[Amps] Wanted - 50pf vac variable

[Bill Turner]

On Sun, 5 Dec 2004 16:22:04 -0600, Dr. William J. Schmidt, II wrote:

>I'm looking for a smaller foot-print (e.g. 4" in diameter or smaller, 6" length body or shorter)  50pf vacuum variable to replace the plate cap in a SB-220 for a conversion to six meters.  I have a couple of 50uf 4kV caps to trade...  Anyone?

_________________________________________________________

If that's what you want to do you can, but replacing the plate cap
isn't necessary at all.

You can install a coil between plate cap and the plate capacitor which
will act as an L-network in conjunction with the tube's output
capacitance.  I did that on my SB-1000 and it works perfectly.

Here's a picture:  http://members.ispwest.com/dezrat1242/1.jpg

The L-network coil is the upper of the two, the lower is the new plate
tank coil. 

You might have to replace the plate choke due to the higher frequency,
but you could use the old one if you are sure it has no resonance near
six meters.  You can see the one I made.  I just wound a bunch of
turns, checked it for resonances with a dip meter, and it worked fine.
I also reduced the inductance of the parasitic suppressor coil to
avoid overheating the resistor.

Details are in any recent ARRL handbook.

--
Bill W6WRT



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