On Wed, 20 Aug 1997 22:13:54 +0000,
w8jitom@postoffice.worldnet.att.net wrote:
<snip>
If the goal is getting close to what you want, it would be MUCH
better to calculate the values of caps. If you know that, by leaving
the tube, choke and stuff disconnected, and putting a small silver
mica from the tube end of the tank to the chassis and another one at
the loading C location instead of the loading C, you can do the
whole tank without ever having the tube filament on!!
The tank is terminated at the tube end by a small carbon or film
resistor, of the same resistance as the estimated tube operating
impedance.
Then the tank can be driven from the load end with a cheapo antenna
analyzer (or a transceiver and the inductor trimmed to perfect size,
with all sheet metal and covers in place, starting from the lowest
band and working up. Safe, fast, cheap, correct, accurate, and easy.
<><><><><><><><><><><><><><><><><><><><><><><><><><><><>
This is the method I used on my just-completed 8877 amp and it worked
like a charm. I used an MFJ-259 SWR analyzer to pump RF into the
output of the tank and had a 2300 ohm 1/2 watt carbon resistor tacked
from anode to ground. All cut and try was done before power was
applied for the first time, and when power WAS applied, it all worked
perfectly. Well, almost. For RTTY (brick-on-the-key mode), I had to
go to a larger diameter wire for the 10/15/20 part of the coil, but
the inductance was the same. #10 gauge is not big enough at the 1500
watt level - guess I should have known that - so I used some .200"
diameter solid copper ground wire and that did the trick. 10 gauge
seems to be ok for the 40/80/160 part.
Incidentally, the SWR analyzer made it really easy to get the input
pi-network set up. I just tacked a 50 ohm resistor from cathode to
ground and tweaked it for 1:1 SWR on each band. The tube isn't
EXACTLY 50 ohms, so a wee bit of tweaking was necessary, but it was
plenty close enough. Don't know how I ever got along without that
little jewel.
73, Bill W7TI
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