Author Topic: Convert a Fetzer Valve to SRPP style Booster  (Read 96 times)

Ralfg

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Convert a Fetzer Valve to SRPP style Booster
« on: January 08, 2018, 11:02:46 PM »
Hey Guys,

I've been playing around with Fetzer Valve style JFet gain stages
http://runoffgroove.com/fetzervalve.html

and was wondering if there is some sort of formula or pattern that could be used to convert this to an SRPP style gain stage.
http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

The Fetzer is nice to control the gain with the source resistor on the JFet, but I was hoping to avoid having to use a trimmer to adjust the bias.  The SRPP style booster is nice because it self biases, but in my experimentation it seems to have much more gain and I would have to really play around with the source resistor and the capacitor between gate and source of the top JFet as it really changes the overall character of the tone either more bass or less depending on the cap.

I assume there is no direct sort of conversion and I'd probably have to rely on my ears to dial it in, but I was wondering if anyone has had any experience here and if there are any sort of patterns or design decisions that would help in the conversion.  Perhaps even it isn't even worth it and you need design your circuit with one or the other.

Appreciate any opinions any may have.
Dr. Von Fuzzbrauer @ Rocket Surgeon Effects Pedals
https://www.smallspacesband.com/

Max

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Re: Convert a Fetzer Valve to SRPP style Booster
« Reply #1 on: January 09, 2018, 02:30:41 AM »
Well, that looks damn close to the third stage of the Thor circuit (Q3 + Q4), still from ROG.
http://www.runoffgroove.com/thor.html
EDIT: It's also used here with a couple of diodes replacing the 10M resistor to ground:
http://www.runoffgroove.com/ginger.html
« Last Edit: January 09, 2018, 04:25:23 AM by Max »

287m

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Re: Convert a Fetzer Valve to SRPP style Booster
« Reply #2 on: January 09, 2018, 09:58:30 AM »
yeah, great topic

for mu-amp, no need to bias and matched. cmiiw

sorry, not for hijack thread
but maybe someone here can give example how to convert Mu-amp style to Jfet opamp, i love that  ;D

tcpoint

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Re: Convert a Fetzer Valve to SRPP style Booster
« Reply #3 on: January 09, 2018, 11:19:59 AM »
http://www.diale.org/triode.html

The mu-Valve is pretty interesting.  I guess you could always add a resistor to make it an SRPP circuit.  Note, the gain is going to be way higher than a Fetzer Valve.  I much prefer using another transistor as buffer after the mu-amp.  See the mu-amp in the Cascadia design by jubal81.  He uses a simple bjt buffer.  The main knock on the mu-amp is the high impedance output.