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[Amps] SB220 - 10 Metre input coil adjustment

Subject: [Amps] SB220 - 10 Metre input coil adjustment
From: "jim.thom" <>
Date: Sun, 2 Jan 2022 08:13:56 -0800
List-post: <>
Date: Sat, 1 Jan 2022 23:01:26 -0500
From: "Tim Duffy" <>
To: "'Sam Ferris'" <>, <>
Subject: Re: [Amps] FW:  SB220 - 10 Metre input coil adjustment

<So two 3-500Z tubes give about 50 ohms feedpoint Z. W1QJ can correct if
need be.

<So take OUT the tubes - put IN a 50 ohm non inductive resistor there. You
know <where it goes?

<Then use the RIGEXPERT and find the match on the input - get it to 1:1
(you can do <this easily with the core).

<Then you will have over 1 KW on 10 meters output.

<Tim K3LR

##  Leave the tubes in their sockets.  Each tube has aprx 8.5 pf of C,
between grid and cathode.  That's 17 pf of input C, that's  directly in
parallel with  C33 and C34
(both 22 pf) on the cathode side of the coil.   With both tubes removed,
you also removed 17 pf....which will throw the tuned input way off.  Input
C is easily measured with a digital  LCR meter. ( grid to cathode ).

## eimac does not spec what the input Z of a  3-500Z is.  Orrs's books
say  130 ohms per tube, and  65 ohms for a pair.   I believe it's closer to
50 ohms for a pair.

##  once u have it tweaked for flat swr,   you can always temp install a
100 ohm pot instead of  your temp  50 ohm resistor.  Adjust the pot for
flat swr, then remove the pot..and measure it.  That will be your real
input Z.

##  I have used that pot trick a few times when the input Z of a tube
varied quite a bit, depending on drive level, B+, bias etc.   A guesstimate
was done, and a single tuned input was built for 40m.   That consisted of a
pair of arco variable caps, and a fixed coil. Tube driven to desired
output, and both caps tweaked for 1:1  swr.  Pot then temp installed
 between cathode and chassis.  Analyzer installed on input of amp..and  TR
relays temp operated.  No B+, no fil V applied, no drive applied, no bias
applied.   Pot tweaked for  1:1  swr.  Pot removed and measured  for
resistance. That value is your real input Z  the temp tuned input is

##  Knowing the actual input Z, the rest is simple...the PI net values are
all done in software.... factoring in the input C of the tube, or
tubeS..and also the INPUT Q.   At the same time, the input  Q can be
selected, and changed, depending on which band the tuned  input is for.
IE:   200 khz wide on 160m is different than 500 khz on 80m, and is
different  than just 300 khz on 40m, etc, etc.   10m is 1.7 mhz wide.  The
10m tuned input may have to be re-used on12m.  The 15m tuned input may have
to be re-used on 17m.   The 40 or 20m  tuned input might have to be re-used
on 30m.  If warc bands not used, the tuned inputs can be designed to
accommodate  just one band.  If 9  tuned inputs used, then each  band can
also be optimized.  EG:   30m band is only 50 khz wide,  its  Q  could be
be increased a bit. Same deal on 12m, where that band is only 100 khz
wide....and ditto with 17m.

Jim   VE7RF
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