[AMRadio] capacitors for tank circuits


John Coleman jc at pctechref.com
Tue Apr 15 19:30:32 EDT 2008


Berrie:
	The size of the spilt stator capacitor that you are looking for may
need to be determined by the voltage and current that you intend to run, The
ratio of I:E determines the capacitance  There are charts in the handbooks
for this purpose.  Be sure to consider that you are doing push pull with a
balanced tank circuit as the required capacitance is only a quarter of that
required for non-balanced. The voltage will determine the needed spacing of
the plates.  If you raise the tank capacitor above the chassis on insulators
and connect the rotor to B+ you can get by with less spacing but you will
need to use and insulated coupler to the knob shaft and be sure the knob
shaft is grounded for safety.  You can also accomplished the same thing by
using two large RF chokes, one per tube, and capacitive coupling the RF to
the tank circuit as is done in most PI net circuits.  This way the plate
tank cap can be mounted to chassis and the RF choke at the center of the
tank coil can be grounded.  The idea is to not have a DC + modulation
voltage across the plates of the tank capacitor but just RF voltage.
Capacitive coupling, as was just described, is a neat way to do this but it
requires the very large long RF chokes and good coupling capacitors.  I
would suggest looking for 812s for the final and modulate with 811As.  I
think the 811A is good to 1500 Volts at Zero bias for modulators but can run
2000 if a regulated bias supply is used.  The 812s, for a push pull final,
are easy to drive and easy to light up.  If you're skeptical of their size
then use 4 of them.

The nice thing about a single band rig, is that you can just wind your coils
and bolt or solder them into place.  Making a adjustable output link may get
creative though.

Good Luck
John, WA5BXO   




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