[AMPS] Tetrode poor IMD performance

[2]

>
>While not claiming to know the answer to Tom's question, I would like to add
>some information using a similar tetrode.
>
>The 4CX1000 was originally not designed for linear amplifier service.  

So why does the grid have a 0W max. rating?  There is but one class of 
operation that makes zero grid current --  i.e., AB1 [linear amplifier 
service]. On the 4CX1000A/8168 sec sheet [5-1-70], the only listings are 
for Class AB1.

>Eimac specifies the 3rd order distortion as -23 db and 5th order distortion is
>specified as -26 db.  

I could find no such IMD information on the 4CX1000A/8168 in my Eimac 
literature.   However, the quite similar 4CX1500B is rated at -43db 3rd 
and -47db 5th ref. 2-tone - which = -49db and -53db ref. PEP 
respectively.  

>The Collins 30S-1 used that tube, but used a cathode
>drive configuration (which has inherent feedback) and applied additional
>feedback to arrive at a clean amplifier.
>
>Without feedback, though, a 4CX1000 with a resistive grid and perfectly
>stabilized voltages ought to yield no better than a -23 db 3rd order
>distortion.
>
It seems a bit doubtful that the 4CX1000A has c. 100-times as much 3rd 
order distortion as a 4CX1500B.

>The 4CX1500 tube was designed specifically for linear service.  Although hte
>physical construction looks similar, Eimac specifies that the performance
>is -44 db 3rd order distortion - a whopping 21 db improvement in distortion
>figures.  I understand the main difference between the tubes is the grid
>construction.

Both tubes have grids that are designed to avoid intercepting electrons.

>The Russian tetrode that is used in the Ten-Tec amplifier tested by QST was
>not designed for linear service.  

Why is the tube-type?  Is it made by Svetlana?

>The main quality is that it is
>considerably less expensive than a 4CX1500 tube.  The distortion levels may
>indicate that construction is similar to the 4CX1000.
>
>Colin  K7FM
>
cheers, Colin

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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