Mosfet or other semiconductor substitute for cathodyne?
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Gnobuddy
diyAudio Member

Join Date: Mar 2016
Quote:
 Originally Posted by skyra Well, then this guy is similarly ignorant because he is using the exact same idea....
No, he's not; the problem with the circuit you showed earlier isn't the topology, it's the values.

In your post #80, your circuit diagram (schematic) shows 131 volts at the FET source, and 137 volts at the drain. That means there are only 6 volts across the MOSFET; if the source rises 3 volts, the drain falls 3 volts, leaving 0 volts across the MOSFET. Even if you had a mathematically perfect MOSFET, the circuit you showed would clip as soon as the output approaches 3 volts peak (6 volts peak-to-peak.)

The problem is the 134 volts at the gate. It's far too high to allow direct-coupling the MOSFET. For a MOSFET source-o-dyne, the gate should be at about one-quarter of B+, in this case, (270/4) or about 67.5 volts. This will allow nearly 135 volts peak-to-peak from each of the two outputs, instead of the tragically small 6 volts you have now.
Quote:
 Originally Posted by skyra BTW the amp (NIVS) works perfectly with the described mod, pumping out 17 Watts of undistorted sine wave from 30Hz to 20k. How is it possible ?
When you say "the amp", I assume you mean the tube amp you showed in post # 88.

How is it possible? Because this one is designed correctly. The schematic shows the MOSFET gate at 75-90 volts. With a B+ voltage of 300 - 360 volts, this will put the MOSFET source at one-quarter of the B+ voltage, exactly where it should be for maximum headroom before clipping. The MOSFET will be able to put out about 150 volts peak-to-peak at each of its two outputs before clipping.

Again, compare this with the roughly 6 Vpp maximum output from the circuit in post #81 (which can easily be reduced to nothing at all if a different 6N2P with slightly different parameters is plugged in.)

-Gnobuddy

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