[Amps] B&W PT-2500A on 10m?

[Joe Subich, W4TV]

> Both references suggest adding a single 133pf (100pf & 33pf in
> parallel) cap with a coil of 6t of 18ga wire, loose spaced (about
> 5/8" long ) on .500" diameter. However, this yields a C-L only
> solution for the tuned input.

That's fairly common for 2 x 3-500 on 10 meters.  The input capacitance
of the tubes provides the missing "C".  The tubes have about 8 pf grid
to filament each in addition to circuit strays, etc.

The L7 used CL networks on both 15 and 10, IIRC.

73,

    ... Joe, W4TV


On 4/3/2014 5:36 PM, bob wrote:
> I think we got off the rail a bit with respect to the history of the
> PT-2500A which has been discussed elsewhere at length.    While that is
> interesting, I need to get back the primary objective -
>
> What is the best way to get the stock amp back onto 10 meters???
>
> Thanks to both Dick W1KSZ and Tom W7WHY for responding with what turns out
> to be the same information (from separate sources, but same text) suggesting
> that only the 10m input need to be installed to get it working on 10m (a
> good thing).
>
> Both references suggest adding a single 133pf (100pf & 33pf in parallel) cap
> with a coil of 6t of 18ga wire, loose spaced (about 5/8" long ) on  .500"
> diameter.  However, this yields a C-L only solution for the tuned input.
>
> Yet, the schematic clearly shows a PI input network (C-L-C) for all the
> other bands where this article suggests a simple single C-L section (I have
> the original printed manual and schematics if anyone is interested, I have
> them on PDF soft form as well.)
>
> Any thoughts?
>
> Anyone recommend a PI (CLC) solution with values or have any idea of the
> factory update kit for 10m ?
>
> I am getting frustrated by some of the Pi matching calculators I have found
> on the web since I get rather large differences (20-100% diffs) in the
> calculated outputs from them with the same inputs, and because of that, I
> have little confidence in them.  This is rather surprising.
>
> I was using 50 ohms in,  and 60-65 ohms out (for a pair of 3-500Z's) for
> Rs/Rl respectively @ 28.5 mhz with a Q of 2 or 3.
>
> Any idea of jXL reactive component of the pair of tubes?
> Is my choice of 60-65 ohms fair for the input of the pair?
>
> What is my feeble mind doing wrong?
> Anyone got a spreadsheet or online calculator for input matching you have
> faith in?
>
> -bob
>
>
>
>
>
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