I've been stuck for awhile, trying to ponder some class-d scheme for a power output stage, plus trying to keep cost way down. Sort of a losing battle. It is ending up like a "me-too" product.
And so a brain fart today got me thinking about a tubed version. Played around with the concept for awhile trying various configurations and finally got something to work using off-the-shelf parts that aren't too expensive. Once again, the classically beautiful PA-10 output stage comes to the rescue!
The design (draft #1) uses a 269EX power transformer for both power supply and output stage coupling. I use the 190-0-190 taps driven by a 6080 triode pair. The 120-0-120 output (normally used as primary) offers a nice 2.5 reduction in output impedance to about 200 ohms. Hey, with your typical 3W TT motor that amounts to 5Vac drop.
I tap the unused 6.3V winding on the output for optional ac feedback. This would provide a little more control and accuracy of output voltage, plus reduce the output impedance. Interesting, the combined mu of the 6080 and output step-down works out to roughly 1.0 gain. I added some local resistive feedback so that can be tuned too. The driver stage is a 12AX7 pair running at 0.5mA each as a PP driving a 124E interstage tranny (such a tranny is required for PA-10 topology).
Pretty simple. Power levels not that high. Two tubes, three trannys (no choke in supply). It should provide roughly 100Vac output for 1Vac input at 60Hz. Stability can be set using rolloff cap across 12AX7 plates and in the series feedback (like any other opamp).
Not sure what the limitations are yet, but it should drive a 5W to 10W motor pretty handily. Standing current of the 6080 is only 25mA each, at 170V (-90V grid). Output voltage should go up to about 130Vac before clipping. Depends on delivered power. Output stage remains in total class-A. The PP stages are designed to eliminate as much even order harmonics as possible.
Now to buy parts and build up an amplifier!
jh
