On Friday 14 June 2002 06:33, Glenn McNeil wrote:
> Hi Guys...
>
> I'm about to start construction for a 2 meter linear using a GU74B (
> 4CX800 ). The socket I have is a military one, with built in chimney and an
> external screen ring. This ring makes contact with the screen area of the
> valve.
>
> At present it has 4 x 1000pf 1kv disc ceramic caps between the screen
> "ring" and ground.
>
> My question....
>
> (a) Is this socket useable at 2 meters
> (b) I note that about 1100pf is usually the screen capacitance in eimac
> sockets for the 4CX250...can I just solder an even number of disc ceramics
> to the socket to total about 1100pf, or am I missing something ??
The key point is whether the screen has a low impedance to ground at
frequencies where the circuit can deliver much gain. In the usual '250 bases
and in the Svetlana SK-1 base, the screen cap is sandwich construction,
giving low inductance whereas leaded caps can give odd resonant effects.
To be sure, you should check the impedance somehow - an antenna analyser
might do if you can't get to professional equipment. My hunch is that it will
probably work with 6 or 8 good quality 1000pF caps with minimal lead length.
The capacitor in a SK-1 here measures about 10nF. It's difficult to guage
exactly what impedance the tube sees, especially at vhf, because I'm
measuring at a single point but I see <10 ohms impedance from 2-200MHz.
Hope that helps.
Steve
|