VTTC - optimal primary coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Herwig Roscher by way of Terry Fritz <twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>


Hi all,

I am running a VTTC, driven by a power tetrode. Plate voltage is 10 
kV filtered DC. Secondary coil is wound onto a 4" form.

In order to find the "best" tank circuit configuration, I wound several 
8" primary coils:
70 µH, 116 µH, 127 µH, 640 µH and 1100µH

After having tuned the system carefully in each configuration, I 
found the results of the first 3 coils to be lousy. With the 2 other 
coils I gained the same 17" discharges and I am assuming, that 
this is, what I could expect.

To my surprise it was not possible, to find the "optimal" coil 
inductance. With the help of list members I came to the following 
conclusions:

If the impedance of the tank circuit was not extremly low, its 
influence on the output power is low. The impedance, seen by the 
plate, mainly is the reflected impedance of the load. The 
impedance of the tank circuit itself becomes infinity at its natural 
frequency.

Different tank capacitances (necessary to keep the system in tune) 
did not affect the performance either (bang size).

Varying the coupling between primary and secondary coil from 0" 
to 2.3" (secondary above primary) had no visible influence neither 
on the length of the discharges nor on the volume of the plasma 
ball. Length of the 1100 µH coil is 4".

While tetrodes have higher plate resistances than triodes when 
operated in A-mode, plate resistance is not an important factor 
when operated in C-mode. Here the tube is driven into plate current 
saturation and acting as a switch.

Any comments/critics are very welcome.

Herwig