[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Steve Thompson]

Tom Cathey wrote:
> I called some ham friends looking for 4CX-350's drivers for the tetrode 
> linear project.
> 
> One of the guys has some 8560A's, with sockets, heatsinks, etc. that I can 
> swap some stuff for.  These are the conduction cooled version of the 4CX-250 
> with 6.3VAC filaments.
> 
> I found the data sheets for it, but nothing is mentioned about the IMD 
> figures.
> 
> Does anyone know what the IMD figures are for 8560A's or how this tube 
> compares against the standard 4CX-350 for IMD?  These tubes were once 
> popular in commercial quarter kilowatt FM rigs.
The STC data for 4CX250B gives typical values for class AB1:
2kV anode, 350V screen, 100mA Iq. 2 tone Po 325W pep, IMD3 -30dBc, IMD5 
-50dBc.

Same IMD at 1500V anode, 225W pep out.

The power levels mean it's not that useful as a guide to what they will 
do at much lower level. -30dBc IMD3 is probably somewhere around 1dB 
gain compression, and the IMD might be somewhat higher than you would 
extrapolate from a compression point prediction. Back the power off 10dB 
and you will probably see more than 20dB reduction in the IMD relative 
to the tones.

If you can find data where someone has used them in a class A TV tx, you 
will get better figures for what they will do as drivers/predrivers in 
your system.

Steve