>
>
>Rich wrote:
>
>> >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.
>
>That is not the *only* way to present a 50 ohm termination to the amp's
>input and still not provide a low Z impedance to the grid.
>
** How ?
>Typical is not terribly relevant to the discussion of amplifier class
>definitions...
>
>Grid current drawn = AB2 not AB1.
>
>>
>>
>> >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.
>
>Sure IM increases, if you've just clipped the amp! The amount of grid
>current in
>the situation I mentioned is so infintesimal as to not make any practical
>difference -
>the tube is not going to conduct any more due to that small an amount of
>current.
>
>>
>>
>> >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.
>
>Ok. :- )
>
>But current isn't going to be sunk by the driver in that circuit.
>
>>
>>
>> >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.
>
>Whatever - I'm sure he mentions amplifier classes and operation... if not
>there
>then
>in another text.
>
>>
>>
>> >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
>
>Which is my point. AB1 != AB2.
>
>>
>>
>> - Rich..., 805.386.3734, www.vcnet.com/measures.
>
>--
> _-_-bear
> http://www.bearlabs.com
> Silver Lightning Interconnects & more...
>
>
>
- Rich..., 805.386.3734, www.vcnet.com/measures.
|