Date: Thu, 19 Dec 2019 12:30:34 +0000
From: Bill <magoo@isp.ca>
To: amps@contesting.com
Subject: [Amps] HIGH INPUT SWR
<I had a query about my finding regarding the higher than expected input
<impedance of the pair of GI7B tubes in an amp I have been reworking.? I
<did some reading and found a post from Frank, W3LPL several years ago
<regarding the same thing. He discovered that if the input tuning
<networks are not located very close to the base of the tube the added
<capacitance of the connecting coax is enough to radically alter the
<input SWR, resulting in loss of efficiency unless the networks are re-tuned.
<That is the case in this GI7B amplifier-the tuned input is fed through
<approximately 10" of coax to the tubes, and it appears that this is the
<source of all the problems with this particular unit.? The elevated
<input impedance of 150 ohms rather than the expected 100 ohms for two
<tubes appears to be explained by a flaw in amplifier design.
<73, Bill VE3NH
## IF.......... EACH tube has 150 ohms input Z, use 75 ohm coax
from tuned inputs to cathode.
## what else screws things up is the stray C from cathode to
grounded grid. Its a fixed value..and
easily measured with a digital LCR meter. That stray C is directly in
parallel with the C2 cap of each
of your tuned inputs....via your 10 inches of coax. Its..free C2.
Eimac + Svetlana will list the cathode to grid C on its data sheets.
## To measure input Z, I guestimate the input Z....and design a TEMP
tuned input PI net for 40M. Both C1 + C2 are adjustable.
C1 + C2 tweaked for 1:1 input swr..... with amp driven full bore.
Amp then shut off. MFJ-259B connected to input side of 40M tuned input
...and a 0-250 ohm pot on
output side of 40M tuned input. Pot tweaked for 1:1 swr. Then
measure pot with a fluke 87A ohm meter... = input Z.
## Once input Z is determined, then use GM3SEK PI spread sheet to
design pi nets..for each band.
## On my HB GG Triode amps I did not want to mess with 9-10
band switched pi nets. I used 2 x broadcast variables + a tapped 4uh
coil... made from 6 gauge solid CU wire.
Both caps padded on 160 M . 6:1 Jackson Bros ball drives on each
cap. 1:1 input swr from 1.8 to 30 mhz continuously.
Jim VE7RF
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