vht valvulator

[euronymous0001]

i would try to make a fet version of this, im not sure tho on some of the values which i copied from a schem i found somewhere  
also, R3 i think would be a 100k trim, C2 is indicated that it is left blank on the original ckt.



Edit: here is the value for R10 

[PRR]

> R3 i think would be a 100k trim

No. This is probably not supposed to be "clean". Those values will leave the JFETs pretty jammed-up, "saturated". Q1 cuts the bottom of the wave, Q2 cuts the top. What it is evidently supposed to do.

I doubt that "all" J201 will give the same action. But to trim you really ought to fiddle R4 and R8. Set Q1 Drain down near 2V or 3V up from ground. Set Q2 Source up 2V down from the supply voltage. Play. Listen. Tweak.

The ratio R5 R6 will surely need tweaking. Make R6 a 50K rheostat.

Some combination of bias and R5 R6 loss will give a symmetrical phatt rounded square wave. Other settings will give asymmetric sweet or reedy tones.

C2 tames the high-end zizzle. Without it, it might be like pins in your ears. If too painful, 470pFd is a starting value. 10nFd will be like cotton in your ears.

There's no need for both C5 and C6.

Some part values are odd.

R1 can be anything 10K to 100K. If you have 54.9K, use it, but it's not magic (unless you think it is). It might better be right AT Q1 gate rather than out to the left of R1 C1, though it may not matter.

R7 is also very un-critical. 220K, 2Meg, whatever you find in your pants-cuff.

C4 does not need to be anywhere near 0.22uFd. 0.01uF will pass full bass. Guitar amps often use 1,000pFd here.

Likewise C1 can be 0.02u for bass and might like to be much less for guitar. Good place to try a 0.01u-470p range.

What is R10, "3MM.2"?? I'd drop anything 100K to 1Meg and call it done. 3-something Meg may be a bit large for some 10u electrolytics' leakage.

On one hand, "conventional design" would have a cap between Q1 Drain and R5. For these values, this "mistake" only shifts the distortion point, no harm done.

[anchovie]

This is probably not supposed to be "clean".

It is. The Valvulator is marketed as a mojo-laden tube buffer. The R5/R6 combination heavily attenuates the signal boosted by the first stage (which would be unlikely to clip a guitar plugged straight in) and it's then to a follower and out, basically a simple tube pre-amp with the equivalent of a gain knob glued at a low setting.

Doing a FET emulation of it isn't the best idea, in my opinion, as that first stage will have far less headroom than a 12AX7 with high B+.

[mac]

Doing a FET emulation of it isn't the best idea, in my opinion, as that first stage will have far less headroom than a 12AX7 with high B+.

Except for the input cap and that strange resistor value, the first stage is quite similar to a fender amp input stage.
I would use 40V **can be dangerous**, trim R3 to get 26V at the drain of Q1, and I would change the follower stage or copy one from a fender schematic with tone control and master volume.

mac