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

Rich 2@vc.net
Mon, 2 Dec 2002 02:09:04 -0800


>> --------- Forwarded message ----------
>:  From: Rich <2@vc.net>
>> To: <bearlabs@netzero.net>, "AMPS"
>>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 permissible 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.  
>
Skipp:  Non-standard attribution (:) marks do not work as well as (>).
-
>In his audio amplifier examples, the "grid circuit" is 
>very often a much higher designed input Z.  
>- 
True, but this discussion group is about RF amplifiers.

>:  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.
>-
RR

>If he wrote that, then he is a little off base... like RF amps, 
>there are rules and guide lines for the amplifiers he 
>described in his examples.  

Like unto wearing football shoes to play a basketball game. 

>The grid resistors are 
>chosen for operation with the phase inverter circuit 
>layout delivering drive voltage to the push pull output 
>stage. The resistors are chosen as loads for the 
>phase inverter, the driving voltage developed across..
>-
>>In practice the output signal is merely clipped when 
>>it reaches the point  where in an AB2 amp grid 
>> current would start to flow.
>-
>I would prefer sage flat topping or hard limiting as a 
>more descriptive "better label". I'm sure the stage 
>is into compression/hard limiting/flat topping or 
>what every label you want to use long before AB2 
>comes to the table. 
>-
>: **  Would ?   The reality is that some grid current Does 
>: flow and IMD increases.
>-
>Big time increase...
>
Good for clearing a chunk of the band and keeping the competition at bay.

>>web I am sure. Guaranteed 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 over driven.
>-
>It does... in both his and your examples. 
>- 
Another amplifier "expert" ?

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