Phase 90 FETs and zener

[Jay J]

Yesterday I finished Phase 90 (Tonepad layout), using matched FETs (BF245) with Vgs of -2,30. Zener is 4,7V. Maybe two years ago I had built this effect, but didn't even know about matching fets. The sound was really nice.

This new one has parts that I ripped off of my own, big and ugly pcb (different FETs). Still I'm not pleased with the result compared to the earlier build. Or I just remember it all wrong...

How should FETs and zener relate to each other? Should I lower zener value or what? Is there a "rule of thumb", that "if vgs is something then zener should be this"?

[R.G.]

The zener just has to be bigger than the Vgs that will turn the JFETs off so that the LFO DC level can be adjusted somewhat lower than the zener and get the JFETs into their variable resistance region.

I believe that many tales of people who love some JFET phaser but can't duplicate it are due to the distortion the JFETs have in their variable resistance region. People get hooked unknowingly on subtle distortions that are not easily heard as distortions.