[Amps] Pi-L Network Question

[Carl]

Id think that a pair of 8877's run at 3CX800A7 voltages would be an ideal 
choice for a reliable amp and a Pi-L included.

OTOH, there are very few plain vanilla 800's available at modest cost and 
the cheaper 3CPX versions are being pushed at more HV in many HB or 
conversions such as the LK-800 series.

Right angle mounting allows a savings in horizontal space and shorter leads 
to the BS but at 1500W and shorting switches I cant confirm if there is any 
benefit. In days of old, etc, it may have been due to the lack of shorting 
switch availability and a lot of mil surplus was used??

An amp such as the NCL-2000 used a one coil 80-10M Pi (2500V/800ma) with 
identical efficiency on all bands and Ive rebuilt one of mine decades ago to 
include 160 using a toroid and some WARC with just coil taps. The switch was 
a non shorting Centralab JV-9000 type from a hamfest. At 1200W it never 
burped.

Carl
KM1H



----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: <amps at contesting.com>
Sent: Monday, June 18, 2012 11:56 AM
Subject: [Amps] Pi-L Network Question


> I've been noodling around various tank circuit possibilities for a 160m
> monoband amplifier. A pi-L network isn't very practical for an 8877 
> because
> the relatively high plate impedance mandates very large values of C1 and 
> C2.
> For an 8877, a simple pi-network is preferred. On the other hand, a Pi-L
> makes a lot of sense for, e.g., three 3CX800a7s or GU-74Bs,  because the
> combined plate impedance is under 1000 ohms and the required capacitance 
> and
> inductance values are quite reasonable.
>
> I know that good design practice is to orient L1 and L2 in a Pi-L at right
> angles to each other (or to use a self-shielding toroid for L2), to 
> minimize
> mutual inductance between the two coils. However,  I don't really 
> understand
> why this is so important. In principle, it would be very convenient to use 
> a
> single coil whose inductance is L1+L2, with a tap for connecting C2 at the
> junction point between L1 and L2. I'm sure that the mutual inductance 
> would
> invalidate this approach, but I don't understand why. Can somebody who's
> thought about this issue clarify it for me. Thanks!
> 73,
> Jim W8ZR
>
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