A Tale of Three Variacs

Chuck D. Bones

Circuit Wizard
Left: Variac with auto-balance & Germanium final stage. Built on a Big Noise board from Cooder.
Middle: Brownout OD.
Right: Boneyard Suburban Decay Maniac. Last stage uses blue LEDs as a soft clipper in place of the differential pair. Built on Vero.

3 Variacs 02.jpg

3 Variacs innards 02.jpg

Like most JFET-based pedals, getting the biasing right is essential. Plan on selecting JFETs and/or tweaking source resistors. On the Brownout, I have the drains of Q1 & Q2 set at 5.4V and 5.0V, respectively. Because the drain resistors are so large, you must use a DMM with 10MΩ input resistance to measure the drain voltage. The cheap DMMs with 1MΩ input resistance will read 10% too low. Q3, Q4 and Q7 should have low Vp (<1V) and Idss > 2mA.

I used selected 2SK193s for all five JFETs in the Big Noise Variac. Source resistors on the first two stages were tweaked to achieve the desired bias. Slight retuning of the Tone Stack. Differential stage has two 104NU71 Germanium transistors. The auto-balance keeps the Ge transistors at the correct bias point. Board has true-bypass relay switching.

I followed the Brownout build docs except for the transistors. I used selected 2N5246s for Q1 & Q2. None of the 2N5457s I had came anywhere close to biasing correctly in the 1st two stages. The other three JFETs are selected PF5102s. The diff pair is a vintage dual matched pair Silicon transistor.

The Suburban Decay Maniac uses the same 1st two stages with selected JFETs and tweaked source resistors. I rescaled and retuned the Tone Stack and added in the MID control. I replaced the diff pair 3rd stage with an LED soft clipper.

Suburban Decay Maniac v1.1b sch.png

The left and middle Variacs sound very similar. The Maniac on the right has more gain and compression in the last stage and is a little darker compared to the two Variacs. I think they all sound great. Distortion is easily controlled from the guitar and the two gain knobs. Note decay is very smooth and natural sounding, no fizziness.

I had a problem with high-freq oscillation in the Maniac due to trace proximity on the Vero. The trace for Q1's gate ran right next to the trace for Q2's drain. One cut fixed that.
 
Last edited:

music6000

Well-known member
Left: Variac with auto-balance & Germanium final stage. Built on a Big Noise board from Cooder.
Middle: Brownout OD.
Right: Boneyard Suburban Decay Maniac. Last stage uses blue LEDs as a soft clipper in place of the differential pair. Built on Vero.

View attachment 19731

View attachment 19732

Like most JFET-based pedals, getting the biasing right is essential. Plan on selecting JFETs and/or tweaking source resistors. On the Brownout, I have the drains of Q1 & Q2 set at 5.4V and 5.0V, respectively. Because the drain resistors are so large, you must use a DMM with 10MΩ input resistance to measure the drain voltage. The cheap DMMs with 1MΩ input resistance will read 10% too low. Q3, Q4 and Q7 should have low Vp (<1V) and Idss > 2mA.

I used selected 2SK193s for all five JFETs in the Big Noise Variac. Source resistors on the first two stages were tweaked to achieve the desired bias. Slight retuning of the Tone Stack. Differential stage has two 104NU71 Germanium transistors. The auto-balance keeps the Ge transistors at the correct bias point. Board has true-bypass relay switching.

I followed the Brownout build docs except for the transistors. I used selected 2N5246s for Q1 & Q2. None of the 2N5457s I had came anywhere close to biasing correctly in the 1st two stages. The other three JFETs are selected PF5102s. The diff pair is a vintage dual matched pair Silicon transistor.

The Suburban Decay Maniac uses the same 1st two stages with selected JFETs and tweaked source resistors. I rescaled and retuned the Tone Stack and added in the MID control. I replaced the diff pair 3rd stage with an LED soft clipper.

View attachment 19735

The left and middle Variacs sound very similar. The Maniac on the right has more gain and compression in the last stage and is a little darker compared to the two Variacs. I think they all sound great. Distortion is easily controlled from the guitar and the two gain knobs. Note decay is very smooth and natural sounding, no fizziness.

I had a problem with high-freq oscillation in the Maniac due to trace proximity on the Vero. The trace for Q1's gate ran right next to the trace for Q2's drain. One cut fixed that.
Cool Builds Chuck, You obviously like this circuit a lot to build 3 variations of it, it had Mr PedalPCB intrigue for a long period of Time also!;)
 

Chuck D. Bones

Circuit Wizard
It's a Marshall emulator that sounds good to me. Some of the others MIAB pedals were a disappointment.
I would like to reduce the compression a bit on the Maniac. But the other two are good to go.
 
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cooder

Well-known member
Of course I had to race in the workshop to change the drain resistors on Q1 and Q2 to 75k and 68k respectively bringing the voltage to 5.1 and 4.7 V, allowing for my crappy meter underread.... cheers! Now it rips even harder, maybe, it did sound good before and it does sound rippa now.
 

Dan0h

Well-known member
Very cool. I have yet to built a MIAB. Need to. I agree that JFETS sound so great when dialed in.
 

benny_profane

Well-known member
Great looking set there! Thanks for the detailed write up as well.

Why do you use the switched jack to ground the input when there isn’t a lead inserted?
 
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Chuck D. Bones

Circuit Wizard
Thanks!

Grounding an unconnected input jack serves a couple purposes.
1. It prevents hum if the input is not plugged in. This is particularly important with high-gain pedals because the hum could be very loud.
2. It can take the place of an anti-pop resistor on the input because the input is never left floating.

If I'm out of grounding jacks, I'll use an ungrounded jack, but my preference is to use a grounding jack.
 
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