[Amps] Designing the Cleanest Linear with RF Negative Feedback

[R.Measures]

On Dec 14, 2005, at 9:49 AM, Tom Cathey wrote:

> 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.

The 3cx3000A7 already exceeds –50db total IMD without RF-NFB.  In class 
A, a 4cx10,000D / 8171 will equal that, and if you need 10db more suds, 
there's no pain, just turn up the mic gain.
>
> 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
>
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org