mode, so you don't have a huge capacitive load to drive. I've used these in preamps being driven by 12AX7 plates and had no tone losses - just the buffering you're looking for.

1/10/2000 7:58 PM

R.G.

Re: Cathode follower drive to output tubes

Sure. Wherever you have a cathode follower, pull out the tube, connect in the MOSFET with drain=plate, gate=grid, source=cathode. Turn it on.

The MOSFET biases with its gate about 3-4V above the source, not 1V less than the cathode like the tube. Almost all cathode follower circuits work with a MOSFET subbed in one for one. An IRF820 for example will do 500V, 3A and 75 W dissipation. It's fast enough and has a sufficientlly wide voltage range that the FET will still be linear when the tube driving it and the tube it's driving are distorting, so the other tubes hide any nonlinearity that the FET might do - the tube distortions limit circuit action before you hit the limits of the MOSFET. You may have to heat sink it if you use it with lots of power dissipation. The TO-220 package will only dissipate maybe 1W in still air.

No heater current, of course 8-)

The effective gate-source capacitance is reduced by the circuit feedback of the device in source follower mode, so you don't have a huge capacitive load to drive. I've used these in preamps being driven by 12AX7 plates and had no tone losses - just the buffering you're looking for.

Replies:

lion Wait a minute RG – let me get this ... -- 1/10/2000 9:14 PM