Don't let the stray caps get ya.
Don W4DNR
Quoting Jim Thomson <jim.thom@telus.net>:
Date: Sun, 8 Jul 2018 20:40:52 -0500
From: "Jim W7RY" <jimw7ry@gmail.com>
To: "AMPS" <amps@contesting.com>
Subject: [Amps] 6 Meter Pi net and Tuned Input Calculator
<Looking for a pi network calculator that covers 6 meters.
<GM3SEK site just goes through 10 meters.
<Thanks
<73
<Jim W7RY
## sure it does. I use it almost continually. Just enter what
ever freq u want.
## I use it for PI nets. I also use it for PI TUNED INPUTS.
## beware, when using his online calculator, the one thing it wont handle is
say 50 ohms to 50 ohms... like on a tuned input, using a tube with
a 50 ohm input Z.
No big deal, just enter 50 to 49.... or 49-50 ohms. Similarly
you cant enter
75 ohms to 75 ohms... or any other combo where the Zs are identical.
## I have used the online calculator for tube amps... from new
475 khz band.....
up to 50.125 mhz ..and everything in between.
## when used for tuned inputs the plate load Z now becomes the 50
ohm xcvr output. The ant
Z now becomes the desired tube input Z.
## The beauty of his online calculator is.. it factors everything
in, like the all important
stray C.....and also the stray L
## On bands like 15-12-10-6m, I typ install a small coil, between
plate block cap..and main
C1 tune cap. That extra tiny bit of uh, + tubes anode to
chassis C, forms a step down L network,
making for a reasonable Q to be used. Without it, the typ loaded
Q on upper HF is sky high.
Then u can also see at a glance how that effect drops off, as you
move down in freq, like lower bands.
## I have found on metal tubes like the 3x3, etc, that the anode
to chassis C will rise, due to proximity
of the lower anode fins..to chassis below.... vs testing with LCR
meter on the wooden bench. On the
wooden bench, I get identical results to Eimac specs. It will
also rise due to proximity to side and rear walls etc.
## All this stray C..and intended or un intended uh can easily
be measured,and factored in.
## It will also factor in the plate choke XL. Use to small a
plate choke on say 160M band, and you will
have to use a lot more tune C, since you are in essence, parallel
resonating the plate choke anyway.
## same deal with the bifilar used on a GG amp. The 10 uh
bifilar on a SB-220 can be used on 160m,
BUT the C2 cap on the tuned input will have to be increased by
aprx 900 pf.
## The VK1SV spreadsheet is ur generic PI spreadsheet..which
wont compensate for all of the above, which is
why we dont use it, or similar generic PI spreadsheets.
Jim VE7RF
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