Experimenting with Fetzer Valve

[percyhornickel]

I´ve been experimenting with the R.O.G. Fetzer Valve with some K68A Jfets (VP: -0.93V / IDSS: 5.83 mA). I tried the drain bias 4.5V and 6V (fetzer cal) with a good response but biasing the drain at 2.5V (my case) I´ve got an amazing crancked tube sound (I´m using a 22u cap with the source resistence).

Trying to have a gain control I took de Deep Blue Delay input and output buffers (taking out the PT2399 stage) and put the fetzer stage to mix it with the dry signal, I put all this in a spice simulation and it seems to work.

According the Boy in Well PDF info (DIYGUITARPEDALS) the mix would be ratio 1.1:1 in favor of the wet signal.

I was thing to put a tone control like (faux ocho does) before the mix pot but this would just affect the wet signal guess, I really don´t want to change the output buffer so maybe I could put it after.

Could somebody help with this idea?, this mod fetzer valve really sounds great (I like it more than my DIY TS!!!)

Thank you


Percy

[antonis]

I'm not sure that I can get your issue but I presume you realize that you mix two signals 180^o out of phase..

P.S.
R13/R14 values should (and had to) be much lower..
(with the addition of high value bias resistors, of course..)

[GibsonGM]

Does the great cranked tone have more to do with the driving by the input opamp than the response of the Fetzer?   A good way to play with these is to cascade a few, and THEN mess around - I think you'll really enjoy doing that

[Steben]

I presume you did not want the use of a gentle input stage anyway? 

[percyhornickel]

I'm not sure that I can get your issue but I presume you realize that you mix two signals 180^o out of phase..

P.S.
R13/R14 values should (and had to) be much lower..
(with the addition of high value bias resistors, of course..)

Antonis you are right the jfet change the phase at the output so maybe taking the fetzer input before the first buffer could work, I am trying to make it work and playing with the gain, HPF and LPF...    ...lot of work with spice, maybe later I´ll be putting all in the breadboard or testing.

GibsomGM..  ..yes I could try to play some jfets in cascade..       

[GibsonGM]

I do like the idea of hitting the stage with the opamp, and the idea of mixing the clipped signal in, too  

If you add another FET gain stage, that will solve your inversion issue, of course.   

[antonis]

Antonis you are right the jfet change the phase at the output so maybe taking the fetzer input before the first buffer could work,

Work for exactly what purpose..??
(meaning, in such a case, you'll have nothing to mix..)

Could you plz post a brute (just building blocks "boxes") diagram..??

P.S.
It should better for C6 to be placed right after U1 output (before R2/R4/R10 branch)..
(U2 DC gain R9/R2 should be negligible..)

[percyhornickel]

I`m just trying to have an alternanive kind of "gain control" over this config of the fetzer valve (Vd=2.5V), The second buffer I thought it could change the mixed phase since both (input buffer and the jfet) change the original signal...        ...as I said, I`m not an electronic expert (petroleum engineer) with some circuits over my shoulders already (TS, Rat, Pulsar, Delays, Big Muff, Fuzz Face, Tone Bender, Chorus, compressors, env filters, etc etc...)

Percy

[antonis]

Much better now..
(at least, from mixing signals phase point of view..)