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[Amps] Tuned Input Answers

To: Amps Newsgroup <>,
Subject: [Amps] Tuned Input Answers
From: Jennings <>
Date: Sat, 04 Jun 2005 22:47:11 +1200
List-post: <>
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 



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




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.



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.

The noblest of all dogs is the hot-dog;
it feeds the hand that bites it.



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