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

[temol]

And some oscilloscope shots.
IN - 1kHz, 200mV p-p. Yellow trace - output, cyan trace - out from TRE potentiometer. Different settings of GAIN and PENTODES pots. TRE @ 15, LOW @ 9.
MMBF5457 and MMBT5088

G @ min, P @ min


G @ 9, P @ 12


G @ 9, P @ max


G @ 12, P @ min


G @ 12, P @ max


G @ 8, P @ max

 

[temol]

By the way - Q1 and Q2 with Vp about -1.4V. I lowered the value of the source resistors from 47k to 43k. Now Q1 and Q2 are biased around 5.4V. With 47k it was almost 5.7V.

 

[Chuck D. Bones]

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.

 

[temol]

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.

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 D. Bones]

That was my thinking too.

 

[bluedmc777]

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

 

[Chuck D. Bones]

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.

 

[Einzeln]

Any experiences using 12 or 18 volts?

 

[Chuck D. Bones]

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.

 

[Einzeln]

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.