[AMRadio] Re: 4-65's

Geoff/W5OMR ars.w5omr at gmail.com
Thu Apr 5 19:23:56 EDT 2007

ne1s wrote:
> Geoff/W5OMR writes:
>> Does anyone have any 4-65's?
>> I think that'd be a neat little ~100w rig to build up.. a single 4-65 
>> tetrode, modulated by a pair.
> Sounds like a neat combination. You oughta be able to get about 150W 
> carrier out of one, run less conservatively (not that anyone will hear 
> the difference, of course).
> Someday I am going to build a rig with a pair of 'em modulated by 
> 811s, when I get a round tuit. Already have the HV PS/modulator deck 
> built, just have to do up the RF deck, which will also contain the VFO 
> (a T-195 PTO I've been saving for this project), and RF and AF driver 
> circuits. I hope to run the pair at 500W input (2500V @ 200 mA), or 
> close to it, depending on the plate efficiency I'm able to achieve.

Oh, now -there's- an idea... a pair of 811's to modulate a single 4-65.  
Yeah, yeah.. your thinking 'what overkill', but y'know... a single 833 
in the final should only need a pair of 250w dissipation tubes in the 
modulator, but no one can argue that a single 833, mod'ed by a pair 
doesn't sound absolutely wonderful!

>> 6.3v @ 3.5a?  Got those transformers, too. 
> Actually, they want 6.0V, so drop the voltage to the primary or run  
> resistors in the primary or secondary circuits of the fil xfmr. One 
> trick is to use appropriate lengths of undersized wire to the 
> filaments (whatever length is necessary to drop the filament voltage 
> at the socket to 6.0 V).

Well, even better.  I've got some 6.6 @ 18amp transformers.  Reduce the 
primary voltage down to 110 vs 120vac and that should bring the 6.6v 
down to 6.0, or, as you say, insert some series resistance to drop the 
voltage on the secnodary.   Maybe even a small value resistor to drop 
the 6 tenths of a volt, back to the center tap of the filament 
transformer... there's lots of ways of doing it.

Good info!  Thanks, Larry!

73 de W5OMR

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