[AMRadio] Home-brewing construction considerations

John Coleman jc at pctechref.com
Thu Jan 17 13:09:45 EST 2008

Hi Geoff:
	In my opinion, the coils should be tied together in the middle as
close to the coils as possible, either on the male bar or on the female bar.
RF current flows on the wires between the coils so it is imperative that the
wire be capable of handling the skin effect and current of the RF which is
much greater than the DC/audio connection from the top of the RF choke. So
rather than making two connections to the RF choke, where you would need to
make large wire and have RF flowing though it, just make one connection with
flexible wire and join the coils together at the coils.

John, WA5BXO 

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of geoff
Sent: Thursday, January 17, 2008 11:49 AM
To: Discussion of AM Radio in the Amateur Service
Subject: [AMRadio] Home-brewing construction considerations

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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