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I'm wondering about using DC coupling for the first two stages of an amp using DHTs. This would be balanced push pull - two diff pairs, the first with a CCS in the tail. Can the second stage be done DC coupled with DHTs?I thought about a simple experiment using 3a5s into 1J6s - both common cathode double DHTs, but it's not so simple since the 1J6 has a max anode voltage of 135v (a lot of small DHTs have low max anode voltages). If it's DC coupled, the cathode/filaments would be raised, so does this make it easier or harder to configure?
Might need to go into a transformer. Or could it be done with an anode load CCS and a resistor in the tail? I could use a pair of DHTs with a higher max anode voltage of course. Couple more sockets.
Haven't constructed this type of circuit yet with DHTs - any ideas how to make it work? Any schematics to refer to? - thanks, Andy
Follow Ups:
Take a look in the archives or google it. It is quite famous.
My memory of the Loftin White is that it's a SE circuit where the driver is direct coupled to the outpit stage. I'm talking push pull and directly coupling to the driver - that's more Mullard in a way. I hadn't thought of direct coupling to the output stage - interesting thought - thanks for bringing it to my attention!
Andy,Don't confuse max. allowable plate to cathode potential with the plate to ground potential.
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. 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.
CCS loading the 1J6 anodes is (IMO) a good idea, as you can set the operating point spot on.
Eli D.
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
> > 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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