[AMRadio] Home-brewing construction considerations

geoff ars.w5omr at gmail.com
Thu Jan 17 17:49:00 EST 2008

ne1s at neandertech.com wrote:

>> 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?

Having a ball playing radio, can ya tell? ;-)

> 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.

I wouldn't have thought so either if it had been anyone else who made
that recommendation.  But, it's hard to argue with a ham who's 'been
there, done that' and has built up -many- kW rigs, always paying
particular attention to minute details.

If John/WA5BXO says it, I believe it. 

> 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?)
No, I meant the part that doesn't rotate.  The 'frame', if you will. 

> , their values would need to be double (not half) the value of a single cap, because they are effectively in series.

I want to add enough capacitance across each side of the butterfly,
equally.  I'm pretty sure I want half of the values.
 If (for example) the total capacitance that's used across the entire
tuning capacitor is 50uuf, then each side has to be 25uuf, with the
single mounting point on the frame of the capacitor... the part that
doesn't rotate.  Is it not called the 'stator'?  Common sense tells me
'rotor' is the part that -moves- ;-)

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

It'll be in addition to.  I believe what I'm gonna do, is use the
double-connected racks to house,
2 power supplies, the 250TH modulator and the original RF deck, on one side.
1 power supply the new RF deck, a 19" rack of (4) 6-position coax
switches and an R-274 receiver on the other side.

The coax switches will be used to select exciters and finals.  With a
totally adjustable bias supply for the new (to me) final, it would do
fine in either Class B, for a linear (being driven by a rice-box), or
Class C for high-level plate modulation, driven by the Viking II.

I've still got a lot of thoughts in my head... this is just the
preliminary thinking.
Bottom line, I'm trying to conserve space in the shack. ;-)


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