Marshall plexi Jfet preamp

[Gus]

One problem jumps off the page.

  Grid to ground resistors for the input fets?

  Next something a bit deeper that people seem to overlook with tube to fet circuits. What is the output impedance of the tube plate of the real marshall and what is the output impedance of the fet stage as you have it drawn?  I can figure this out but you should google etc. this might have an effect on bright channel to 1meg volume pot.

  Look for duncan's tone stack calculator note the program has source resistance and load resistance as something you can adjust.

What voltage supply will you use? you need to note the fets max drain to grid voltage rating Vgs.

[fuzzo]

Hi,

Actually I cannot answer you about the impedances of both preamps (mine and Marshall valve) I know nothing about theory things (even if in some cases ,like here, are important ).

The only thing I can say, it's I'll use a 15 or 24VDC with this project to have a more "headroom"  (Actually it will be put in a rack box).

Beside, I don't want a really extremely similar sound (I think isn't impossible I'll use Jfet and not valves) but something which sounds like the marshall preamp.

[slideman82]

Well, I use 24V in my solid state amp's preamp, all JFET, and I must say it's cleaner, sounds excellent, but doesn't distort pretty soon. That's because any stage becomes more linear with higher voltage. My 9V version of  the Plexi sounds cool, that's because the 9V.
Believe me, with more than 12V some of that magic (very presemt 2nd harmonic) is lost...

[snap]

Well, I use 24V in my solid state amp's preamp, all JFET, and I must say it's cleaner, sounds excellent, but doesn't distort pretty soon. That's because any stage becomes more linear with higher voltage. My 9V version of  the Plexi sounds cool, that's because the 9V.
Believe me, with more than 12V some of that magic (very presemt 2nd harmonic) is lost...

amp too clean?
put a booster in front!



where have we seen this scenario before?
oh, it was with those real tubed amps.

[DougH]

I suspect you won't get much distortion out of it. Distortion from those kinds of amps comes from driving the power stage.

[slideman82]

Yes, it break really nice at full gain, talking 'bout the clean preamp (based on Mesa MKI), sound realy rocker, and I have to put my 9V JFET booster as last stage just to sound similar to Santana's Abraxas (I always say this)... distorted channel is a 24V Dr Boogey, it has no contendant! I must try it in a 4x12 cabinet... videos will come soon!

[fuzzo]

Hi guy,

I decided which preamp I'm gonna do, I'll make a plexi preamp like. I drew a new version :



First, I'll do it without "boost" circuit, if I need more drive i'll add it with a push pull pot .

You see mistakes on coponement value ? especially the Resistors on Jfet source ?

[fuzzo]

Yeah I made it. But the sounds not really good (but not bad too).

There's any crunch sounds, just clean.

I juste made it with the 1st and 2nd gain stage and the buffer before tonestack.