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Old 2nd August 2008, 02:57 AM   #1
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Default heater-less Pentodes

hey-Hey!!!,
Anybody ever try building a tube-style circuit with MOSFET's? Something like the last two stages of a Citation II?
cheers,
Douglas
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Old 5th August 2008, 03:29 AM   #2
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I've duplicated the Tabor local feedback pentode in SS using high voltage cascode connected MOSFETs. Very good performance. Transconductance that tubes can only dream of.

A couple of things to keep in mind... Even though the characteristic curves of a MOSFET looks like a pentode, the MOSFET has high gate capacitance. Using the cascode connection resolves the issue of input capacitance and makes a very good emulation of a pentode. For best performance you need to bias the circuit so that you have a minimum of 20 volts across the lower MOSFET in the cascode.

Comparing the distortion of the tube vs. transistor versions of the same amplifier was interesting. Similar harmonic profiles but the MOSFET version was over 10x less distortion.

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Old 5th August 2008, 12:20 PM   #3
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hi Gary,
Thanks for the insight. I had planned this circuit with just 10 MOSFET's. Two in each half of a LTP/diff amp input stage, two for the tail load and two in each phase of the power stage. E-Linear connection to 50% taps on the plate winding. First cut will run stock S-258-Q 5k a-a outputs. Next cut will run rewound/modified with lower percentage taps for less FB.

Probably use these 900V MOSFET's everywhere except the CCS:
http://www.fairchildsemi.com/ds/FQ/FQP4N90.pdf
cheers,
Douglas
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