Super Hard On with 18v

[nero1985]

Hey guys, i have a SHO on my breadboard, sounds great BUT out of curiosity i wanna put in 18v and see how that sounds, before i dod that i want to understand what can happen, I have a 9.1v zener between the Source and Gate, I'm not sure what will happen with the zener since it's lower wattage, also would the MOSFET handle it? all my caps are rated 25v and up so i should be ok in that area.

all the zvex stuff and the AMZ booster have zeners between the G & S so i'm curious if changing that zener for something else would allow the use of higher voltage

[knutolai]

Accorrding to the datasheet from ON-semiconductors the max Drain-Source voltage is 60Vdc, so you will probably be OK.

[nero1985]

thanks! but should i remove the zener?

[knutolai]

thanks! but should i remove the zener?

Im guessing you still need over-voltage protection whatever the supply-voltage. I dont understnd how the protection diode work, but Im eager to suggest a 18v zener as the 9v zener matches the supply voltage in the original circuit. You'll just have to wait for someone more knowledgeable to chime in.

[Keppy]

The MOSFET can only survive about 20v gate-source. The protection diode needs to be there, and should be larger than any possible G-S voltage caused by normal use but smaller than 20v. I tend to use 12v because I have a bag full.

The DC voltage on the gate will be half the drain voltage (referenced to ground). With the gain maxed, the source is grounded and the drain tends to sit a little under half supply, lets say 9v to be safe. 9v D-S means 4.5v G-S, so any reasonable signal won't diode clip at max gain, since it would take another 4.6v to cause the zener to conduct, which means over 9v peak-to-peak signal before the input clips. An min gain, the drain biases to about 2/3 - 3/4 supply voltage, but the source is also elevated, to about 1/4 - 1/3 supply. If the drain voltage is about 13.5v (3/4 supply), then the gate is about 6.75v (half of drain voltage) and the source is about 4.5v (1/4 supply), so diode clipping at the input has become even less likely as it would take almost 7v positive signal or 14v peak-to-peak input. These numbers are kind of fuzzy , but they should give you an idea of why a 9v zener will not have any effects on a guitar signal when run at 18v.

The zener doesn't conduct during normal operation, by the way. It's only there in case of static discharge, to which MOSFETs are extremely sensitive. Taking it out doesn't change a thing, until one day it doesn't work because of a static discharge that you probably didn't even notice.

[nero1985]

Thank you for the info! I'll put 18v on it and mess with it tomorrow

[PRR]

A 5V Zener should be ample for any MOSFET.

20V is the upper limit.

Anywhere between is fine. If you have one, leave it alone. If you are building, and have a pile of 12V, that's cool.