Q About Zener Diode on the Slow Gear SG-1

[Paul Marossy]

On the Slow Gear, there is a 5.6V zener diode called out on the schematic (D1). What is the function of this diode? Do you think it would it work with a 5.1V zener?

[Zero the hero]

The zener diode will keep the FET "closed"... Well, a little explaination about how the SG-1 works.
The fet in this configuration acts like a variable resistor.
If you feed the gate pin of a fet with a voltage that exceed the max Vgs(off) parameter of the fet, no current will pass between drain and source of the transistor. So, looks no signal will pass through it!
But, if you lower the voltage on the gate, a few current will pass, and the more you lower the tension on the gate, the more current will pass from drain to source: now the guitar signal passes through the FET.
The slow gear works in this way: when no signal is applied, the fet stays "closed" because the zener diode keeps 5.22 volts on the gate pin of the fet (that's a reply to your question). When a signal is applied, the envelope detector (IC1, TR4-5-6) charge a capacitor which slowly decrease the voltage on the fet: it passes from 5.22 (stuck fet) to 2.88 (the fet allows guitar signal to pass through it). Again, as the guitar signal lowers, the voltage on the fet goes slowly to 5.22.

So, the zener diode keeps the fet stuck.
If you plan to use anothe fet, you have to choose an appropriate zener value: 2N5459 has Vgs(off) parameter of -8.0 volts, so a 5.6 zener diode doesn't provide a sufficient tension on gate to keep the fet stuck: the guitar signal will pass even if the note is not played...

More questions? Feel free to ask, I'll reply when I'll have time... I'm supposed to work now  :oops:

[Paul Marossy]

Thanks Zero, I just read the other thread with your reply and it's starting to make sense to me now.  :wink: