[AMPS] PA Recommendation Wanted

[Rich Measures]

>Rich Measures wrote:
...
>>Why not adjust the grid bias V so that the grid can not draw current with 
>>max. drive applied? 
>>
>Not all tetrodes require grid bias voltages that conveniently match up
>to the available drive power and the grid swamping resistance. Adjusting
>the grid bias may work for Rich's amplifiers because they use big
>tetrodes that require more than 100V of grid bias. Since the peak RF
>voltage from the nominal 100W exciter is already pretty close to the
>manufacturer's recommended bias voltage, a relatively small adjustement
>to the bias voltage may make sense. But that strategy obviously can't
>work for tetrodes that only need 35-50V of bias.
>
  My strategy, Ian, is to choose a value of grid termination resistance 
that roughly affords the needed peak RF drive V to the grid with exciters 
that develop 100v-p (100w rms) to 141v-p (200w rms) across 50 ohms. 
-- For tubes that require  50v to 70v of grid bias, like the 4CX800A, a 
voltage-halving bifilar stepdown transformer driven, 12.5 ohm grid 
termination is used. 
 -- For tubes that require 100v to 140v of grid bias, a directly-driven 
50 ohm grid termination would be used.  
-- For tubes that require 200v to 280v of grid bias, like the 5cx1500a 
and 8169, a voltage-doubling bifilar stepup-transformer driven, 200 ohm 
grid termination is used.   
-- For tubes that require greater than 300v grid bias, like the 8171, a 
trifilar stepup-transformer driven, 450 ohm  grid termination is used.  

>>>I also like Ian's suggestion of building the grid supply 
>>>like the PA is for AB2 (regulated with a stiff shunt regulator), even 
>>>though you plan on running it on AB1. 
>>>
>>Eimac says you don't need a regulated bias supply..  
>
>Rich, please can you quote exactly what Eimac says on this topic, and
>where. 

Eimac does not state such in so many words.  The max. allowable R of the 
supply is found in the technical specifications for each tube.  For 
example,  on  page 4, column 1. of the specs. for the 4CX15,000A, the 
maximum R is stated to be 100k ohms.  In your opinion, Ian, is a 
regulated supply, or an unregulated supply, most likely to have an output 
R of 100k?  

>Similarly, please can you quote what and where Eimac says that the
>correct method to configure a class AB1 amplifier is to adjust the grid
>bias for zero grid current, 

You've got to be putting me on, Ian.  AB1 means zero grid current. .  

>and the adjust the screen current for the
>recommended ZSAC.

I thought I read it in *Care and Feeding....*.  After spending about 15 
minutes therein, I couldn't find it.  However, the practice makes sense 
to me.  To explain:  There are two means of setting ZSAC: by adjusting 
grid V, and by adjusting screen V.  Grid V adjustment is constrained by 
the requirement that zero grid current be maintained---which leaves 
choosing to adjust the screen V.  Does this make sense to you?
Rich---

R. L. Measures, 805-386-3734, AG6K   


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