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Re: [TCML] VTTC-tuning. John got me started ;-)



Thanks Dr. John.

That is a fantastic list of tuning issues which should come in handy for VTTC coilers. If the top terminal is making that much of a difference, then it is certainly a finicky coil (it may be quite difficult to make a tube coil portable). The tuning issues themselves are fascinating to me from an LC aspect. I've noticed this need to detune with VTTC's which is why I asked the question. It appears there is an inductance or capacitance that is in series or parallel with one of the coils (not related to a resistive component). Is there an LC aspect regarding the tube itself? I'm not a tube guy, so that question may seem silly.

Regards,
Bart

Dr. John W. Gudenas wrote:
Hi Bart
Here is a short list of tuning issues:
1) The tube impedance needs to be considered in the tank circuit
2) The coupling is high and the secondary height (like in any coil) needs to be adjusted empirically
3) Good RF capacitors are essential considering both voltage and current
4) Tapping the primary helps, but the adjusting tank capacitance in LC plays a big role 5) Consider tuning an old AM radio where moving pico farads found your carrier on the AM band.
6) As John F. indicated sometimes you resonate initially on a harmonic f.
7) Grid coil and grid leak need to be adjusted for a fully loaded secondary 8) Grid coil placement and number of turns is tube dependent (usually adjusted by experiment) 9) Tuning self C on the secondary with a top load adjustment of different size toroids can put you in or out of resonance. 10) Pushing high power can result in grid coil flash over (solved by raising the grid coil to a nodal area or my pancake coil grid under the primary that has a loose coupling to secondary and tighter to primary 11) Conservation of energy: To preserve tube life and have ultimate performance the secondary must be in such perfect tune as to use (resonate) as much energy in the tank circuit as physics allows. Any unused goes to heat or picked up in the grid circuit causing many different unwanted things to happen.

For example, I thought my coil was really running great, but just to see what would happen I put on an inch larger toroid, I lost streamer length and the plate got red hot (conservation of energy) So, I put on a smaller one by about an in from what I started and streamer length went up 6-8 inches. I had to drop the coil from my work table as it made white hot power arcs to the floor joists on the ceiling on full 140 v variac.

I also use a separate variac and stable filament transformers (scrapped from an AM radio Station) to set the tube filaments to their proper voltage.

Anyhow, that is just for starters from a brain fried from correcting freshman natural science papers.

Thanks to John F. I have a Christmas tree of toroids to choose from. I also picked up an English Taylor metal spinning lathe that I am slowly rebuilding to make various HV terminals. I have a TIG welder too so I can produce seamless terminals with some grinding & polishing. It all just takes time. Now its time to hit the sack so I can act rationally in tomorrows morning class until the coffee works.
I am sure John F. will add to the list.
John W. G.

John W. Gudenas, Ph.D.
Professor of Computer Science

On Mar 30, 2009, at 10:29 PM, bartb wrote:

Hi Dr. John,

Dr. John W. Gudenas wrote:
If you are tuned at 250 KHz stay there as you have already discovered VTTC tuning is tricky.

What is it about VTTC's that makes tuning so tricky?

Bart B.
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