[Amps] Pi network tuning

[Tom W8JI]

73, Tom W8JI
----- Original Message ----- 
From: "Mike(W5UC) & Kathy(K5MWH)" <w5uc at cox.net>
To: "Tom W8JI" <w8ji at w8ji.com>; "AMPS" <amps at contesting.com>
Sent: Monday, July 24, 2006 7:54 AM
Subject: Re: [Amps] Pi network tuning


> Good Morning:
>
> The tubes are Gi-7b's. (output capacitance = 4.6 pf/tube) 
> The grids are tied hard to the chassis.  I cut holes in 
> the chassis just big enough for the grids to fit through, 
> and then made clips (3 per tube) similar to those used to 
> mount 4CX250B tube sockets. (no grid fuses here HI).  The 
> strap from the anodes to the blocking capacitors is brass 
> (tinned at the connecting points) about 1/4" wide and 
> approximately 1.5" long.

All sounds good. If the tubes have conical grid supports and 
very low lead length it should be very stable. You probably 
won't even need a suppressor if the grids are compact with 
short wide connections.

>The plate capacitor is a standard air variable.

There's the rub. You not only have stray capacitance to 
contend with, series inductance increases the effective 
capacitance even more.

Did you look at the estimated capacitance required compared 
to the tube output capacitance? I've never used that tube so 
I don't know what typical output C is.

73 Tom