Fetzer valve as buffer?

[PBE6]

Hi all,

Long time listener, first time caller! I have a question about using the Fetzer valve essentially as a buffer, and whether or not it's a worthwhile addition to a distortion design. After drowning my sketchpad in ink, I thought I'd turn to the experts. 

I'm trying to design a new distortion pedal that will feature both pre-distortion and post-distortion EQ. In coming up with design I decided to use two identical James EQ stages (i.e. bass/treble controls) as they are straightforward to implement and surprisingly flexible. The James EQ requires low input impedance and high output impedance for predictable operation, which I have implemented before with good results using a single TL072. However, I noticed that if I end up using a JRC4558 for the gain stage then I can save myself a chip by replacing the input opamp buffer with a JFET-based one - thus the Fetzer valve idea.

It seems from reading various posts here that the Fetzer valve does its best imitation of a triode when not being driven hard, so I thought using it as a buffer would be ideal to add some quasi-tubish character to this pedal. I would use an MPF102 as it has a higher clipping threshold than a J201, and let the gain stage handle the volume and grit duties.

My question is, would the character of the Fetzer valve be completely swamped out by TS-style distortion? Would moving it to the output buffer stage retain any more of the character, or will the gain stage overdrive it (even if clipping diodes with a forward voltage less than Vp of the MPF102 will be used)?

Any thoughts would be appreciated!

[Davelectro]

Does your circuit run on 9V?

If yes, an MPF102 fetzer might not have enough gain. Not even unity.

[PBE6]

It will run on 9V.

Any attenuation from the MPF102 will be made up later in the chain as the James EQ sections require about 20 dB of makeup gain each and there will be a separate gain stage for the distortion part. I could see this being a problem if it's moved the the output buffer though.

[pappasmurfsharem]

Does your circuit run on 9V?

If yes, an MPF102 fetzer might not have enough gain. Not even unity.

I made a "revisited" Fetzer Valve ala the ROG implementation with a J201, it had much better headroom than I expected, but you need to do al the measurements recommened.

I ended up needing to bias mine for a little over 5 volts for the ldss and Vds for that particular J201.

It didn't clip until I hammered the strings.

It was much over unity too.

[wavley]

I used a Fetzer as the input buffer for my pedalboard until I got a little tube mixer that I modded to be the input stage of a fender and it worked pretty great.

[PBE6]

Thanks pappasmurfsharem, I'll see if I can find a J201 with a high Vp in my bunch.

What kind of pickups were you using? My friend has a PRS with fairly meaty humbuckers, I wonder if even a high Vp J201 will do the trick.

Is the Fetzer even worth it as the front or back end of a Tubescreamer-style pedal? Will the tube character get squashed by the clipping diodes?

[PBE6]

wavely, did you notice any change in the sound of your overdrives/distortions when using the Fetzer buffer vs. no Fetzer? Or was the Fetzer character basically swamped out?

[blackieNYC]

I didn't find the fetzer to be clean.  I think it measured 5-6% thd.  100-120mv input.  I would think you want neutrality.

[wavley]

wavely, did you notice any change in the sound of your overdrives/distortions when using the Fetzer buffer vs. no Fetzer? Or was the Fetzer character basically swamped out?

The biggest difference was my clean sound, of course I like my cleans to have just a little bit of dirt and my pedalboard in general reacted a little more like plugging straight into my amp, but I can't say that I remember any huge differences in the sound my dirt boxes, there was a bit better touch sensitivity.  Of course the caveat is that my Fuzz Face has a pickup simulator built into it, my Truly Beautiful Disaster and Big Muff both pretty much destroy any nuance, and my modded English Muffin already has a tube for an input stage, so my experience here might not apply to your dirt box and it's certainly worth breadboarding.

[PBE6]

blackieNYC a strictly neutral buffer is not what I'm looking for. I'd like the Fetzer to add a quasi-tubish quality to the sound. Some distortion of the waveform is desirable (and necessary) to achieve triode-like harmonics, but overt clipping and fuzz would work against it. According to other threads I've read, driving the Fetzer too hard will cause it to lose whatever triode-like qualities it has.

My question is whether or not the tube-like Fetzer character will be lost if I put the tube-like signal through a Tubescreamer-style gain stage with clipping diodes. Will all the diode clipping and extra harmonics essentially destroy the Fetzer sound? If so, I could just use a simple JFET buffer like one on the AMZ website and save my trim pots for another circuit.

[pappasmurfsharem]

I didn't find the fetzer to be clean.  I think it measured 5-6% thd.  100-120mv input.  I would think you want neutrality.

Thats strange using the recommend value mine was super clean.

[Ice-9]

You certainly can use a Fetzer circuit as a buffer, try out a J201 or a 2N5457. depending on the values of the other components it can be set up to be clean whilst adding harmonics valve like. Try it with a 12k drain (or a 20k vr) resistor and a 1k2 source resistor, take some voltage measurements from drain to gnd and source to ground. With the J201 aim to a get a voltage of 5.8 to 6.2 on the drain to gnd and about 200-300mv from source to gnd. Adjust the resistances till your in that ball park or simply fit 1k ish source resistor and adjust the the drain vr until it sounds right.

At 9v you will get plenty gain which if you need to you can keep it clean whilst it should add that valve like warmth. Not really a buffer in this config to be honest

[PBE6]

wavley, good to know. From the sounds of it heavy distortion is going to crush anything fed into it. I wonder if the Fetzer might be better as an output buffer then? Essentially just coloring the output of the pedal. Provided it doesn't get driven too hard.

wavley, have you ever tried putting the Fetzer last before the amp with a distortion pedal in the chain?

Will have to try bread boarding this on the weekend.

[Pojo]

I may be missing something here, but when I think of a jfet buffer I think of a source follower which is meant to NOT color the signal but providing the advantages of high Z in and low Z out with as little change in gain as possible. The benefits of a Fetzer valve depend on the stage being an actual common source gain stage with carefully selected source and drain resistors. So if I got this right you're not actually looking for a true buffer, but rather a couple fetzer gain stages to impart a tube-like coloring to the tone?

[Davelectro]

I don't recommend a j201 for a Fetzer-type input stage. It will probably clip too fast.

You can use an MPF102 if that's all you have, but you should select a proper one (based on the fetzer calculator results). I'd shoot for a voltage gain between 1 and 2 if subtle colouring is what you are after.

[Ice-9]

I don't recommend a j201 for a Fetzer-type input stage. It will probably clip too fast.

You can use an MPF102 if that's all you have, but you should select a proper one (based on the fetzer calculator results). I'd shoot for a voltage gain between 1 and 2 if subtle colouring is what you are after.

J201 can be configured to not clip at all if you don't want it to.

[Davelectro]

ROG's article says the maximum clean input signal for a fetzer stage is roughly equal to the jfet's pinch off voltage (+ / -) . Since most j201 have -or at least they used to- a pinch off voltage lower than 1V, i'd say there could be unwanted clipping with certain signals.

That is, of course, if you don't apply some sort of attenuation.

[PBE6]

Ice-9 Won't deviating too much from the calculated values lose the tube character?

[PBE6]

Pojo yes you're right, I was a bit sloppy with the terms. I do want a single Fetzer "gain" stage to impart the sound, but it was in the back of my mind that the MPF102 has unity (or less) gain if used in this circuit.

[PBE6]

Davelectro I thought the MPF102 would be the best choice because it has the highest Vp of the JFETs I have right now. I want to keep the JFET as close to the "tube" operating conditions as I can which means it shouldn't be clipping at all. I think if I go lower with a 2N5457 or 2N5458 I might run into trouble with humbuckers.