[Amps] Fwd: Pi-L In-circuit Adjustment Question

[Ian White G3SEK]

David C. Hallam wrote:
>All of this has been "informative", but  I had planned to do the 
>following with the tank circuit of my amplifier under construction:
>
>Replace the tube with a noninductive resistor of a value equal to the 
>plate load impedance; set the tuning and loading capacitors to the 
>values from the tank circuit calculations; connect my Rx meter to the 
>output and adjust the tap position on the coil until the Rx meter reads 
>50 ohm and j0.
>
>Will this result in anything meaningful?

With a plain Pi-tank, it would do exactly what you want - in fact, you 
wouldn't even have to pre-set C1.

If R1, R2 and C2 are already pre-set to the correct values, then the 
values of L1 and C1 must *both* be correct in order to bring the SWR 
down to exactly 1.0. When you vary C1, you will always see some dip in 
SWR, but only the exactly correct value of L1 will take that dip all the 
way down to 1.0.

This is a property of L-networks: *both* arms need to be correct to 
achieve a perfect match. It is very convenient that you don't have to 
pre-set C1, because you wouldn't actually know what values of stray C 
and L to allow for anyway. Normally you would leave the tube connected, 
so it is contributing its correct value of C_out.

Unfortunately that simple procedure will not work for a Pi-L, because 
you need to adjust L1 and L2 separately. However, you could do it in two 
stages, starting with L2 and then moving on to adjust L1.

First, disconnect L1 from C2 (which also disconnects L1's bandswitch, C1 
and the tube). From your Pi-L design calculations, determine what the 
'intermediate' value of R is (usually 300 ohms) and connect a 
low-inductance resistor of that value in parallel with C2. Now swing C2, 
and adjust L2 for a perfect match at the bottom of the dip in SWR, as 
described above.

Disconnect everything from C2, and pre-set this capacitor to the correct 
calculated value for use in a Pi-L. Re-connect L1 and L2 to the top of 
C2. Don't touch L2 or C2 after this.

Now adjust L1 by swinging C1, as described above.

I think this will work, about  as well as any practical method can.

But a couple of points about test equipment...

You need to be able to see clear indications of SWR right down into the 
dip at 1.0. Not every meter will show this well: the digital display in 
something like an Autek or an MFJ-259B/269 is useless for this 
application because it only goes "1.1 - 1.0 - 1.1". That is not enough 
resolution to show you what's really happening down in the dip. The 259B 
and 269 both contain a good resistor bridge, but the little analog meter 
is too small to read accurately, so hook up a larger moving-coil meter 
in series.

You will need a capacitance meter for pre-setting C2. The AADE L-C meter 
is excellent for this, and even the 0-1999pF capacitance range of many 
DMMs might be OK. Strays don't matter too much here, because the 
capacitance that you are trying to measure and set is usually quite 
large. It also helps that one side of the meter is connected to the 
grounded frame of C2. However, you should always zero the meter with 
both test leads in *exactly* the same layout that you will use to 
connect to C2. Arrange the 'hot' lead so that only the smallest movement 
of an alligator clip is needed to make the final connection to C2, 
keeping all the rest of both leads fixed in the same position.


-- 
73 from Ian G3SEK