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[AMPS] Another angle on cathode-matching ckts

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Subject: [AMPS] Another angle on cathode-matching ckts
From: (Jon Ogden)
Date: Fri, 12 Feb 1999 19:54:25 -0600
>Although I have not seen a great many amps in my day, the few I have always
>"pigtail" the coax connectorization inside the amp at all.  It
>would seem to me (and I've measured it as well) that the shield "pigtail"
>is poison to achieving good input matches at 21 MHz and above.  Obviously
>the introduced inductance would be that much worse at 3rd and 5th harmonic.
>My question:  is it really the coax itself that causes such pain in the
>input, or is it the lack of proper coax termination?  Would using quality
>bulkhead/panel fittings allow for remoting the input network the
>traditional 6-12" from the tube filaments?


I've not fiddled with moving my networks yet.  However, I can tell you 
one thing for sure.  Inside my amp I have a piece of thin coax coming off 
the bandswitch and going to a BNC panel mount.  The "matching network" 
end is where you would solder the coax.  The connector end is in the tube 
chamber.  I had grounded the shield of the coax initially on one end on 
the same ground as some of the matching caps.  Yes, the ground leads were 
a little long.  While playing with the matching circuits and the MFJ-259, 
I noticed that if I connected the OTHER end of the coax to ground (by the 
connector) the SWR improved a good deal (I'd been planning on doing this 
all along, BTW).  So  I would bet coax termination is a big issue.

You can bet I'll let everyone know once I start playing around with 
moving mine.



Jon Ogden
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