[Amps] 6 Meter Pi net and Tuned Input Calculator

[donroden at hiwaay.net]

Don't let the stray caps get ya.
Don W4DNR

Quoting Jim Thomson <jim.thom at telus.net>:

> Date: Sun, 8 Jul 2018 20:40:52 -0500
> From: "Jim W7RY" <jimw7ry at gmail.com>
> To: "AMPS" <amps at 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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