On
Sat, Jan 6, 2024 at 05:21 AM, Robert W5AJ wrote:
Jim,?? is there public domain software program that
helps with Pi network design?
?
aka,? I'm looking at 8877 50Mhz project in 2024
73 Robert W5AJ
Don't mess with the PIEL software.? The gold standard is
the GM3SEK? PI / PI-L spreadsheet.? ?It's better since it
factors in everything, like stray C....and also the all
important stray L? Any stray L (intentional or otherwise)
between the anode and the C1 tune cap? will form a step
down L network....which will reduce the plate load Z to a
lower value.? This L network uses the tube's? anode to?
grid C + the extra fractional? uh before the C1 tune cap.?
The GM3SEK software also factors in the parasitic
suppressor if one is used.? ?It also factors in the plate
choke.?
I use the GM3SEK software for all amps? from 160-6m.?
?
I also use it for PI tuned inputs.... but there is a
trick to this.? The software won't match 2 x identical
Z's...so the 'workaround'? is to alter one a tiny bit.?
IE: 50 ohms to 49.999 ohms..... presto, works superb. In
the case of the tuned input, everything else is zeroed
out....( except for the plate choke....enter '9999 uh'?
for the plate choke). .? Where the plate load Z normally
gets entered, you enter 50 ohms..... then enter 49.999
ohms for the load end.
?
You can also tweak the total network Q.? ?Total network Q
= sum of the input and output Q.? Both Eimac and older
ARRL books only used the input Q.? Typ with an amplifier,
with a total network Q of? '12'.? The input Q = 10..and
the output Q? ='2'.
?
On tuned inputs, where both? ends are 50 ohms,? typ a
total network Q of '4' is used....with the input Q =
'2'..and the output Q = 2.
On the high power output tank, (160-6m)? in most cases,
the extra coil has to be installed between the plate block
cap assy...and the C1 tune cap...forming a step down L
network. The plate load Z is stepped down to a much lower
value, low enough that a practical? PI network can be
built, using a much lower Q.... on the highest band? on a
multiband amp.? ?Ditto with any mono band? 12/10/6m amp.?
?With out the extra fractional uh installed, the resulting
loaded Q is sky high, due to the tube's anode to grid C?
(which rises a bunch when tube plugged into the grid
ring..... or grid flange bolted to the chassis).? ?The
extra C comes from the proximity of the lower anode fins
to the chassis below it.? The u can also get even more
stray C between anode to rear / side walls....if tube too
close.? ?All that stray C is easily measured.? ? WE call
that network a L-PI? since it has the extra uh.? ?An?
L-PI-L also works.? I stay away from PI-L networks since
plenty of harmonic attenuation is obtained with the simple
PI (or L-PI) setup.?
?
On the 6M? ?3CX-6000A7 amplifier, the extra fractional uh
was obtained using a 1" wide cu strap in a semi circle,
forming an arc.? ?Main inductance between the vac tune and
vac load caps was done the same way....and both? straps
are at right angles.? Eff is sky high across the entire 6m
band, 72.5%...... and an easy 10 kw CCS? brick on key. By
forming the strap into a semi circle, the strap will
conduct current on both sides. Input swr is dead flat from
50-54 mhz...since it uses a manually tuned, PI tuned
input.? ? Again, it ample harmonic suppression.? If more
is required, a simple shorted 1/4 wave stub can be added,
BUT it has to be inserted exactly an electrical 1/4 wave
length downstream from the C2 load cap.? In the case of
the 3x6 6m amp, the vac load cap is mounted to the inner
front panel, then into the paralleled RJ2B's..... then a
long length of SFT-600? to the rear panel 7-16 DIN
connector.?
?
The only time u can insert the shorted 1/4 wave stub
directly on the output of an amp, is if it's PI-L config.
( or a L-PI-L).?
