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Mosfet or other semiconductor substitute for cathodyne?
Mosfet or other semiconductor substitute for cathodyne?
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Old 23rd July 2019, 04:03 PM   #91
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Originally Posted by skyra View Post
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.
Originally Posted by skyra View Post
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.)

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