Class D tube amp from Japan

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#4
Kjeldsen said:
There were tubes in the first class D amps in the 50's
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Class C but not class D. There was no way of perfecting the PWM and the valves wouldn't switch off fast enough as they are thermionic. So class D was never produced using valves. Clive Sinclair was the first to produce class D using XB113 (Edison/Westinghouse) and OC71/2 (Mullard) germanium transistors with the "X10".
 
#6
JonSnell Electronic said:
the valves wouldn't switch off fast enough as they are thermionic.
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So what?
Tell that to people who built microwave equipment in the 40's .
Or radio equipment since the 20's. (or earlier).
Thousand times higher frequencies (both on and off switching) than any used in any Class D amp.
So class D was never produced using valves.
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That has a different reason, mainly that tubes must be coupled to a speaker load.
If using transformers, the classic solution, we can't wind one which both passes the Audio frequency component we are trying to amplify and the HF pulse width modulated chopping frequency.
But it's not a tube problem, at all.
In fact, we can very well build a working Class D amp if we use a tube OTL circuit.
The reason they are not built (except maybe as an experiment) is that the efficiency increase gets swamped by high saturation voltages, power wasted in heating filaments, etc.
 
#7
Crowhurst did a series of articles and built prototypes of tube class D amps about 50-60 years ago. Plus ca change...

Although not really class D, the Berning amps of recent years share much of the same features- MOSFET bridges, high frequency carrier, high efficiency. Clever and creative design there.
 
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