Chuck D. Bones
Circuit Wizard
The Variac is a versatile pedal and sounds great in stock trim. But I can't leave well enough alone so I thought I'd experiment with alternate circuits for the last stage. What Subdecay is doing with the transistor diff amp is they are emulating the long-tailed pair phase splitter in a tube amp. BJTs are not triodes; their transfer function is completely different. Triodes clip much more gradually. But that's not the whole story because Subdecay is also emulating the pentode output stage and that clips fairly abruptly. Still, transistors are not tubes and neither are LEDs, but I tried using LEDs and an opamp to make symmetric clipping that behaves similar to the transistor diff amp. It works well, uses fewer parts and is inherently balanced.
Ladies & Gentlemen, Presented for your dining and dancing pleasure: The Suburban Decay Maniac
I left the 1st two stages more-or-less intact. I fiddled the biasing (R3 & R7) to accommodate the different JFETs. I really like the 2SK193s because they are transparent. They have very low capacitance and do not color the tone. I used a PF5102 for Q3 (other FETs will work) because it has a higher Idss and I'm running Q3 at a higher drain current than in the Variac. I reduced R10 3x to get the higher drain current req'd to drive the tone stack. I rescaled and retuned the tone stack, adding a MID control. Just about everything after the tone stack is new. I used a CA3130 for U1 to ensure maximum headroom, however a TL071 will work. The POWER AMP control takes the place of the Variac's PENTODES control and does the same thing. The POWER AMP control has 6dB more range than the PENTODES control. The high Vf of the blue LEDs ensures the 3rd stage plays clean at low to moderate signal levels, just like the Variac. The low LED current causes the distortion to come on gradually. At high gain settings, the LEDs glow visibly, but not brightly. I found the L/C switch on the Variac to be a little too subtle. The addition of the MID control and higher gain capability of the POWER AMP control more than make up for its absence. There's some top-end roll-off after the 3rd stage, tuned the same as the Variac. The opamp's low output impedance made an output buffer unnecessary.
PREAMP - Controls the gain and distortion before the tone stack, just like on a Marshall. Turning it up makes lots of compression as well as plenty of even and odd-order harmonics. PREAMP influences how the pedal responds to picking and guitar volume.
TREBLE-MID-BASS - It's a Fender tone stack, which means if you turn all three knobs down to zero, no sound gets thru. It's tuned warmer than a Fender tone stack. If you want it to sound like a Variac, then set MID to zero.
POWER AMP - Acts like a tube amp's Master Volume control. Controls the gain and distortion after the tone stack. Turning it up makes smooth odd-order harmonics, like the output stage on a push-pull tube amp. Use it in conjunction with PREAMP to set the overall compression and distortion.
VOLUME - Depending on the PREAMP and POWER AMP settings, unity can be anywhere from 9:00 to 3:00.
Turn PREAMP and POWER AMP all the way down, VOLUME all the way up, and it's clean boost (ok, almost clean). For a bluesy tone, keep PREAMP below 10:00 and use POWER AMP to control the distortion.
This pedal is pretty much an amp-in-a-can, but it's not trying to sound like any particular tube amp.
Ladies & Gentlemen, Presented for your dining and dancing pleasure: The Suburban Decay Maniac
I left the 1st two stages more-or-less intact. I fiddled the biasing (R3 & R7) to accommodate the different JFETs. I really like the 2SK193s because they are transparent. They have very low capacitance and do not color the tone. I used a PF5102 for Q3 (other FETs will work) because it has a higher Idss and I'm running Q3 at a higher drain current than in the Variac. I reduced R10 3x to get the higher drain current req'd to drive the tone stack. I rescaled and retuned the tone stack, adding a MID control. Just about everything after the tone stack is new. I used a CA3130 for U1 to ensure maximum headroom, however a TL071 will work. The POWER AMP control takes the place of the Variac's PENTODES control and does the same thing. The POWER AMP control has 6dB more range than the PENTODES control. The high Vf of the blue LEDs ensures the 3rd stage plays clean at low to moderate signal levels, just like the Variac. The low LED current causes the distortion to come on gradually. At high gain settings, the LEDs glow visibly, but not brightly. I found the L/C switch on the Variac to be a little too subtle. The addition of the MID control and higher gain capability of the POWER AMP control more than make up for its absence. There's some top-end roll-off after the 3rd stage, tuned the same as the Variac. The opamp's low output impedance made an output buffer unnecessary.
PREAMP - Controls the gain and distortion before the tone stack, just like on a Marshall. Turning it up makes lots of compression as well as plenty of even and odd-order harmonics. PREAMP influences how the pedal responds to picking and guitar volume.
TREBLE-MID-BASS - It's a Fender tone stack, which means if you turn all three knobs down to zero, no sound gets thru. It's tuned warmer than a Fender tone stack. If you want it to sound like a Variac, then set MID to zero.
POWER AMP - Acts like a tube amp's Master Volume control. Controls the gain and distortion after the tone stack. Turning it up makes smooth odd-order harmonics, like the output stage on a push-pull tube amp. Use it in conjunction with PREAMP to set the overall compression and distortion.
VOLUME - Depending on the PREAMP and POWER AMP settings, unity can be anywhere from 9:00 to 3:00.
Turn PREAMP and POWER AMP all the way down, VOLUME all the way up, and it's clean boost (ok, almost clean). For a bluesy tone, keep PREAMP below 10:00 and use POWER AMP to control the distortion.
This pedal is pretty much an amp-in-a-can, but it's not trying to sound like any particular tube amp.
