The Valve Wizard

[tubetvr]

Morgan Jones only describe the 2 cases as class A1 loading and class B loading "only one output loaded" what he obviously is referring to is class B2 i.e with grid current, he doesn't even mention AB1 or B2 in the text describing the concertina and for now I can not find anywhere else where he say anything about AB1, AB2 or B2, (or A2 for that matter). So I think what he want to stress that is important, (and the only thing that is relevant in this case) is if the following stage draw grid current or not, i.e put different loads on the outputs of the concertina.

Regards Hans

 

[EC8010]

Don't forget that a Class B output stage switches Miller capacitance between its valves. As one valve is on and has gain, it has Miller capacitance, but the other is off and has zero Miller capacitance. At the next zero crossing point, the two valves swap roles, so you effectively have a capacitance being switched between the two outputs of the concertina.

 

[hemgjord]

Hello, maybe it is starting to be a little OT but I would be intrested in some more elaboration on the switching of miller capacitances.
In a single ended stage it is very clear, when you leave class A the tube stops conducting-no amplification-Av=0, Cag(Av+1)=Cag, OK.
In a PP-output stage on the other hand the other tube still working and through autoformer action the potential between anode and grid should still be Av+1 also on the non conducting tube hence miller capacitance should still be present.
Could anyone with some experience pls enlighten me.

Olof

 

[EC8010]

In a PP-output stage on the other hand the other tube still working and through autoformer action the potential between anode and grid should still be Av+1 also on the non conducting tube hence miller capacitance should still be present.

That is a very good point. The Cag capacitance still exists within the valve and it still has the large voltage across it, causing a current to flow that is indistinguishable from the current that would flow into a larger capacitor from grid to ground.

 

[ray_moth]

That's what was confusing me too. I realise that the book can't be expected to explain every statement it contains, otherwise it would probably be ten times as big, but this statement left me trying to visualise what was happening and I couldn't grasp it.

Anyway, I was just curious. It doesn't really matter to me, because I much prefer to have a buffer between the splitter and the OP stage, in the form of either a diff gain stage (like Williamson) or cathode or source follower.