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Re: mosfet in place of triode

9/9/2005 2:56 AM
R.G.	Re: mosfet in place of triode I checked too - dang, you're right. Use of a protection zener is so necessary that many MOSFETs have them built it - but not all. I mentally add a protection zener to every MOSFET I use, but apparently I did not draw it in to the article. They are there in the MOSFET Tremolo article. I'll correct the schemos. There's a 12V zener from gate to source on every discrete MOSFET - always. The gates are always about 20V thick, and a 12V zener is proof against almost any transients. There's always a 470 ohm to 1K gate stopper resistor there as well in all linear audio circuits. Otherwise the device can oscillate at frequencies too high for a cheap o'scope to see. So, if you use a discrete MOSFET, always use a 12V zener, with the zener reverse biased in normal operation to limit gate voltage to +12/-0.6V. That will guarantee that the gate voltage limits will not be exceeded. The MOSFET tremolo is exactly a replacement for the Fender tube driven tremolo, almost part-for-part, with a phase shift oscillator driving a current into a neon bulb which modulates an LDR. I'm not familiar with the TOV at all. Your description sounds like it has an LED/LDR module with the LEd driven by a 555. That could work, I guess.

9/9/2005 2:56 AM

R.G.

Re: mosfet in place of triode

I checked too - dang, you're right. Use of a protection zener is so necessary that many MOSFETs have them built it - but not all. I mentally add a protection zener to every MOSFET I use, but apparently I did not draw it in to the article. They are there in the MOSFET Tremolo article.

I'll correct the schemos. There's a 12V zener from gate to source on every discrete MOSFET - always. The gates are always about 20V thick, and a 12V zener is proof against almost any transients. There's always a 470 ohm to 1K gate stopper resistor there as well in all linear audio circuits. Otherwise the device can oscillate at frequencies too high for a cheap o'scope to see.

So, if you use a discrete MOSFET, always use a 12V zener, with the zener reverse biased in normal operation to limit gate voltage to +12/-0.6V. That will guarantee that the gate voltage limits will not be exceeded.

The MOSFET tremolo is exactly a replacement for the Fender tube driven tremolo, almost part-for-part, with a phase shift oscillator driving a current into a neon bulb which modulates an LDR. I'm not familiar with the TOV at all. Your description sounds like it has an LED/LDR module with the LEd driven by a 555. That could work, I guess.