Date: Mon, 05 Apr 2010 23:03:33 -0700
From: Vic K2VCO <vic@rakefet.com>
pi-net spreadsheet question
I'm using the GM3SEK pi-net design spreadsheet
<http://www.ifwtech.co.uk/g3sek/>. It
requires that you enter the inductance and resistance used in the parasitic
suppressor.
What I'm wondering is how to do this for two tubes in parallel, where there are
two
suppressors.
Do you enter the value of one coil and resistor, or do you treat it as though
these
components are in parallel?
## excellent question.. and I have been using his pi sheet for yrs now...
but only
1 x tube amps... or 2 x GS35bs... where no suppressor is used at all.
## I'd say to parallel the values of the 2 x suppressors. If it's a GG amp,
the grids
are in parallel at one end. The plate block cap is where the 2 x suppressors
come
together at the other end.
This makes a big difference at 28 and 21 MHz.
I am finding this exercise very interesting, playing with the value of "lead
inductance".
I'm trying to add a small "l network" before the main pi network which will
cancel out
some of the large amount of output capacity of a pair of 813's so they will be
reasonably
efficient on 10 and 15 meters. The parasitic suppressors become part of this,
too.
## that feature is the BEST part of the spread sheet ! Start with just the
paralleled
suppressors, NO lead uh. The tune cap may end up neg value.. or very low..
like
just a few pf. [say for a Q=14-18] Take what ever value it spits out for
the main coil..
and then divide by 2. EG: spits out say .. '.8 uh' .8/2 = .4uh
Ok, then make the
lead uh .4 uh. Main coil value may well change. Juggle the lead Uh value,
till BOTH the
lead uh and main coil are identical.
## Ok, now you can probably DEcrease the loaded Q. This all depends on Min C
u have
and whether a vac cap is used, etc.
The idea here is to obtain the lowest Q you can, and still not have the tune
cap totally
unmeshed.. and keep the main coil and 'lead uh' the same value.
## I call the result a ' L-PI' and it works very well ! On my 160-15m
[ no 10+12m]
3CX-3000A7 amp, sure, it would tune 15m.. with out the extra .66 uh b4 the
main
pi-net.. but loaded Q was WAY too high for my liking. Also, my 15 M position
is also
used on 17m. So loaded Q on 17M is through the roof. The 3x3 tube has
24 pf of
anode to grid C... and rises to 33 pf.. when tube in the socket. Min C of
the vac cap is
12 pf... so I have 45 pf.. with the vac cap unmeshed !
## with .66uh b4 the pi -net..... plate load Z drops way down. Now I can
get the loaded Q
on 15m down to just 8....... which rises to Q=12 when on 17M.
## In this case.. a .66 uh coil was made from 7 x turns of 3/8" od
tubing... on a 1.5" ID.
The .66 uh coil is at right angles to main tank coil. I THINK the
harmonic suppression increases
having the 2 x coils at right angles. Main tank coil was 3" OD.. also made
of 3/8" OD tubing.
## so the extra coil doesn't have to be the same diam as main coil. In my
case, I wanted it
long and skinny, to bridge the gap between Plate block cap assy.. and vac
tune cap.
## on another amp, the vac tune cap was tapped one turn into the main coil.
In this case, it's
all one coil. So both coil orientation schemes have been tried, and both
work very well.
## On older desk top amps, where loaded Q is sky high on 10M... adding a tiny
bit of uh between
plate block cap and C1 cap does wonders. The extra uh can also go anywhere
between the anode
and C1 cap. Then, using the stock value 10m tank coil... the loaded Q
drops like a rock, way less
circulating current, more eff, and main tank coil and the poor band switch
don't get cooked! But the
extra L coil has to be same OD as main tank coil.. or bigger, if main 10m
tank coil is marginal to start
with.
## I designed a 6m 8877 tank coil setup for a fellow 2 yrs ago. It used a
.5 uh coil... wound with
1/2" wide flat strap... and tapped dead center in the middle of the coil..
with the tune cap. The strap
coil is wound like a suppressor.. with the advantage that there is zip C
between turns.. since they
look like knife edges facing each other. 3/8" wide strap is the eq of
1/4" tubing, since the
circumference of both is aprx the same. 1/2" wide strap = .318" OD tubing,
etc. He ends up
with .25uh on either side of the tune cap. This works very well. Two
separate .25 uh coils
can also be wound... and at right angles, or anything from in line to 90 degs
between em.
## That extra L coil gets installed on all my amps, since I discovered the
spread sheet, 10-12
yrs ago. b4 that.. the maths by hand was tedious.
later....... Jim VE7RF
--
Vic, K2VCO
Fresno CA
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