Vic,
with some care and a bit of experience in correct positioning and wiring,
the problem to find or realize proper components to have and keep a
standard PI network whose loaded Q is 10 is a fake one.
The problem is that on 10m the residual of plate capacitor plus tube/s
internal plate capacitance in some cases are so big that the first usable Q
is not 10 but a much higher value. In that case it's really hard to find out
the proper components (coil) and do not destroy Q with multiband amps
related items like wires (even short) and/or switches (even small).
If the practical situation allows the design of a PI whose loaded Q is 10
and no other parameter is required such network is a totally superflous
complication.
Go ahead with a symple PI network.
73,
Mauri I4JMY
-----Messaggio Originale-----
Da: Vic Rosenthal <rakefet@rakefet.com>
A: Amps reflector <amps@contesting.com>
Data invio: domenica 1 agosto 1999 18.45
Oggetto: [AMPS] L-Pi net...downside?
>
> OK, another question. I'm designing the output network for my amp, and
I'm
> facing the issue of getting a reasonable Q on 10 meters with practical
parts.
> The 1999 ARRL handbook has a discussion of this, and they mention putting
a
> small inductance in series with the plate of the tube, before the pi-net
input
> capacitor. This has the effect of transforming the plate load impedance
to a
> much lower value (in their example, from 2200 to 1200 ohms), so that the
pi-net
> can be designed with a more practical value of input capacity on the
higher
> bands. The effect of this inductance decreases with frequency, of course,
so
> you can ignore it on the lower bands. Sort of a "L-Pi" (or L-Pi-L)
network!
>
> Recently I was looking at a 30S-1, and I noticed that the plate tuning
capacitor
> was a big air variable, right next to the shielding. "Collins should know
> better", I thought, until I saw the little coil in the anode compartment!
>
> 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.
>
> 73,
> Vic, K2VCO
> Fresno CA
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