[HomeBrew] To short or not to short, that is the question

[Philip Atchley]

Hi,

Today I wound the coil for my "Rockin' '50s MOPA" transmitter (12AU7 Osc and 
6AQ5 Final).  In scouting around the web I found as many different Pi Network 
coil designs as I found QRP transmitters.  Since I want this unit to "look" 
and basically be something that would have been built in the '50s (when I 
first started building radios) toroids or similar were naturally out, and I 
didn't have any AirDux coils.  Wanting to make this a bandswitcher (80-20M) 
with a switch rather than plug in coils (no room for an octal socket), and I 
have a nice heavy duty 2 pole 7 position switch I opted for the tapped coil 
idea.

It seems the number of turns varied from about 40 to 50 turns for 80M, with a 
center tap for 40M (depending on the particular design).  So, on a 7/8s inch 
form I wound a coil of 60 close wound turns with a tap at every 10 turns.  The 
LAST 10 turns are spaced a small distance from the others and have a slight 
air gap between turns to use for the highest band.  It 'may' need to be 
tweaked slightly, but I think that the taps will allow those 4 bands to be 
worked ok.

The FORM is a piece of white PVC pipe, 7/8s inch outside diameter.  A long 
solder lug was mounted on each end to solder it directly to the lugs of the 
two tuning capacitors, supporting it securely ABOVE those capacitors 
(capacitors are mounted on their sides with lugs sticking up).  Since PVC pipe 
wasn't around in the 50's, or at least wasn't common, the entire coil (except 
the taps) was given a generous coat of Varnish.  This makes it look "vintage", 
and just looking at it you can't tell it from a ceramic coil form!

Anyway, the QUESTION is this.  I notice that some (most) designs short out the 
undesired turns to change bands, while a few leave one end of the coil OPEN 
and just select the number of turns with a switch.  WHICH IS BETTER?  Will 
shorting out the turns lower the "Q" of the entire coil and thus the output of 
the transmitter?

73 de Phil,  KO6BB
SWL QSL GALLERY: http://photobucket.com/albums/y123/KO6BB/
THE BEACONEER'S LAIR:    http://www.geocities.com/ko6bb/
Merced, Central California, 37.3N  120.48W  CM97sh