FET as cathode follower...Sorry. Not for valve fanatics...

[Gregg]

Thanks for the informative explanation as usual.

Running the source resistor up makes it easier for the preceeding stage to drive the high gate-source capacitor at all frequencies and I think that's what happened for you.

For those of us who are not very much into theory dose it mean that 33k is too low or too high for SF source resistor? I'm currently running this SF at 363V, 173V at the source.

[R.G.]

The big power MOSFETs have big gate-source cacitances, up to 1000pF or so. All by itself, this would prevent the high-impedance plate of a tube from driving it at even modest audio frequencies. But with a big enough resistor in in the source, the voltage feedback effectively reduces the capacitance that the tube plate sees. In this case, the source-ground resistor does make a difference in what the driving stage sees. So it sounds like the 33k was too low. A higher resistor increases the local feedback that's hiding the gate-source capacitance from the tube plate.

Or you could go with one of those Zetex ZVN0545A's. Those have gate capacitance of only 55pF, much lower than the IRFs. But they can only dissipate 0.7W, so you have to keep the current through them down.

With 363V on the drain and 173V on the source, there is 363-173V= 190V across the MOSFET. The highest current you can use and not burn it up is 0.7W/190V =3.6ma, and that's a source resistor of 173V/3.6ma = 46.7k.

With the IRFs, they can dissipate (much!) more power, so you could run them with more current, but then you lose the audio response. So you still have to use a bigger resistor.

It looks like you're forced to 47K to 100K for the source resistor.

[Gregg]

It looks like you're forced to 47K to 100K for the source resistor.

Thanks for this answer. To keep it short and simple that means that in general for a SF/buffer we better stick to the "standard" 100k value we see in almost every CF amp.

[chocopower]

Hi!!

After some time learning more about tubes and FETs, think that i finally understand this mod.
Iīll would like try it in Pro-Reverbīs Normal Chanell (is not vintage, is a clon)

Please, check this schematic.




And this is a second version, with Reverb and Tremolo added to this new chanell.




I would like too if i should remove the .047 cap after the tone-stack.

regards, and, again, sorry for put the S.S. demon inside a tube amp

[chocopower]

here.... the layout, for the bone-heads (like me)