mosfets or JFETS with low Ciss

This is the problem.

I want to make a pre that uses a tube input i.e. at 15-25mA operating current.

This is driving a MOSFET or JFET follower running at 100mA Class A. running off +-20vdc.

the tube is obviously cap coupled to the gate of the MOSFET.

Any recommendations on MOSFETS (N channel) that would work with low Ciss to minimize HF roll-off.... oh and high S.

gate vs source

I thought the gate is positive wrt source in a source follower configuration?

So if The gate is at 150vdc the source is at 146vdc or approx.

So plate resistor on tube to B+ At the plate a cap (value?) to a resistor to ground. gate connected at point of resistor and cap.

Like a 100K to ground?

Maybe a 10uf?


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
If you capacitively couple the output of the plate, then
the Gate of the Mosfet follower should be biased at the
midpoint of its swing +4 volts or so. In your case it
sounds like +4 V referenced to ground. Any resistance
that works is OK for this, as the DC input impedance of
the Mosfet is near infinite. 22K would probably be the
kind of value you want to load the plate after the cap,
and 10 uF or more film cap is what you probably want
for coupling.