[AMRadio] Home-brewing construction considerations

Larry Szendrei ne1s at neandertech.com
Fri Jan 18 13:54:10 EST 2008

geoff wrote:
> Having a ball playing radio, can ya tell? ;-)
As it should be, then!

>> 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. 
As John replied to me earlier (forget if it was on- or off-list), I 
guess it depends on your own particular situation. I'm using a single 
50pF vacuum cap to augment the (rather small, electrically) split-stator 
cap on the Old Buzzard Rig on 80/75M. Push pull finals, works FB. It 
gets removed for 40M and above. YMMV, of course.

>> 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.
I'm pretty sure you want double the values :-) The capacitance in the 
tank circuit is plate-to-plate. From a tank circuit resonance point of 
view, it makes no difference whether the center point (frame/rotor of 
the air variable, center tap of the coil) are connected to anything or 
not, as long as they're isolated from each other for RF, which is 
necessary to avoid two independent tank circuits in series, which in 
practice would never track each other. This is usually accomplished by 
way of a plate choke in series with the modulated HV to the coil 
center-tap. So both sections of the variable cap are in series: stator - 
rotor/frame - stator. If you are going to use two fixed caps, one in 
parallel with each section of the variable, they are also in series. For 
the capacitance of the series combination of two to be equal to that of 
a single cap plate-plate, each of them need to be double the value of 
the single cap. Theoretically it makes no difference if the center 
connection of the fixed caps are connected to the frame/rotor of the 
variable or not, as long as the layout is symmetrical and both fixed 
caps are equal in value, and both sections of the air variable are also 
equal in value, since the junction of the fixed caps is at the same RF 
potential (= 0 RF Volts) as the frame of the air variable. I'm trying to 
make sense here; it'd be much easier (and a lot more fun!) to draw 
diagrams and discuss this over a couple of tasty beers!

>  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- ;-)
In the example above, two 100uuF caps would be needed to have the same 
effect as a single 50uuF cap, by virtue of what said above. The part of 
the cap that rotates (rotor) is common to both sections of the variable, 
and is connected to the frame. The stators are the plates that don't 
move, which are each tied to a plate of the PA tubes. In the type of cap 
I'm visualizing, anyway, which is by far the most common. Maybe you've 
got a strange bird there.

>> 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.
Cool. You might consider what's called a RF "transfer switch" to direct 
things t0 & from the two different rigs. That would be ideal, if you can 
find one.

> 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. ;-)
Know the feeling!


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