[AMPS] Linear with 4CX800A

[Rich Measures]

>The Communications Quarterly Winter 1995 issue have one article on using 
> the 4CX 1600B.
> While on this subject, do anyone know if there is any problem related to
> using 2 each 4cx 800A in class AB1. 
- Considerations -
1.. Grid current -/- make the bias V adjustable, and set it for virtually 
zero grid current with max. drive applied.   /  Using the bias V adj. to 
increase ZSAC  results in grid current, and distortion.  The best way to 
increase ZSAC is to increase screen V, as long as you do not exceed the 
max. rating.  
2.  Terminate the grid with 12.5 ohms, and drive it with a 4 to 1 bifilar 
toroidal transformer.   /  Connect the 12.5 ohm grid-R as close to the 
grid as possible, triple-bypass the cold end to ground with c. 100pF, 
1000pF and 10, 000pF.  Connect the grid bias supply to the bypassed end 
of the R.  
3.  Screen supply:  Due to the reverse Screen current tendency, obtain 
the screen voltage from a zener-string shunt-regulator.  20v 5w zeners 
are ok.  A multi-position switch to short/unshort zeners is a good way to 
adj. the screen V.  Feed the zener string though a high power R connected 
to the anode supply.  
4.  VHF stability: Do not put an inductor in series with the 
grid-termination R - - unless you want to build a VHF oscillator.  /  
Design a VHF suppressor with a low VHF Rp & Q.   /  Place the tune-C 
close to the anodes.  
5.  Tuning - Loading:  Adjust the amplifier for Svetlana's specified 
screen current.  Remember that for a Class AB1 amplifier, you always tune 
with the screen current meter - - not with an RF output meter, as is the 
case with g-g amplifiers.   

> I have two (not matched)( is that a problem ?)

Not likely.  If you put roughly 5 ohms of low-L R in series with each 
cathode, the currents tend to equalize.  

>of them, but remember 
> someone saying that it was hard to get low enough C on
> 10 meters. 

This is just another case of someone propagating an 'old wives tale', 
Harald.  Whenever you use two tubes, the output load R is 1/2 as much as 
it is with one tube, which means that you Need 2x as much C on 10m to 
achieve the recommended Q of 10 to 15.  The same thing applies if you use 
three tubes. 
-  For additional information see *Amplifiers* at:
http://www.vcnet.com/measures
Rich---

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


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