Question about IRF820, R.G ?

[fikri]

It seems like i have another question for the IRF820. I have build the fender tonestack for the hotbox clean channel, but i need more gain stage after the TC, because i loose alot of volume level. My question is, can we just replace a triode gain stage with an IRF820 ? what about the biasing ?

Thanks

[MetalGod]

I would replace the CF in your dirty channel with the IRF820 as explained in RG's Mosfet Follies article (http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm)

This would then leave you with a free triode.  I know that you shoud generally only share triodes between stages on the same B+ 'node' so you might have to rearrange your layout.

OTOH, just bang the IRF820 in there as a direct gain-stage replacement and see if it flies.

[R.G.]

In general, you can't just replace a triode with a power MOSFET because the gate-source capacitance is so high. In a cathode follower setup, this works OK because the feedback from the source reduces the effective value of the gate-source capacitance as seen by the stage driving the gate. In a gain setup, you are likely to lose a lot of treble to this loading.

The idea of using the MOSFET instead of the tube to drive the tone stack is much better. The MOSFET used as a source follower works fine. Then you have the tube left over for level recovery, as MetalGod says.

There exist lower capcitance MOSFETs like the Zetex ZVN0540A. It's input capacitance is only 55pF, so the treble losses are much smaller. This is as compared to the input capacitance of a 12AX7 of about 2pF. If you're going to just try it, get some ZVN0540As to try it with. They're 450V, 1W devices in a TO92 package.