[Amps] Re: AB1 v. AB2

[BEAR]

Rich wrote:

> >> >You can build an audio amp or modulator that is capacitively coupled
> >> >to the grids of the output tube and there is nothing you can do to drive
> >> >it into AB2 (grid current).
> >>
> >> How is DC grid bias applied to the grids?
> >
> >Separate minus voltage power supply fed via high impedance (resistors) to
> >prevent loading down the drive voltage...
> >
> **  What stops DC grid-current from flowing through such resistors
> whenever the grid is driven positive?

The grid current that flows is so insignificant that no appreciable current
*can*
flow, so the tube does not reach beyond AB1 in practical terms.

In practice the output signal is merely clipped when it reaches the point where
in an AB2
amp grid current would start to flow.

A good example of an audio amp like this is the good old Dynaco Stereo 70... the
schematic
of it and the MkII, MkIII and Mk IV which are all virtually identical circuits
are on the
web I am sure. Guarranteed this amp will not run into AB2 no matter how much
signal
you put in the front end... : -)

If I get around to it, I guess I'll have to look and see what Bill Orr had to
say about this
topic with respect to RF amps... I'm guessing there is some difference in the
circuit
requirements in general, but honestly I can't recall anything about it at the
moment.

In the end, if the amp can be run into grid current, then it is by definition an
AB2 amp,
no matter how you cut it.

    Regards,

                _-_-bear

>
>
> -  Rich..., 805.386.3734, www.vcnet.com/measures.




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