Date: Fri, 22 Dec 2017 14:11:13 -0700
From: Steve London <n2icarrl@gmail.com>
To: amps@contesting.com
Subject: [Amps] Operational Input Impedance
<Several years ago, I modified my Alpha 76PA, replacing the 8874's with a
<pair of 3CX800A7LG's, purchased from Alpha. These were new, Chinese made
tubes. Other than that, I know very little about them.
I have never been happy with the input SWR - it's in the 3:1 to 4:1
range, with about 25 watts of drive (and 500 watts output). The input
network is the standard Alpha 76 2:1 unun. The 95 ohm swamping resistor
pack has been removed from the circuit. Despite the high input SWR, the
amp seems to work satisfactorily (when using the transceiver tuner to
"fool" the transceiver into putting out adequate power).
Today, just to make sure the input network wasn't fowled up, I pulled
the tubes and put 22 ohms and 50 pf across the cathode pin to ground, to
simulate the published specs for a pair of Eimac 3CX800's. Using an
antenna analyzer, the SWR was close to the expected value - 1.2 at 3.5
MHz, rising to 1.8 at 21 MHz. Nothing like the 3 to 4 SWR observed
during the operation of the amp with the Chinese 3CX800's.
So, this begs the question: What is the actual input impedance during
operation ? How would one go about measuring it with something like an
antenna analyzer or VNA ?
73,
Steve, N2IC
## An UN-UN is not a tuned input. You need a PI net to achieve any amount of
fly wheel effect.
Henry radio tried the 1 : 1 un-un trick with their 8K ultra..which was a
total disaster. The 3CX-3000A7
tube they used already has a 50 ohm input Z. SWR sucked, and henry said NOT to
use the auto tuner
in the xcvr, go figure.
## GG triodes are only driven on aprx half cycles, like typ 210 deg
conduction in class AB. The Z
is sky high during the remaining 150 degs !
## You cant put a VNA or anything else in there to measure the input Z either,
it never works. Tube has to
be driven, and the more drive applied, the lower the input Z becomes. That
explains the difference between
your VNA and when actually driven.
## heres what I have found to work, if installing conventional PI net tuned
inputs. I use GM3SEKs PI
and PI-L spread sheets to calculate the C1, L, C2 values. The total tube C
has to be subtracted from the
calculated C2 value the software spits out. The software was made for a kw PI
net. It can also be used for
tuned inputs. More often than not, the C1 + C2 values ultimately used are a
bit off from the spread sheet.
## another method I use is to install a small pair of air variables like arco
compression trimmers, and padded
for the lower bands + the coil value that the spread sheet spits out. I
measure the values of the caps + coil
with a B+K 875-B digital LCR meter. Caps tweaked for 1:1 swr, when amp
driven to full output. Ok, then
power down amp, leave tubes installed. Put your VNA, or MFJ-259B on the
INPUT of the amp, then with TR
input relay activated, put a 0-100 pot or 0-500 ohm pot between cathode and
chassis. Tweak pot for 1:1 swr.
Then measure the dc resistance of the pot. Say its 36.5 ohms. Thats what
your tuned input is matching, that,
plus the total tube C from cathode to chassis.
## Ok, at this point the PI tuned input works dead on... at least on one freq.
By varying the C1, L, C2 values, you
are now tweaking the Q of the tuned input. You can get extremely close, by
entering the desired Q on the
spreadsheet! Beware, the Q on the spreadsheet is split into input Q....and
output Q. When the various handbooks
talk about an input Q of 2, its actually the input Q. Output Q will typ be
aprx 1.2 Total Q will be 3.1
3.1 to 3.5 is the typ Q value you feed into the software. When the
spreadsheet spits out the values for C1, L, C2,
it will also spit out the input Q...and output Q. The input and output Q
always total the desired total Q you enter
very early one.
## same deal with KW PI net... or PI-L networks. Old books said to use a Q
of 10. New books say to use a Q of 12.
When the old books said to use a Q of 10, thats actually the input Q....and the
output Q is typ 2. Total Q = 12..which is
what the newer books use.
## Your 2:1 un-un will probably get you pretty close to the pair of tubes
input Z, but the 50 pf Tube C from cathode to
chassis is what will screw up your swr...esp on the upper HF bands. The only
way to tune that out is a coil between chassis
and cathode, to cancel out the 50 pf of tube C. Problem then is, that
value of coil will be different for each band. Which is
probably another reason the swamping resistors were installed.
## You cant do this on a table top amp, but on my hb amps, I was not prepared
to install 9 tuned inputs. I instead used
a pair of 64-2180 pf broadcast variables.... both padded on 160m only. And a
tapped 4uh coil. 4 uh used on 160m,
2uh used on 80m, etc, etc. 6:1 jackson bros ball drives, which come in a 4
inch diameter, and also 3 inch and also 2 inch.
Skirts are calibrated 0-100 over the 180 deg arc. Set tuned input
bandswitch for desired band. Then tweak the 2 x caps
for flat swr. Repeat for each band. Write it all down. After that, its just
dial up by the numbers. swr is always 1:1 across
each of the 9 HF bands.
Sri for the diatribe. Tuned inputs over the years have caused a lot of grief
for folks.
Jim VE7RF
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