[AMRadio] Home-brewing construction considerations

geoff ars.w5omr at gmail.com
Thu Jan 17 12:49:23 EST 2008

In a balanced push-pull Class C RF Final with plug-in coils, I've seen
things done a couple of different ways.

One is, to split the B+ so that the potential is fed with the B&W HDVL
jack-bar to the inside of the two coil's.  The outside of the coils is
then connected to the tubes.  That's the way the home-brewed 250TH final
I'm running was done, back in the mid '50's.  There was also a couple of
finger-stock pieces across the B&W butterfly tuning condenser to
facilitate adding a vaccuum capacitor across it for allowing the circuit
to resonate on a lower frequency (like 75, or 160m).

I understand that the circuit would no longer be 'balanced' were that
condition to occur.  It would go back to being balanced if a pair of
capacitors at half the required value were used, and where the two meet
in the middle were tied to the stator of the Butterfly.  THAT would keep
it balanced.  That's the reason for the split-stator capacitor across
the final, now.  But, that's not my question...

I recently picked up another home-brew 250TH final that has had more RFI
considerations involved in it's construction (than the one in current
operation) as it was built sometime in the '60's.  Screen and mesh
aluminum, etc... It's nice and hopefully will keep me out of the
neighbors TV's, microwaves, blenders, etc... (grin) But the plug in coil
is what prompted me to this question:

Is there a difference in feeding each of the inside sockets on the
jack-bar with B+ from the top of the RF plate choke, vs having the two
coils tied together, in the middle, and feeding B+ there?

I understand that the potential is the same, but for the sake of having
less stray capacitance and a 'cleaner' RF environment, which would be
better, and please... tell me why?


73 = Best Regards,

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