[NCC] Amplifier troubles

[tleek8az]

On Wed, Jul 27, 2011 at 7:38 PM, KQ8M-1 <KQ8M at kq8m.net> wrote:

> I have an AL-1200. I had to replace a band switch due to arcing trouble
>
Tim,

Just a guess -- but I strongly suspect that you may have mis-aligned the
bandswitch wafers and are incorrectly switching in the wrong input network
on the wrong band.  (This could lead to bad problems, since, for example,
dumping 40m RF into the 30m input network might not make those components
very happy.)

One thing you can do is figure out what the input impedance of the tube
should be, and then substitute a resistor of that value for the tube.  (Data
should be on the Eimac website).  If things still look screwy when you
measure input impedance, you'll know it's not the tube.  (Use an MFJ if you
can.)  And if you do this test, you can also see if, for example, the 40m
signal gets "happy" on the 20m bandswitch position.

Another thing:  Given the arcing problem, I'd also consider installing an
AG6K parasitic kit on the plate of the tube.  3cx1200s are known to be
fussier than many tubes, and I know this helped calm down my Henry 3K way
back when.

73,
Tom, K8AZ