[AMRadio] Legal limit AM amplifier, homebrew

[Geoff]

On 11/13/2011 8:06 AM, Rick Poole wrote:
> Thanks to all for all of the wealth of information you have here.
>
> So, it sounds like a reasonable amp would be a pair of 833's in
> push-pull running class B linear for AM and class C for CW, and
> perhaps forget the high-level plate modulation idea.  1500 watts
> output on CW and, as a starting point, shoot for 375 watts carrier
> out on AM for 1500 watts PEP at 100 percent modulation.  I have read
> K4KYV's paper on "The 375-Watt AM Carrier Power Myth" and more or
> less understand the issues, but 375 watts carrier out on AM sounds
> like a good starting point until I understand the issues a little better.

Rick, if you're going to go through the trouble of building up such an 
amplifier, you might as well plate modulate the critter.

Don't let the power 'myth' stop your creativity... a single 833A will 
load to (at least) 1kW DC input!  You're math is right - 70~ish % 
efficiency would give ya around 700w of Carrier.  A pair of 833's will 
properly modulate that power level (or a pair of 450TH/L's) quite 
nicely, but the "legal limit" is 1500w PEP *output*.  With that in mind, 
build it as a kW - put a variac on the power-supply input voltage, run 
the thing at 500w DC input, and let 'er rip!

A little grid circuit for the input, push-pull Class C (balanced 
circuit, cross neutralization) 10v at 10A filament xfmrs,  a modulation 
transformer
and a B+ plate transformer and you should e ready to go.

> Bernie makes a good point that the difference between such an amp and
> the barefoot output of the DX-100 on AM probably will be just barely
> noticeable, if at all, on the other end.  The DX-100 is rated 100-125
> watts out on AM, and there is a general guideline that says if you
> can't increase power by at least a factor of 4, it's rarely worth
> it.  For CW, though, it will make a significant difference.  Plus it
> gets mighty cold here in the shack in the winter and I need something
> to keep the place warm.  :-)

For CW, just put a strap across the modulation transformer secondary, 
and crank up the variac. :-)

> 833's at least seem to be available... RF Parts has Taylors for
> around $120... hope they last longer than the Chinese 811A's and
> 572B's do... :-(
>
> Now... where's a good place to get a power transformer and filament
> transformer for such a beast?

Regional Swapfests are the best bets.  Lots of stuff happens there, but 
bringing a little V.O.M. would flush out the truth of any possible 
'story-tellers'.

Good luck and keep us posted.

-Geoff/W5OMR


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