[AMRadio] Subj change - HB designs


Geoff/W5OMR ars.w5omr at gmail.com
Tue Jul 24 13:03:27 EDT 2007


KX5KW wrote:
> 811's make great modulators, don't they?
>
> I've had 2000 volts on 811A's with a 9v transistor radio battery for bias, 
> just to see how/if they would work.  They were very sensitive.  Had to reduce 
> the audio drive considerably from the previous 1100 volt/0 bias.  No 
> oscillation or any weirdness though.  They actually seemed to sound a little 
> cleaner, but that could have been anything - reduced drive - better impedance 
> match - wishful thinking?
>
> Sounds like a good project Geoff.

Well, there's a multitude of reasons for going with 811's in the 
modulator... heck, I might even run 4 of 'em, in push-pull parallel, 
just to drop the plate-to-plate impedance down a little more, and 
provide a little more ability for the modulator to handle a naturally 
peaked asymmetrical voice.  Not that I would -consistently- be running a 
kW of Audio... just to have the ability to properly modulate the plate 
of the final, without distortion. 

Good Engineering Practices.  Isn't that what it's about?

Besides, I might wanna change out the 4-125 for a 4-250 on Heavy Metal 
night, and run 350w of carrier (with 4 811's in the modulator)
:-)

The more I think about it, the more I like that idea.  a single 4-125, 
modulated by (4) 811's in push-pull parallel with the option of going up 
to a 4-250 or even a 4-400.

Check http://www.g8wrb.org/data/Eimac/4-125A.pdf for what Eimac says 
about screen modulating the 4-125 final.

--
73 = Best Regards,
-Geoff/W5OMR




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