This Week on the Breadboard: the Subdecay Variac (Brownout OD)

Good info!
The asymmetric clipping from the preamp section is quite apparent. Interesting that higher setting of PRE causes the preamp clipping to become more symmetric. The clipping at the output becomes more and more symmetric as PENTODES is turned up.
 
Chuck D. Bones said:
Good info!
The asymmetric clipping from the preamp section is quite apparent. Interesting that higher setting of PRE causes the preamp clipping to become more symmetric. The clipping at the output becomes more and more symmetric as PENTODES is turned up.
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We have two inverting stages here. I'm sure both are capable of clipping the signal. So maybe at some point we have more or less symmetric signal. I'm on a vacation now so can't test the circuit to check what's going on.
 
Chuck, what do the following changes do to the circuit? “After playing with it a bit, I bumped C7 up to 1nF, C11 up to 15nF and changed the TREBLE pot to B250K.” I assume it’s altering the voicing but in what way?

We need an official PedalPCB “Boneyard Edition” of this circuit. 😁 @Robert
 
It's minor retuning of the tone stack. Moves the mid notch down in freq. You can check it out using Duncan's Tone Stack Calculator. Changing TREBLE from A to B taper changes the knob sweep, that's all.
 
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No, but I can tell you analytically what to expect.

Raising the rail voltage can improve the headroom or gain of the JFET stages (Q1 & Q2), but only if you adjust the bias accordingly.

Raising the rail voltage will have little or no effect on the BJT stage (Q5 & Q6) because the clipping threshold of a differential amplifier is determined by Vbe, not rail voltage.
 
Ah thanks for that. Had to google Vbe and take a better look at the schematic. So this pedal is a high headroom stage into a low headroom stage?
By raising the voltage the headroom of the preamp part increases (the JFet parts) but poweramp part (transitor) is not effected.
 
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Einzeln said:
So this pedal is a high headroom stage into a low headroom stage?
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Yes. The PENTODES knob controls how hard the output stage is driven.

Einzeln said:
By raising the voltage the headroom of the preamp part increases (the JFet parts) but power amp part (transistor) is not effected.
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That's correct. Raising the power supply voltage does not automatically increase the preamp headroom. We also need to readjust the bias to take advantage of the higher rail voltage.
 
I think the designer of this was really onto something unique with this pedal. Can’t believe it’s discontinued. I just finished my second build of it and will probably do a third with Chucks mods. I wish more designers thought outside of the box like this instead of just tweaking the same old designs. Or maybe some do and I just don’t know about them. Do boards exist with your mods @Chuck D. Bones?
 
gtt said:
So why the BJT transistors for the phase inverter? Why didn't the designer stick with JFETs throughout? Is there a tonal advantage here?
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For a diff pair to work, the Vbes need to be matched. In the case of JFETs, Vp needs to be matched and we know what a PITA that would be. I also think that the BJT's exponential transfer function is better than the JFET square-law transfer function in this application. But hey, break out the breadboard and give it a try.
bluedmc777 said:
We need an official PedalPCB “Boneyard Edition” of this circuit. 😁 @Robert
Click to expand...

Cooder already made one. Take a look at the violet box.
 
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