Driver question

[Rikaro]

I think that 50W clean indicates at least some AB2.
The 15W per pair with AB1 I mentioned is already quite optimistic. Of course depends on B+ measured at full output.

 

[Diabolical Artificer]

In case of doubt about operation class, class AB2 starts when the grid signal Vpp exceeds twice the grid bias voltage

I thought AB2 is when the control grid starts pulling current which starts to happen around 0.5V Vgk and then definitely by +1v Vgk.

Re designing a a good driver AFAIK it has to be able to drive the OP valves grid capacitance, where 1w is sufficiant in most cases but it obviously depends on how many valves your driving and coupling caps come into play too. Pat Turner used EL84's configured as LTP's with a CCS to drive paralleled PP OP stages. I've found 6N6P's/12BH7's paralleled in a LTP configuration also do a good job.

Andy.

 

[jhstewart9]

I thought AB2 is when the control grid starts pulling current which starts to happen around 0.5V Vgk and then definitely by +1v Vgk.

When G1 of a tube is driven +ve in an RC coupled circuit the rectification that occurs drives G1 more negative
as charge accumulates on the coupling cap. That further cuts off plate current.
The effect is referred to as blocking, CF or IT coupled cct fixes the problem but needs some power to do it. 👍

 

[Rikaro]

I thought AB2 is when the control grid starts pulling current which starts to happen around 0.5V Vgk and then definitely by +1v Vgk.

How would that be conflicting with my statement?

Re designing a a good driver AFAIK it has to be able to drive the OP valves grid capacitance

Using datasheet values, the grid input capacitance of a 6L6 in triode mode (including Miller effect) should be below 80pF per tube.
So a driver source impedance of 30k should suffice for good audio response with 3 6L6s.

 

[Rikaro]

When G1 of a tube is driven +ve in an RC coupled circuit the rectification that occurs drives G1 more negative
as charge accumulates on the coupling cap. That further cuts off plate current.
The effect is referred to as blocking, CF or IT coupled cct fixes the problem but needs some power to do it. 👍

Yes, capacitor coupling with class AB2 is likely to cause blocking distortion.

 

[catalin gramada]

That clarify a lot. Back to homework. Thank you gents !

 

[Diabolical Artificer]

Thanks JHS, I was forgetting the effect of the coupling cap and yes to drive a valve into AB2 direct coupling to a CF or whatever is needed. You also need a negative supply. CF with a CCS works well in this case.

Andy.