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Old 15th September 2003, 01:13 AM   #11
SY is offline SY  United States
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Coincidently, I've been using a 2B today to drive the ESL panels for watching football. The 'Niners lost, but it sounded just fine.
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Old 15th September 2003, 01:21 AM   #12
dhaen is offline dhaen  Europe
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SY,

I'm waiting for you to show us details of your direct-drive ESL amp design. If I remember right you theatened us with it in the tubes forum at the beginning of the year.
Class AB or not, it must be worth a view!

Cheers,
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Old 15th September 2003, 01:55 AM   #13
SY is offline SY  United States
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I've got a low power version (900 volt) of it prototyped and working- not much SPL, but it demonstrated the concept. I'm trying to gather fundage for the Real Deal. High voltage tubes and caps are expensive, drat it, but if I can pull it together, I think it will be awesome.

In outline, what I've got going is a pair of 6LF6s (I'll need something bigger for the 5kV version, a HV zero-bias triode is the goal) running screen drive near class B. The plates are loaded by a CT choke with the B+ fed to the center tap. The stators are connected directly to the tube plates. In my proof-of-concept, I used one of the Acoustat transformers (a treble one) as the choke, connecting to the secondary and letting the primary float. I'm sure there's a commercial choke that would work better, but this is something I had laying around.

The drive is a tube-FET hybrid cross-coupled circuit I've been using for at least 20 years, using a 5691 and a pair of 2N5462s; this couples to a 5692 diff amp to get enough juice to run the output tube screens, though a nice step-up transformer would be even better (though costly). Screen drive gives you great linearity at low currents, but the effective mu is 5 or so, so you need a lot of gain and a lot of swing from the driver. The output screens are driven by a pair of high voltage Hexfets configured as source followers.

The trick to making this work is to connect the diaphragm bias supply "backwards."

And, as usual, I'm two months behind schedule
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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