[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Tom Cathey]

Hi All,

I'm planning to build up a 75M monoband amplifier that has at least -45db 
to -50db 3rd order IMD using heavy RF negative feedback. 10MW in, 1500W out.

I wanted to get opinions here what others thought was the best general 
configuration and perhaps suggest some real world tubes.

It seems that the best amps that use RF feedback start with a much larger 
tube than generally used in open loop linears and run it about half power .

I plan to use the 10 milliwatt output from my FT-1000D as a source. This is 
the low level stage just before it goes into the power amp board. I'm hoping 
this is class A stuff and a  much cleaner starting point than its -32db IMD 
200W solid state power amplifier board output. Any opinions on that?

For this amplifier I am thinking of either a chain of grounded grid stages 
so that they have individual built in feedback and an additional loop of 
feedback around the whole chain.

OR, use a tetrode final with regulated screen voltage with stages going back 
to amplify the 10MW level. Hopefully, use up to 15-20db of  loop feedback 
assuming the gain is available.

I would appreciate any pointers to URLs showing working schematics of the 
big commerical amps that Collins, Harris, etc use, and ideas you may have 
for a design.

For example... I don't know what good low level tubes to use, but lets just 
say it went something like,  6AU6, 12BY7, 6146, 811A, 3-500Z,  GS-35B  - all 
in grounded grid with an additional feedback loop.

Or, for grid driven:  The same low level stages, then 4CX-150, 4-1000A, 
4CX-3000.  Looking for conservative ratings at 1500W and high IMD figures.

Another thing I wonder about is if a tube like the 6146 or 4-1000A which has 
modest IMD figures open looped,  can successfully participate in a -45 
to -50db IMD chain when big feedback is used?

 I will be building this amp over the next few months and will post the 
results once completed. Thanks in advance for any help!

73,
Tom, K1JJ