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[AMPS] 4-1000A Input Impedance

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Subject: [AMPS] 4-1000A Input Impedance
From: (Jon Ogden)
Date: Sun, 1 Mar 98 19:52:07 -0600
Howdy all.

Got a question on my 4-1000A.  What should the input impedance of this 
tube be?  I have heard that it is approximately 110 Ohms in parallel with 
27.2 pf.

My problem is that I had build an input network to match to that 
impedance.  Everything seemed to be working fine.  Suddenly however 
though, the impedance changed. The matching networks no longer gave good 
SWR. I can match up the amplifier with a match box and get full power out.

All the components in the input path of the amp are fine.  Today I have 
been playing with this tube (bought it about a month ago from Surpluss 
Sales of Nebraska) and with an old 4-1000A that was going bad (which is 
why I got the second tube).  The old tube appears to have the impedance I 
have expected or at the very least different from the newer tube.

I've managed by trial and error to match up 80 meters and 40 meters.  I 
am using an L-network (series L/shunt C - lowpass configuration).  Both 
circuits take larger inductors and smaller caps than I had expected.  I 
am still working on 20, 15 and 10.

Is there something wrong with this newer tube?  Did I do something to 
damage it?   Do different tubes have different impedances and/or can this 
change over time?

Should I not use this tube and stop working on the circuitry that I am 
working on or is everything OK.  Do I need to get a new tube (again!)?  
If I need to get a new (or different tube) where can I get one cheaper 
than what they sell them for at Surplus Sales of Nebraska?

I don't know if I am making myself clear here or not, so I am describing 
it as best as I can.

Can someone please get back to me with anything they know.



Jon Ogden

"A life lived in fear is a life half lived."

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