David G4FTC wrote:
>Searching the amps archives, I know this question has been asked before
>Apart from squeezing/expanding the coils of the input T network to obtain a
>satsifactory match, has anyone tried using a PI-L or similar network to
>simply the input tuning?
>The original W6PO design made one of the L in the T network variable to
>simplifiy obtaining a match - but the chances of getting a suitable
>ajustable core are probably nil, so adding a second variable C into the
>network would seem to be a logical move.
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.
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