16/04/2013 16:34
May I ask a question re my construction of a tuned input circuit? I am
converting a Henry 2000D RF Generator on 27.12 MHz to (initially) a
single band (20 meter) linear amp. I am trying to build the tuned
input circuit, after checking it works on 27 MHz. The 3CX3000A7 valve
has a cathode input impedance of about 48 Ohms. It has a stock Henry
Pi input tuned circuit comprising a slug tuned coil on a half inch
diameter former (about 4 turns 20 gauge wire), and two sets of
paralleled dipped silver mica caps each side of the coil, to ground. I
have acquired a new henry former and have wound it to give .475 uH on
my AIM 4170 analyser. I have a pair of air variable caps of about 350
PF max each. I have built it as per the photos at:
http://www.chriswilson.tv/pi/input_pi.zip
The idea of this Pi circuit came from my reading and is to
is to add Q and give a flywheel effect, ideally needed in Class AB GG
linear inputs.
http://www.eham.net/ehamforum/smf/index.php?topic=12665.0
See last post by W8JI Tom Raunch.
You all know this of course, don't get me wrong, just showing my source!
:)
I am unsure if the antenna matching "stuff" in my Kenwood TS-590
exciter precludes the need for this, though? I am trying to match 50
ohms to near 50 ohms,which is the cathode impedance of the 3CX3000A7
valve, I guess the antenna matching unit in the Kenwood would do this,
if I just removed the stock Pi input on the Henry, but would I then
lose the desired "flywheel effect"?
Measuring my circuit on the bench I see ZERO Q. With it terminated
into a 50 ohm dummy load I can get a perfect impedance match, but Q is
zero! Should I be inputting the required frequency span from the AIM
(say 14 to 15 MHz) into the input and have the output terminated in a
dummy load, or with the output (linear amp) side of the circuit open?
I also measure the stock Pi input components, with both a 50 ohm
carbon composite direct to ground on the output side, and at the
cathode, after the de-coupling cap. The AIM gives nonsensical
readings, so I am either doing something wrong, or misunderstanding
things.
I am similarly confused by advice I have had regarding *measuring* the
stock 27.12 MHz input circuit in the Henry, as a test of my measuring
methods. I am advised to put a 50 ohm carbon composite resistor from
the cathode of the 3CX3000A7 to ground and use the AIM on the RF input
socket to the RF Generator RF deck to measure how it is now. But
looking at the circuit diagram of the Henry I see there are isolating
capacitors between the PI input circuit and the actual pins for the
cathode. Shouldn't the loading resistor be direct to the output side
of the Pi tuned circuit? If not I now have doubts over whether a dummy
load DIRECT to the output side of my own Pi circuit, on the bench, is
correct, or if it needs the de-coupling capacitor before the dummy
load?
If required, the stock Henry RF Deck schematic is at
http://www.chriswilson.tv/pi/RF_Deck.jpg
In other words I am very confused, could anyone spare a few minutes to
help me out? Thanks.
--
Best Regards,
Chris Wilson.
mailto: chris@chriswilson.tv
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