[Amps] Plate Choke Issue

[Rob Atkinson]

There are a number of factors affecting choice of plate choke.

Most hams know that series resonances must be keep safely out of the ham
bands for which the RF amplifier will operate in.  It must also handle the
sum of the dc plate supply current and RF current and withstand the peak RF
voltage at the positive peak of the sine wave.  The whole point of the
choke is to reduce that peak voltage to a level the power supply can
tolerate, and power supplies with capacitor input filters made with
electrolytic caps are most vulnerable.  A certain amount of inductance is
needed for this along with bypass capacitance.  This combination varies
with the frequency coverage needed, and the type of RF amplifier or
transmitter.   For example a plate modulated AM rig has a different set of
requirements than does a RF linear amplifier for the high bands.  I
recommend doing some reading--Rauch and Measures both have web pages with
some interesting statements that can get you thinking, but once you have
some knowledge, it is a good idea to get some test equipment, such as a
signal generator, inductance meter, oscilloscope, Grid Dip oscillator, a
bunch of clip leads, and an assortment of plate chokes of different types
of construction, and resonances, and simply spend a month or two examining
their characteristics and values.   Measure the inductance of a choke, put
RF into one end and load down the far end and put it on an oscilloscope and
sweep the generator up and see where you begin to observe RF on the scope
trace.  Use the GDO to find series resonances.

The one factor many forget about is the ability of the choke installation
to dissipate some heat.  Pie wound chokes have a lot of inductance but are
poor at dissipating heat.   Study manuals and schematics for higher power
ham rigs, medium wave broadcast transmitters, and any QRO ham linear amps
you can find on-line and use google images to look at plate chokes on
chassis.   You'll find solenoidal wound chokes do much better at
dissipating heat and are usually located where air will flow over them, but
they have lower inductance so for example you may find one in series with a
pie wound choke, the solenoidal taking the brunt of the RF voltage at the
anodes, followed by the pie wound to clean up what's left.  But it's a lot
easier to set up something like that with low bypass capacitance if you
only have one band to worry about.

Or, if the plate v. is high or the current demand is high, you may have to
make your own choke using an epoxy glass coil form and AWG 21 or 22 magnet
wire.   A 1 1/4 inch outer diameter and 150 turns should get you around 220
microHenries.   I've heard the popular Ameritron 1.5 A choke melts down at
6 KV and continuous carrier.   I'd also steer clear of that thing with a
ferrite rod in delrin.

73

Rob
K5UJ