[AMRadio] 833 AM rig


D. Chester k4kyv at charter.net
Thu Feb 12 11:44:35 EST 2009


> From: John Coleman <jc at pctechref.com>

> Ok here's the deal.  Otis runs a single 833 in the class C final and 
> modulates with a pair .  The single 833A is in a circuit for push pull but 
> only has the one tube in it. and only one nuetralizing capacitor and only 
> one coupling capacitor to the ballanced tank with swinging link.  The grid 
> circuit is a push pull tank but  any other typewill work  as it's only 
> function is to supply the proper driver to the final grid.  The plate tank 
> is a push pull balance type because the phase inversion for the 
> neutralizing cap is accomplished by the ballanced plate tank.  The tank 
> circuit is shunt fed to keep the DC and audio off of the tank circuit. 
> The center tap of the >coil does have a RF choke but it goes to ground to 
> insure that there is no audio or DC on the tank components.

I could have sworn when I lived in Houston that he had a pair of 833A's in 
the final of that  rig.  Did he once run a pair and later take one out?  I 
also remember the unique spiral wound output coupling link as opposed to the 
more common solenoidal form, with each turn the same diameter and all turns 
side-by-side.

I have thought about duplicating that link, because it would take up less 
space and allow more spacing between tank coil and link.  I series feed my 
tank coil, so it has the full modulated DC on it.  It has never arced over 
in the 36+ years since I built that rig, but if I finally get "round to" 
building the push-pull 833A rig I have had the parts for ever since I moved 
back here, I have thought about constructing the link in similar spiral 
fashion.  But one thing I don't recall is exactly how he gets the lead from 
the central turn back outside the coil, past the other turns.  Did he use 
some kind of insulation in between or just air spacing?  There shouldn't be 
much voltage across the link, so I wouldn't think a lot of insulation would 
be necessary, but winding in that form would allow 6-8 turns of link to fit 
into the same space as a 2-turn link.

I prefer to run the entire tank circuit above ground at full modulated plate 
voltage because that eliminates the problem of the plate choke(s), which 
must bear the full rf output voltage from the plate of the tube on all the 
bands that the transmitter works.  The RF choke to the midtap of the tank 
coil is connected at an rf null point, so it is much less critical, and 
about any choke will work if it is capable of carrying the plate current.

What did he ever do with the crapped-out 1930's vintage VM-5 modulation 
transformer he used to have sitting behind the transmitter at his old QTH? 
It would have been a shame to throw it away because that 600-watt 
transformer core with the audio laminations would be good for custom 
rewinding a new mod xfmr or mod reactor.  I understand UTC used a different 
grade of iron for winding audio transformers from what they used to wind 
power transformers.

Don k4kyv 



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