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

[Ian Roberts]

Hams are meant to use grip-dip meters and not worry about "Q" factors.
On the input circuit (amp "OFF"!), get the deepest dip (check the
harmonic response), on the output circuit, check for dips across the
widest frequency range while twiddlying the "load" and "tune" caps. Then
hope that the tube's input and output impedance doesn't change much as
soon as its switched on. 
If any dips (other than a nice deep one at the wanted response) are
noted, Murphy dictates that in the heat of a contest, that combination
will be tuned by the operator, then possibly bye bye amp.

To tune the thing, if all seems in order:
Leave the ouptut tuning on best match found, switch on and warm up,
twiddle the input tuning a bit (exciter's automatic tuner OFF) while
keeping a beady eye on the location of the HV post, the exciter's full
power output, the exciter's SWR, and the amp's power output by tweaking
the "load" and "tune" caps. Compromise for best output power vs input
SWR. Go and have a cup of coffee while the old nerves unjangle, count
yourself lucky to have survived such a frightening ordeal.
Ian, ZS6BTE

Maurizio Panicara wrote:
> 
> 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

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