Thanks for the replies to my original post, both privately and via the
lists. There were some good ideas but none of them seemed to work for me.
After a little experimentation and frustration I finally discovered why
I could not get the T network to resonate at 144 MHz. I thought I would
share my experience for the benefit of others;
It turns out that the way in which I wired the Cathode connections on
the bottom of the socket was the reason for my problem. I've seen
various photos of homebrew amps of this design, some with the Cathode
pins connected in series by one or two wires and others where strips of
tin or brass were used to connect all of the pins to a common point in
the middle of the socket. I discovered the hard way that the latter
method is the better way to do it, providing much lower impedance
between the Cathode pins.
When I connected the Cathode pins together on the bottom of the socket
in my amp I paralleled 2 pieces of 12 AWG wire and neatly soldered them
to each Cathode terminal around the perimeter of the socket. Then I
connected the output of the T network input circuit to one end of the
common Cathode connection. (FWIW in this design one side of the filament
is also connected to the Cathode connections). It would seem the
additional impedance of the series connected wire was enough to throw
off the resonance of the matching network. So I temporarily shorted the
open ends of the Cathode connection (U shaped loop) with some heavy wire
and sure enough, the resonant point of the T network fell into place. On
Monday I will fabricate some shorting bars out of brass and solder then
onto the Cathode connections of the socket in the shape of a star with
the common point in the center.
Lessons learned:
- All of the Cathode pins are not necessarily connected by ow impedance
within the tube (at least on an 8877).
- It really is important to practice good VHF/UHF techniques by
minimizing lead inductances on all RF connections.
73 de Hugh VA3TO
www.va3to.com
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