[AMRadio] Ongoing Quest: Legal Limit AM Amp

[Rick Poole]

Looking for a good design for a legal limit amplifier for my DX-100 
(looks like I'll be homebrewing it).

I've pretty much narrowed down the choice to a linear amp capable of 
375 watts carrier output or 1500 watts PEP output at 100 percent 
modulation, vs a Class C amp with high level modulator driven from 
the DX-100's audio output.
.
I was thinking about a pair of 3-500's in Class B push-pull.

Everybody keeps telling me that I'll get maybe 30 percent efficiency 
at best during dead carrier times.  At 100 percent modulation and 
class B I should get 60 percent at worst.  I really, Really don't 
understand that at all... seems like if I can get 60 percent at 1500 
watts PEP I should be able to get 60 percent at 375 watts dead 
carrier.  But, I'm accepting it for the moment, and anyway, I digress...

375 watts unmodulated carrier output at 30 percent efficiency means 
1250 watts input, or 875 watts dissipation or 437.5 watts per tube.

1500 watts PEP at 60 percent efficiency means 2500 watts peak input 
or 1000 watts dissipation, 500 watts per tube peak.

So it seems to me like a pair of 3-500's should be able to cut it, 
especially if I give them plenty of air.

I want to stick with glass tubes because I'm planning to put them 
behind a window so I can "feel the glow" during operation.  Anyway, 
it'll impress the living crap out of the XYL :-).  Hence, the 
ceramics have been ruled out for now.

So, a few questions...

1.  Will a pair of 3-500's in Class B push pull really handle the 
load with no problem, assuming adequate air?

2.  This may be a rookie question, but google hasn't given me a clear 
answer... is there such a thing as class B push pull grounded grid?

3.  There are some commercial class AB pair-of-3-500 amps (e.g. 
SB-220, TL-922) that are limited in what they can do by the power 
supply.  So, I plan to make sure the power supply can easily handle 
the load.  Any suggestions on the best places to get power 
transformers that are up to the job?  So far I haven't found one 
under about $600.

4.  I'm actually not that certain that I'm competent to design 
something like this from scratch.  Any plans available on the net 
that I can use for a starting point?

5.  Current tentative plans call for the power supply to be separate 
from the RF deck, and they may be separated by as much as 6 feet 
(there are compelling reasons for that that go way beyond the scope 
of this post).  Is that practical?  Where is a good source for high 
voltage (3500 VDC) cable and connectors?

6.  As noted above, I plan to put the tubes behind a window so I can 
see them.  Is there a glass that can be used for the window that will 
maintain shielding?

Thanks...

Rick WA1RKT
Londonderry, NH
Catch me on 14.300 MHz Intercon / MMSN most days