split push-pull output stage

[Brian Beck]

cerem,

You're right that this won't work with pentodes - the sensitivity (of plate current to plate voltage) is way too low. And yes, in that case varying the screens is an option. But he's showing triodes here. I don't see this concept as fundamentally flawed vis-à-vis a servo for the grid voltages. The heater voltage changes would work and I’ve seen that done before, but there you might be changing other parameters too much, upsetting symmetry. The servo has a long history in grid driving, but it's not for everyone, either on principle or due to the suspected "sound" it might introduce. I happen to think that with some care, servos can work fine. I would be more tempted to drive grids with a servo, but this idea is an interesting variant worth tossing around.

[andrew_whitham]

Originally posted by Brian Beck
Wheeww. That’s more than I can handle! Maybe you’re making all this too hard. I think the important thing is that once these servos converge, the “DC” current through one primary winding is forced to be exactly the same as the “DC” current through the other primary winding. If the two windings have the same number of turns and balanced DCR values, then the “DC” voltage drop across each winding will be identical too. If they don’t have balanced DCRs, then there will be a slight voltage imbalance, the same as in regularly configured OPTs – but it doesn’t matter since it’s the amp-turns that count – and that is balanced by the servos. Then, it’s up to AC signal (which the servo is insensitive to) to make the usual flux changes needed to pass the signal through the OPT. I don’t see a fundamental problem here.

Indeed, BudP I watched that go SOOO far over my head it was like watching a Shuttle Launch. And I used to think that the electrical engineering part of my course was run by a transormer sadist. Honestly I need to study the reply. I think you answered exactly what I was asking with theory in spades. Honestly it's like being in Dr Whalley's class again...

BrianBeck. the IDEA is to avoid noise introduced through the marginally more normal output servo which equally can be arranged to get p-p balance. BUT I figured if you can alter the shape of things in the pre-amp stage it OUGHT to maybe perhaps, possibly scale up. It seemed like an elegant solution to that problem. Driving the scheme here is to me not so easy to me but thats a different story... I Just want to know at this point if its worth persuing. If I spend £400+ on OPT's I'd like to think they were pretty OK. Regardless of experimental configurations.

Cerrem, thats the Idea, But I dont see how it will mess up the P-P operation. I'm going for balance - with triodes (or UL ) the whole thing is to equalise the magenetising currents in each half. Am I being real dumb here? & missing the importance of the voltages? (s'possible, actually more than possible)
I disagree with the thought of contolling heater voltage to alter the characteristics of the tube though - YES I am aware that this may not be completely logical but it seems wrong to me, there are plenty of published documents saying that its BAD when you run Vf low, or high...
OTOH I like the Idea of the servo that sleeps. Worth some investigation. How do you 'fix' the value you arrive at though...

So Thanks all, I think Its worth me seeing if I can get these OPT's in the way that I want, I can try then, even if I end up doing this in the normal way. 'course any results and you'll know first.

Andy

[Brian Beck]

Andy,

OK, just to play devil's advocate, what noise are you wanting to reduce? Can you be more specific? I'm not aware of noise generally being a problem with a well-implemented grid-driving servo. Bias drift if there is a transition to Class B, is a problem, but there are solutions for that too (in either implementation).

[andrew_whitham]

Brian

before I need to go sleep, I want to say that "plan B" IS the bias servo's. I have no problem with those at all, Maxim have published some info on very low noise regulators which ought to make it pretty much equivalent to battery bias. (6nV * Mu in a couple hundred volts?)

Basically I want to try this, If it works great, If not then servo's re the way. I will need a maintenance free output stage. I dont have the kind of patience to play with bias settings regularly, I'd forget and cook something

Andy

[Brian Beck]

With servos, I will tend to add an additional low-pass filter after the silicon, at perhaps an order magnitude higher frequency than the servo corner. This gets rid of any noise and tiny opamp signal outputs above, say, 10Hz. You don't want to get the two poles too close, because this will raise the Q of the resultant second-order filter, but 10:1 or even more is probably OK. I just don’t think that noise needs to be a problem with either type of servo. I’m still seeing only disadvantages to the implementation (but not fundamental flaws) of the B+ servo, relative to standard grid-driving servos. A B+ servo is possible, but I don’t know why you’d bother when the grid servo would be so much easier. I’m open to advantages of the B+ servo that I haven’t thought of yet, though, and that’s why asked. And I would not want to discourage experimentation here. Let us know how it works.

Have some sleep, Andy - you've earned it!

[andrew_whitham]

Brian this was just one of those ideas. I want to give myself some flexibilty. I'm a bit noise paranoid, the idea of amplifying grid noise is... well, enough to have me looking at this. Like so many things in audio I just want to give it a go

I'll admit this is not a properly thought through thing, If it had been I'd be reporting how well it worked, or not reporting how badly it sucked. For instance How far the voltages need to be off to compensate for tube mismatch - got no idea, Do you have cathode bias or a fixed negative voltage - No idea!

Also got some of these "ideas" for class AB in Bias servos, but you can do that with a normal transformer

Cheers

Andy

[SY]

FWIW, I've compared using B+ servos to grid servos in a low level cathode follower. At this point, I'm with Brian- a grid servo makes much more sense and a good one is very easy to implement.