Hi All
Jorge, EA2LU asked for a copy of my replies to my question below:
"Hi all I am currently building an HF amp using a GS35B. It would appear that
the input impedance is around 50 ohms + or - and most circuitry that I have
seen does not use a tuned input circuit. I have plenty of drive from the
FT1000D available.
Any pros and or cons one way or the other?
Lee ZL2AL"
I received replies as follows. One respondent disagreed with the general trend
but did not want his reply posted. I respect his wishes and thank him for the
advice.
A grounded grid amplifier does not represent a constant load
to the driving stage, it will vary from very high when the
tube is cut off to quite low (a few ohms) at maximum anode
current. Although the average driving impedance may be close
to 50 Ohms, the driving stage sees the instantaneous swings.
Those swings cause the operating conditions of the driver
to change constantly ... as the instantaneous SWR goes high
at both ends of the range, the exciter tries to cut back,
etc. This constant change results in greatly increased IMD
from the driver and a relatively "dirty" output.
You would do well to add a good input network as close to
the tube as possible. A tuned input with a Q of 2 or more
will provide a bypass for the harmonics generated in the
tube's input and provide a "flywheel" to smooth out the
impedance variations seen by the driving stage.
73, Joe, K4IK
Using that tube without input circuits will cause
distorted audio - "splatter" - and your ham-friends
will not like it, hi.
If you are using only CW it may be all OK. That is
if your transceiver doesn't produce any key-clicks!
I'm using a 3CX1000A7 - 50 ohms input - without
input circuits on 80 and 160, but only on CW.
73/DX de Osten SM5DQC sm5dqc@areteadsl.se
Since you are going to all the effort to build an amp, why not build in
the input filter? The GS35B does not present a constant input impedance
of 50 ohms and the input filter can act as an integrator that "smooths"
out the effect. It certainly can't hurt if done correctly, and it could
help the quality of your signal.
I purchased the parts for mine from RF Parts for <$40 (ARCO compression
trimmers). Search the archives for more information on this topic,
there is lots to be learned regarding placement, sources, and such.
Tom, KD7QAE
If you are grid driven, then the input resistor is fine. If you are cathode
driven, the tuned input helps in a couple of ways. It reduces imd by 4 to 6
db and provides a load over the entire waveform for the driver transmitter.
Without it, half of the cycle presents no load for the driver.
In short, if it is cathode driven, it will work, but not as well as if there
is a tuned input. The input is 50 ohms only on half the cycle. The other
half is infinite.
Colin K7FM
If you decide on an input circuit take a look at Paul's wd7s.
http://wd7s.home.att.net/TU-6B.htm
73
Lee ZL2AL - ZL7AA, ZL8RI, ZL9CI, VE3LJ
The noblest of all dogs is the hot-dog;
it feeds the hand that bites it.
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