JFET Rs question

Started by mattm, February 11, 2021, 01:58:06 PM

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Hey guys,

Fairly new to pedal building but learning as fast as I can.  Anyway, currently looking at JFET versions of tube gain.


In the fetzervalve they've removed the cap from Rs so it's just a resistor and the calculator reflects that.

Looking at this JCM800 schematic (is there a better one, anyone know?):

From the fetzer page: "This resistor should not be bypassed by a capacitor to produce the appropriate amount of negative feedback to change the square law of the FET into an approximation of the three-halves law of the triode"

So they removed the cap to get some negative feedback.

Looking for an opinion:
-Remove all the source caps (if there is one) and replace all the set Rs values with trims (or calculated values based on the calculator/my ears)?  Any implications there that aren't obvious (meaning I'll miss them for sure)?
-Would that change the tone too much and if so how would I calculate Rs when there's a cap involved?

I fully admit that I'm a noob so go easy on me.  I'll mess around but I like learning the why behind things (as well as tinkering).

By the way this is my first post.  Love this forum.

Anyone ever try a valve caster type version of something like the JCM emulator?  Should I avoid that rabbit hole?  I built a valve caster as last year's winter project (and my first foray into pedal anything) and I thought it sounded pretty cool.


Hi & Welcome..

The only Rs bypass cap is the first stage one (680n) but its value can more be considered as "frequency selective gain" rather than fully bypassing Rs..
(22 - 47μF should be needed for the later..)

Rs value is primarily calculated for desirable Drain current so it should be better to implement trimpots for respective Drain resistors..

In JMC800 Emulation, first stage Rs bypass cap absence doesn't affect tone due to JFEts negligible Gate current (no impedance reflection effect)..

IMHO, it might be interesting dealing with 100nF feedback cap value to get frequency responce close to your taste..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..


first, Welcome to the forum,

the exact value of Rs is there to set the DC operating point, capacitors don't change that, so weather you have a source cap or not doesn't matter for the Rs value.
we generally recommend using a trimmer to set the Drain resistor to bias it right, since Fets generally are different enough to not work with the same value drain resistor as another of the same type.

there are a few Marshall In A Box type pedals floating around on this forum, just type in ''MIAB'' in the search box on the top right of the screen, you'll find dozens of similar pedals.

one of my first builds was a Valvecaster inspired pedal too, i added an opamp gainstage in front, an optional crossover distortion in between the stages and a big muff style tone stack in a feedback loop of the 2nd triode. both triodes had emitter follower transistors directly coupled to the plate, this cleaned up the sound in lower gain settings. i used an E80CC, which is an expensive milspec tube, slightly higher gain than a 12AU7, and uses double the heater current, glass is twice as thicc, and it produces a really nice forward mid range when over driving.

cheers, Iain
friendly reminder: all holes are positive and have negative weight, despite not being there.