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Re: [Amps] W6PO 2m amplifier - input circuit adjustment

Subject: Re: [Amps] W6PO 2m amplifier - input circuit adjustment
From: "David G4FTC" <>
Date: Sun, 18 Jun 2006 07:45:18 +0000
List-post: <>
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.
>jeff, wa1hco

Hi Jeff,

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 - - 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

Take care

David G4FTC

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