[AMPS] L-Pi net...downside?

[Phil Clements]

A 3CX3000A7 is a prime candidate for this method, for all the reasons
Tom states
below. There is a "free lunch" if you don't go for more than the
bottom four bands.
(10, 12, 15, and 17 meters)  For 20 meters and above where the Q is
reasonable
on this tube, I use a SPST vacuum relay about the size of a  25 watt
wire-wound resistor
across  the coil and short it out. These can usually be found at flea
markets for $10- $25
because they are not multiple pole nor are they fast enough for QSK.
I resolve another problem that rears its head in 9 band amps by using
one of these relays
to short out one of the two plate chokes above 40 meters. This
prevents those pesky
choke fires, and allows plenty of L on 160 to keep the RF out of the
filter caps.

The above paragraph is specific for the 3CX3000A7....your mileage may
vary.

(((73)))
Phil, K5PC


> I know this was discussed a few months or a year ago, and I recall
that
> KE9NA used this in his amp.  My question is:  what is the downside
of
> doing this?  It looks too much like a free lunch to me.
>

1.) It requires a tube that is unconditionally stable at all
frequencies, since it impossible to have an "in series with the
anode lead" parasitic suppressor system be effective.

2.) You have to watch low order harmonics if you leave the L
section in place on other higher bands.


73, Tom W8JI
w8ji@contesting.com

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