>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.
>
** Eimac rates the maximum permissable grid bias ps source resistance
for its tetrodes and pentodes. This determines how much grid-current can
flow. In my experiences with AB1, there is a tradeoff in IMD when grid
current flows -- i.e., the amplifier is driven into AB2.
- The argument that high resistance is used to prevent loading down the
drive voltage does not hold water because, in RF service, the typical
grid-terminating resistance is 50-ohms (or 200-ohms through a 1:4
transformer) in order to present a 50-ohm termination for the radio.
>In practice the output signal is merely clipped when it reaches the point
>where in an AB2 amp grid current would start to flow.
>
** Would ? The reality is that some grid current Does flow and IMD
increases.
>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... : -)
>
** I can not believe that no grid current will flow when the amplifier
is overdriven.
>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.
>
** I recall nothing Orr wrote about this subject.
>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.
>
- indeed
- Rich..., 805.386.3734, www.vcnet.com/measures.
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