Fw: Re: [Amps] Re: AB1 v. AB2

skipp isaham nospam4me@juno.com
Sun, 1 Dec 2002 20:18:26 -0800


-
: --------- Forwarded message ----------
: From: BEAR <bearlabs@netzero.net>
-
> >> >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?
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: 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.
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Beyond expected AB1 operation ... distortion goes up big time.
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: In practice the output signal is merely clipped when it reaches 
: the point where in an AB2 amp grid current would start to flow.
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In practice, the stage gain is probably more than enough that 
flat topping occurs well before AB2 happens. 
- 
: 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. Guaranteed this amp will not run into AB2 
: no matter how much signal you put in the front end... : -)
- 
Most all the above amplifiers would flat top well before 
AB2 occurs.  AB2 might not be possible because of the 
limitations of the preceding stage(s).   I'm under the 
impression you are comparing apples and oranges. 
-
The driver in your audio amplifier is often a phase 
inverter stage which is not a large drive power provider 
by design.  It can only do so much...
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: 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... 
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Both are pretty well explained in his 18th Edition Radio 
Handbook. Green Cover in Hard copy. 
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: I'm guessing there is some difference in the circuit... 
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Yep, there is...
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: ..requirements in general, but honestly I can't recall anything 
: about it at the moment.
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Different animals...
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:     _-_-bear
> -  Rich..., 805.386.3734, www.vcnet.com/measures.
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-
cheers
skipp

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