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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