[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Jan Erik Holm]

Tom,

Tom Cathey wrote:

> 
> 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?
>
Sounds smart.
> 
> 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.
>
Forexample the old Collins 204C-1 amplifier first has a 4CX250B in Class A
that drives a 4CX250B in clas AB1 that drives a 4CX5000 in AB1, I bet that
thing would show pritty good numbers if one stays inside AB1. This thing
needs 200 mW to drive it to full power which is an awful lot.
> 
> 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?
>
Collins uses 6CL6 and 6146A set up with negative feedback to get IMD
numbers around -55 dB I think it is. That might be a way to go and maybe
drive a 3CX3000 which has good IMD numbers.
> 
>  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
> 
Only my 2 cents from the top of my head.

73 Jim SM2EKM