Jeff Millar wrote
>I had the problem with an 8877 on 2M. The input tuning drifted somehow
>and having only one adjustment wouldn't bring it back in line. I wanted
>to design a pi or pi-L network with two cap's to ground and inductors in
>series (to keep the cap shafts at ground). Some work with a smith chart
>program produced several acceptable circuits...and the amp is running
>today in contest service with "tune" and "load" caps on the input.
>One problem is that the Q of the pi-net is quite low and the range of
>adjustment is also low. I had to fiddle with the inductors to ensure the
>match takes place within the range of the variable caps. It also seems
>that the small variables have enough stray inductance to throw off the
>model...or else the small inductor formulas don't work too well.
>Here's the Pi-L model I used as a starting point, working from cathode
>to input connector:
>cathode resistance = 40 Ohms
>cathode capacitance = 41 pF
>series inductor = 39 nH
>parallel cap = 32 pF
>series inductor = 15 nH
>parallel cap = 19 pF
>50 Ohm input
>One last problem. The caps live in the cathode compartment and have a
>lot of air flow across them. This caused bits of stuff to get in the
>plates and kill the drive. During the first contest, we used a sharp rap
>on the back of the amp to get back running. I changed the caps to wider
>spacing for the next contest but the stray inductances changed and it
>required more fiddling with inductors.
Thanks for the info. A problem with a Pi or PI-L network is that the values
of the L's become increasingly at 2 metres small whereas the values of the
L's in a T network are somewhat more manageable.
The problem with my W6PO is in adjusting the T network for the best input
match. Without being able to adjust one of the L's from the front panel
necessitates squeezing or expanding the coils which are housed in a
pressurised cathode compartment - which is impossible under operating
I'm certain that if I could adjust it with the amp under power then I could
trim the input SWR to 1:1. However at the moment for a forward power of 60
watts I only get 4 watts reflected power so the exercise is a little
acedemic. So for the moment I've decided to live with the current T network.
Since my last posting, one of the things I did was to check that the values
of the network components are correct, and in my searches I found two bits
of software which members of the reflector may find helpful.
The first is the Triode Application Program by KD9JQ -
http://home.mchsi.com/~kd9jq/hamradio/TAP.html - although a DOS based
program it is extremely useful for calculating operating parameters etc.
with most of the usual amateur (and not so amateur) triodes included in the
The second is a Windows based Smith Chart program - this is only a demo
version and allows networks to be easily created but with a maximum of 5
elements but for T, PI and PI-L networks it's more than adequate.
It can be downloaded from
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