Re: Tube Impedance Matching

["Malcolm Watts" <M.J.Watts-at-massey.ac.nz> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

On 3 Aug 00, at 0:03, Tesla list wrote:

> Original poster: "Herwig Roscher" <herwig.roscher-at-gmx.de> 
> 
> 
> Ross Overstreet wrote to the list:
> 
> > that you have to match the input and output Z of the tube, but I'm 
> > not really sure how to do this in the VTTC. 
> 
> Ross,
> 
> I've got the same problem. IMO the dynamic output Z of the tube 
> has to be adjusted to the the Z of the "brush". Terry (thanks, Terry!) 
> has found it to be about 20 kOhms + (3 kOhms/100 W delivered) in 
> series with 0.58 pF + (0.07 pF/100 W delivered). As the tank circuit 
> and the secondary circuit are parallel resonant circuits, they should 
> have an impedance much higher than the output Z of the tube and 
> the Z of the discharge as well. So, matching should be a question 
> of the TC transformation ratio.
> 
> Please correct me, if my assumptions are wrong.

A note on the subject of matching:

(1) Treat the tank impedance as being a shunt in parallel with 
the actual load (spark). You can see that a low L/high C tank 
will shunt the tube's internal impedance like crazy. You are 
not actually matching to the tank.
(2) I have seen a range of claimed output impedances for 
sparks of one sort or another. I suggest that in doing this 
people are attacking the problem from the wrong angle. I 
suggest instead that what really happens is that the spark 
"chooses" its own impedance based on the ability of the source 
to deliver. It will be seen from this that attempting to match 
a coil to a spark is an exercise in futility: the approach 
should be "what kind of spark do I want - ergo what kind of 
supply do I need to generate that spark?"

Regards,
malcolm