[AMRadio] Re: capacitors for tank circuits

D. Chester k4kyv at charter.net
Wed Apr 16 12:41:56 EDT 2008

> From: "John Coleman" <jc at pctechref.com>

> The size of the spilt stator capacitor that you are looking for (will) 
> 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.

For a balanced tank circuit, the capacitance is one fourth, but the voltage 
rating must be double that of single-ended.  The reason for this is, that 
for a single-ended final, the tube is working into only half the balanced 
tank circuit, but the coil acts as a step-up autotransformer and the induced 
voltage across the other half of the coil is approximately equal to the 
voltage on the half that the tube works into, giving an rf voltage 
end-to-end that is twice that which is actually generated by the tube.  It 
is exactly the same in the case of push-pull, since in class-C or even 
class-B service, only one tube is working into the tank circuit at a time, 
and each tube is working into one half of the tank circuit.  Or you could 
think of it as each tube generating equal rf voltage, but the outputs of the 
two tubes are in series as they work into the tank circuit.

But a capacitor with twice the voltage rating and one fourth the capacitance 
is equivalent to taking the original single-ended capacitor, splitting it in 
half, and wiring each of those halves in series.  That is exactly what we 
mean by a split-stator capacitor.  For example, take the BC-610, which runs 
the final at 2000 volts @ 250 mills.  The tank capacitor is split stator 
with 150 pf per section.  If the final were changed to unbalanced output, 
for example by substituting a 4-250 for the 250TH and converting to a 
pi-network, or by changing the grid tank to balanced and running the plate 
tank unbalanced,  the final tank capacitor would need to be 300 pf in order 
to keep the tank circuit Q the same.  That could easily be accomplished by 
wiring the two sections of the 150/150 split stator capacitor in parallel. 
The parallel connection gives 300 pf at approximately 7 kv rating.  The 
series connection as used in the balanced tank, renders 75 pf at 14 kv 
rating.  A single ended capacitor rated 75 pf @ 14 kv would be about the 
same size as the dual 150 @ 7 kv, and when new, the cost would have been 
about the same.

The point is, for a given power level and plate voltage/plate current ratio, 
the physical size and cost of the tank capacitor is approximately the same, 
whether the tank is balanced or unbalanced.  If a split stator capacitor is 
used, it can easily be connected up as a balanced or unbalanced tank.  Of 
course, the number of turns in the balanced tank will be double that of the 
single ended one, since 4 times the inductance is required to maintain 
resonance at the same frequency.    OTOH, the wire size in the unbalanced 
tank will need to be heavier, since the circulating rf current is higher 
with the higher L to C ratio.

I always raise my entire pushpull tank above ground and put the full HV on 
the whole capacitor, and series feed through the tank coil.  The rf choke 
has to do much less work than with parallel feed, because with series feed 
the rf choke connects to a zero rf voltage point on the tank coil, while 
with parallel feed, the choke goes to the plate of the tube and thus the 
full rf voltage appears across the choke.  Theoretically,  with series feed 
the B+  lead could be connected directly to the cold spot on the coil with 
no rf choke at all, but in a practical circuit, the B+ leads needs to be 
isolated from the tank coil with a choke since it is virtually impossible to 
maintain perfect balance in a nominally balanced tank circuit.

Don k4kyv 

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