Brass shim is what I will be using Dan.
I said "shorting bars" but I didn't mean to imply that I would use brass
bar ;)
73 de Hugh VA3TO
www.va3to.com
Dan Hearn wrote:
> Howard, Your comments about using heavy wire or brass bars makes me
> uncomfortable thinking about thermal stress on socket contacts. I would
> think about using some thin material such as copper flashing or even brass
> shim stock which is widely available. FWIW. 73, Dan, N5AR
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
> Behalf Of Hugh Duff
> Sent: Saturday, April 28, 2007 8:59 PM
> To: amps@contesting.com; vhftubeamps@yahoogroups.com
> Subject: Re: [Amps] 2m W6PO input T network tuning....Problem solved !
>
>
> 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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>
>
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