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In Reply to: Re: Can you DC couple two DHT stages? posted by Eli Duttman on April 10, 2007 at 06:36:39:
Don't confuse max. allowable plate to cathode potential with the plate to ground potential.>>no - just wondering if being a DHT makes any difference to anything!
I think you can DC couple both legs of your diff. amp to a 1J6. You'll have to dedicate an alkaline "D" cell to the 1J6 filament and construct a CT from a resistor pair in parallel with the filament.>>
Something like a couple of 100 ohms or 470 ohms? I could put in a pot for exact balance, have 47 ohm and 250 ohms in higher wattages.
The CT gets connected to a RC network that puts the filament an appropriate number of V. more positive than the grid is, with respect to ground. It's the same old cathode bias setup, with the cathode positive with respect to ground for DC and at ground (courtesy of the cap.) for AC.>>I have a good floating DC filament supply - don't need batteries. So you reckon its "business as usual" - just put a suitable bypassed resistor on the bottom of the second diff pair. Sounds easy enough.
CCS loading the 1J6 anodes is (IMO) a good idea, as you can set the operating point spot on.>>
I haven't built one of those yet, but I have some designs suitable, like in Morgan Jones book. May thanks for the suggestions! Andy
Follow Ups:
> > I have a good floating DC filament supply - don't need batteries. So you reckon its "business as usual" - just put a suitable bypassed resistor on the bottom of the second diff pair. Sounds easy enough. < <Floating is only part of the requirement. Each channel's 1J6 needs a separate, dedicated filament supply. The only connection the filament can have to "the outside world" is via the RC bias network.
Balance pots. sound like a good idea to me. You might get away with AC heating, which is a LOT easier to set up dedicated and floating than DC is. The 2 V. filament (I checked TDSL) is not convienent for readily available batteries and separate, regulated, DC supplies are a PITA.
Eli D.
separate, regulated, DC supplies are a PITA> >I'm OK for those - they're fairly easy to build when you sit down and make a few over the weekend, and they work very well. I've long given up on batteries - now THAT's what I call a PITA! Sounds good - I'll build it and see how it works. I've used both tubes before but not together. Andy
Sorry if I'm saying the obvious, but I think what Eli was trying to explain in particular is that the filaments of the two double triodes need to be at different potentials if you use direct coupling - they are acting as cathodes, after all. The only way you can arrange this is with separate filament supplies, one of which could be a battery, of course, but it doesn't have to be.
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