[Amps] Designing the Cleanest Linear with RF Negative Feedback

[w6ah@comcast.net]

-------------- Original message -------------- 
From: Steve Thompson <g8gsq at eltac.co.uk> 

> 
> 
> wc6w at juno.com wrote: 
> 
> > P.S. -- Steve T. posited in another post that there was little intrinsic 
> difference between a vanilla 4CX350A and the 4CX350FJ. The data sheet claims 
> 10dB lower distortion at equal power. That's a lot. I wonder if Eimac uses a 
> striped cathode aligned grid scheme, as in the 4CX600J & 4CX1500B, in the 
> '350FJ? 
> In the data I have, only the FJ has IMD data in the specs - but it's at 
> either 100 or 250W out, even if that's average power it's less than the 
> tube is capable of. It reminds me a lot of the ratings I'm used to with 
> semiconductors where generic parts are respecified at lower power in 
> order to get linearity for specific applications. I stress it's just my 
> guess, and I'm wide open to correction. 

OK then here it comes Steve. :-)
First let's start with an excerpt from Care and Feeding of Power grid Tubes which is posted on Eimac's Website page 96:
 [I]It is possible by clever manipulation of electron ballistics within a given tube structure to alter the transfer characteristic and minimize the distortion products. Several tubes developed recently by Eimac have transfer characteristics which significantly reduce distortion.[/I]
Now go back to page 93 which will reveal performnce specs of various Eimac tubes. Included in the listing is the 4CX350F. The IMD numbers fpr the 350F is -29 db 3rd order products and -30 db for 5th Order products. Now go back and check out the data sheet for the 350F. It includes two sets of measuring points. One set optimised for a clean signal the other for maximum power. If you look at the numbers for max signal, the dirtier of the two, the 350F's numbers are -40 db 3rd order products and -45 db 5th order products. The 3rd order products for the 350FJ are 10% that of the 350F.
Now matter how you look at it the 350FJ is clearly a cleaner tube.

Bob W6AH