In days of old, etc it was said that adding a tuned input improved IMD about
5dB by providing a flywheel effect and a steady load to the exciter.
Leave the suppressor alone, a 4X1 needs all the help it can get if used
above 20M especially.
Carl
----- Original Message -----
From: "Randy S." <rj3819@yahoo.com>
To: "Amps" <amps@contesting.com>
Cc: "Randy" <swartr@twcny.rr.com>
Sent: Saturday, January 29, 2011 3:23 PM
Subject: Re: [Amps] Tuned Input - IMD and efficiency
I realize this is returning to an older topic but wanted to add something I
tried today.
I have a homebrew amp on HF that runs a pair of 4-400A's in GG.
Untuned input. So it fits the topic I guess.
After reading thru all the great posts I decided to insert a low pass filter
right
at the inpt connector of the amp, no cable and so the only other cable is
from the rig FT-897D to the amp. The cable is RG-8 I believe .. or 213..
OK .. without the loe pass filter .. I get about 1000W output.
I inserted the lowpass filter and the output went up to about 1150W.
I have not tried to check IMD and the only way I have to do that is to
use my Rigol 100 Mhz Digital O'scope in FFT mode. Its all I have ..
So I will try to run the test again and see if I can see a difference in IMD
or not with the filter. I realize .. after reading all .. that is a complex
issue
and topic with an asundry of variables .. But figured what ever I can add
to the mix with real time data in my case would hopefully be some value
added.
I did notice on this amp. that the original builder ( not myself ) put what
looks like
a parasitic suppressor in series with the RF input right at the end of the
coax
feeding the RF to the tubes. Would that be for the higher bands like
mentioned
in the posts? Or is it "old school " and can be removed and perhaps try some
of the methods mentioned in the posts on this topic?
I am also going to at some point buy an LDG autotuner and try to attach it
with an SO-239 / PL259 type Angle adaptor, again, so there is as little
coax between the tuner and the input to the tubes as possible.
I wish there was enough room in the amp to install it .. but the only place
I
have
will disrupt the airflow and so I am not sure thats sucha great idea.
I will try to measure IMD with the LDG Tuner scenario as well to see the
affects.
Any constructive comments or suggestions are always welcomed.
Randy
N2CUA
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