[AMRadio] Home-brewing construction considerations

ne1s at neandertech.com ne1s at neandertech.com
Thu Jan 17 14:41:24 EST 2008

----- Original Message -----
Subject: [AMRadio] Home-brewing construction considerations
Date: Thu, January 17, 2008 12:49
From: "geoff" <ars.w5omr at gmail.com>

> 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&amp;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&amp;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...

Hi Geoff, how ya' been?

I understand it's not your question, but if I understand you correctly, I
beg to differ on a few things you stated above:

1) A single capacitor plate-to-plate wound probably not upset the balance
appreciably - in fact if the cap, and connections to it, were totally
symmetric it wouldn't at all. The fixed vacuum caps I've seen have an
outside and an inside cylinder, so stray capacitance to the surroundings
from the outside cylinder would probably be larger than the strays to the
inside cylinder. However, as long as the cap was reasonably spaced from
surrounding stuff, I don't think it would be of any practical consequence
on the lower bands where you'd be using the auxiliary fixed cap anyway.

2)If you were to use two caps, with a common connection tied to the rotor
of the butterfly cap (you said stator - did you mean rotor?), their values
would need to be double (not half) the value of a single cap, because they
are effectively in series.


> 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?
> Thanks!

John already answered this, so I won't bother.

Good luck with the new RF deck - are you replacing the old one, or
completely different/new transmitter?


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