Scott,
They custom make some air variables with a low min C by the way the rotor
plates are designed but those are expensive. The reason you need low C is first
the tube has so much capacitance. Next you add the stray capacitance between it
and tune C. Those two thing added to tune C just keeps raising the capacitance.
I've built several amps where the tubes C and stray C was high enough that I
didn't need a tune C and just tuned it with load C. That was generally on some
grid driven amps that were single banded. At some point though going down in
frequency, you'll need more tune C so you have to have one. You'll also fine a
lot of amps with fixed capacitors switched in parallel with the load for 40 and
80 meters. This is so you can run a smaller load C as if you had one that would
fit those bands, it would be so big, you'd have problems fitting it in the
chassis. The only good point to a vacuum variable is its voltage rating. They
can make them more compact this way only because th
e plates (cylinders really) are really close together because they cant arc
over a vacuum. The surface area drops some because the air gap is minimized
raising the capacitance. When selecting Tune C, I always used one with 1-1/2 X
the voltage rating of the plate voltage, some use the voltage rating itself. On
load C, I always did the same but that voltage is the peak voltage across the
load, not the plate voltage. Hope this helps somewhat.
Best,
Will
*********** REPLY SEPARATOR ***********
On 10/2/05 at 9:02 AM Scott Townley wrote:
>Regarding air-variable capacitors:
>Most folks refer to air-variable caps by their max C value, and sometimes
>include the plate gap and/or rated working voltage (particularly when
>selling them!)
>But much less frequently mentioned is the minimum C value, which of course
>is quite important in Ctune applications.
>It's surprising (to me, the uninitiated) how few "vintage" caps (Johnson,
>Millen, etc.) are suitable for say a single 3-500z amp without
>compromising
>the loaded Q at 28MHz. Or perhaps I am missing something. (I am
>purposely
>excluding vacuum variables here). To me "suitable..." means around
>10-130pF at > 4kV.
>So a few questions:
>1. What are the primary driving factors behind achieving small minC in an
>air-variable? Say I would homebrew one...what are the mechanical/layout
>considerations in the capacitor itself that tend towards small (c. 10pF)
>min C?
>2. Are there other sources/manufacturers beyond those I've previously
>named? I have several old Johnson catalogs and it seems that only 4 of
>the
>many, many caps they make/made even come close.
>3. Or is padding for the low bands more common than I think? There are a
>few more Johnson caps that would work if you padded both 40m and 80m.
>Thanks,
>
>Scott Townley NX7U
>Gilbert, AZ DM43di
>http://members.cox.net/nx7u
>
>
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