Through serendipity I acquired a set of transformers for a McIntosh 2100. Having built several tube/power MOS hybrids over the past 5 decades, my thoughts started to turn in that direction. Hmm an amp having tubes and output transformers; a Mac no less. Then for less than $60 (including shipping) an MX112 power xfmr popped up on the trading site. That sealed the deal. What I was doing before was to run a voltage amp stage followed by a frame grid cathode follower driver, and then the MOSFET output stage, which was of course configured as a source follower complementary output. I also obtained a pair of anode chokes (which I will run as cathode chokes) to provide for the drive stage interface. Will provide updates as this project proceeds.
WOW I checked out the power transformer; it has a 5.3V 4A winding and a 170V 110mA winding. Since I just bought 11HM7s from Doug I am good to go with just the existing transformer set. The 11HM7's can be configured as 5.5V heaters. A perusal of Mouser and Digi-key turned up the power MOS in TO-247 packaging. Configured as source followers they are going to have about 25nF input capacitance. At a radial velocity of 200K that will present an impedance of -j2000 to the drive tubes. To provide 40V peak drive then I need 20 mA; 30 would be better to reduce distortion. Now that 170V winding will be running about 250V to the plates, providing a plate dissipation of about 7.5W.
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