[Boatanchors] Physics of plate current dip

[D C *Mac* Macdonald]

I seem to remember that the neutralization procedure for my
NCX-5 and NCX-3 transceivers basically involved adjusting
the neutralization capacitor to obtain the maximum output
power on 10m and 20m respectively simultaneously with the
"plate" (actually cathode) current minimum.
 
I remember doing the same with my Drake T-4XC transmitter.
 
73 - Mac, K2GKK/5
(Since 30 Nov 53)
Oklahoma City, OK

 
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From: km1h at jeremy.mv.com
To: k2gkk at hotmail.com; ianmwilson73 at gmail.com;
boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Physics of plate current dip
Date: Thu, 13 Aug 2009 09:42:56 -0400

With a linear amp it is often hard or near impossible to correlate the Ip
dip with maximum power out.

It is easier and more accurate to watch grid current in triodes and screen
current in tetrodes and pentodes. Tune for a peak in those currents by using
the load control and tune control AND keeping currents within tube specs.

Oft times commercial (and homebrew) amps are not completely neutralized on
10-15M and plate current dip then does not coincide with maximum power out.

Carl
KM1H


----- Original Message -----
From: "D C *Mac* Macdonald" 
To: ; 
Sent: Wednesday, August 12, 2009 11:11 PM
Subject: Re: [Boatanchors] Physics of plate current dip

Resonance is DEFINED as the frequency of the energy being applied
to an LC circuit when the Capacitive Reactance (Xc) is exactly equal
to the Inductive Reactance (Xl). The "c" and "l" SHOULD be shown
as subcripts to "X" but I don't know how to do that.

One way or another, the final amp tank circuit is usually in parallel
with the tube or transistor doing the amplification. Current existing
in a parallel circuit is minimum at resonance, but maximum at resonance
in a series circuit.

73 - Mac, K2GKK/6




Date: Wed, 12 Aug 2009 16:12:15 -0700
From: ianmwilson73 at gmail.com
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] Physics of plate current dip

Tuning a grounded-cathode power amplifier with a pi network goes as
follows:
 1. set loading to minimum (maximum capacitance on loading capacitor)
 2. tune plate capacitor for resonance as indicated by plate current dip
 3. increase loading (reduce loading capacitance)
 4. go to 2 until there is no increase in output power

I have done a lot of reading on tube output stages and have searched for
information online about the mechanism of the plate current dip but with
very little success.

Thinking about what happens as the plate circuit is tuned through
resonance, the thing that is special about the resonant condition
is that the plate voltage and current are 180 degrees out of phase
only at this point (in other words, the load presented to the tube
is purely resistive at resonance).  However, I can't quite make the
connection between this condition and the DC plate current.

Grateful for any enlightenment.

73, ian K3IMW
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