[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: 
> > Hi Tom, 
> > That '5106 is probably for Cable TV use and likely cuts off around 40 MHz 
> (on the low end!). 
> > 
> > If I recall correctly, some of the general purpose units that go down to a 
> MHz use numbers from CA28XX group. They come in both single ended and push-pull 
> versions. 
> 
> CA2820 and 2830 are good ones, 0.5-1W capability and only a couple of 
> power line decoupling caps needed. Motorola equilvalents are MHW590 and 
> 592 - these replaced the TRW parts after Mot bought TRW. Many other 
> CA28** or MHW parts will also suit, but some need external chokes or 
> transformers. Beware the gain - around 30dB. If you're running about 
> 1-200mW out for linearity, it means only -10dBm in (ideal if you build 
> your own exciter). If you're going to do that, you could consider going 
> the whole hog and incorporate polar or cartesian loop feedback. 
> 
> > I've seen a few of the 350J's for sale on the web over the years. I don't 
> believe the demand is very high for those as they have a 26V heater and as such 
> can't be dropped into a 4CX250B socket. There is also the 4CX600J/JA/JB. 
> 
> I'd bet that the 'FJ is barely different from a std '350 - just tested 
> for linearity under class A. They might be a few dB better than 'any 
> old' '350, but I'd be surprised if the difference was more than a few dB 
> - not 10 or more. 
The IMD data for the 4CX350FJ looks quite attractive ! -45 db 3rd order products and -50 db 5th order products. Without negative feedback. http://www.g8wrb.org/data/Eimac/4CX350FJ.pdf  The price of 4CX-350FJs new from Richardson Electronics is $ 472.
The 4CX600J also shows similar numbers but the data sheet clearly states these numbers were obtain with the help of a negative feedback resistor and a phasing capacitor. The problem with the 600J is the price, $ 3244 for the cheapest version. That's Pricey, even by Eddie Richardson's confiscatory standards ! I believe this tube was developed expressly for DOD Applications and the price was set according to Cost Plus Contract Standards. I have never seen this tube used in Ham Radio Applications in any form.
As to J variants over Vanilla versions I haven't had much time to compare the 350Js numbers to the 350 but most J versions offer a substantial improvement in IMD over the stock version of the same tube. Electron Ballistics are key in determining IMD performance. Both the 4CX5000J and the 4CX10000J filaments consume nearly 50% more power (750watts) than the 4CX5000 and the 4CX10000 (562 watts). Negative feedback is one way to improve IMD numbers, but it isn't the only way.

Bob W6AH ex K1TA