[Amps] 4CX1500B project

[Bill, W6WRT]

ORIGINAL MESSAGE: 
On Thu, 09 Aug 2012 08:15:00 -0700, you wrote:

>Good points. I had thought that maybe I would need a little inductor in the grid circuit 
>on 10/15m.
>
>I wonder if it would work to put it in series with the swamping resistor?
>
>The idea would be to tune out the tube's input capacitance on the high bands, but be small 
>enough to have little effect on the lower bands.
>
>If this doesn't work it would be easy to use a relay to switch it in/out.

REPLY: 
A few years ago when I built my GU-43b amp I experimented with several
broadband, untuned input circuits but I could never get one to work right. They
worked great on a SPICE emulation but in the real world they didn't. I'm sure
the reason was stray capacitance which was unknown and which SPICE therefore
could not emulate. 

On the website http://pa0fri.home.xs4all.nl/Lineairs/Frinear1500/FRI1500eng.htm
the author claims very low SWR, but I could not duplicate his success. I spent
many hours trying to get it right but never could. I finally gave up and just
used a simple parallel resonant input circuit and that worked with no problem.
The drawback of course is it has to be switched for each band. In my case that
was 17/15/12/10. The lower bands did not need any compensation. 

The odd thing is when I breadboarded the input circuit without the tube, just an
equivalent capacitance, it worked. In a real circuit it wouldn't. I still don't
understand why. 

I don't like to give up on things but I have to admit that one beat me. Maybe
someday when I have nothing to do I'll go back and try again. 

Better luck to you.   :-)

73, Bill W6WRT