Today I tried changing the primary tank circuit capacitance from 2.1 nf to 2.5 nf. I didn't really expect any change in performance and, after re-tuning, I didn't see any. It does seem to generate the longest sparks more consistently though. I am going to stay with 2.5 nf because it puts me right in the middle of my 9 tuning taps giving me more leeway for future tuning adjustment. With the 2.1 nf capacitor I was on the next to last tap point. So my next test is going to be with my bigger MOT to see if core saturation is the problem. My best performance currently comes at 130 volts at the MOT input.
Upon closer examination one thing that I have noticed is the appearance of the sparks. In the VTTCs that I see on Youtube with the longest sparks, they appear to be almost completely straight with sharp edges (so-called sword sparks). Mine are not quite like that. The long ones are quite straight but all along the spark there is a fine "fuzz" of tiny sparks about 1/2 inch long emanating from the spark. I wonder if this is why I can only attain a 19" spark length? Maybe some of the spark energy is being siphoned off by this "fuzz". If this is a a source of loss, are there any ideas to eliminate this fuzz?
----- Original Message -----
From: "Steve White" <steve.white1@xxxxxxxxx>
To: "Futuret" <futuret@xxxxxxx>, "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Tuesday, February 6, 2018 10:26:01 AM
Subject: Re: [TCML] VTTC MOT question
I say 1/2 turn accuracy because that is the coarseness of my tap point accuracy as is almost every VTTC primary that I have seen. Since no one provides infinite tapping positions, except those that have wound an air-spaced bare-copper primary, I have one tap per turn available like almost everyone else that has built a VTTC. So if the optimal tap point was, worst-case, 1/2 turn away from my best available tap point then that is the worst-case tuning inaccuracy. This would correspond to a few kilohertz on my coil.
I have tried the tap points just above and just below the position where I get the longest (19") sparks. The position above (one less primary turn) yields shorter sparks. The next higher tap (one more primary turn) offers no improvement. Before all of that I measured the resonant frequency of my primary and secondary using my arbitrary waveform generator and oscilloscope. I measured and recorded the resonant frequency at all 9 tap points. I measured the secondary with various simulated sparks using thin wire extensions from the top load breakout point. In summary, I have the primary and secondary circuits well-characterized with regard to resonant frequency.
The only significant difference between my VTTC and Steve Ward's that I can see are two. The first is my primary. I used 27 turns of 8 gauge wire instead of 12 gauge. This causes my primary to be taller. It is about 8 inches in height. This should actually yield better coupling. I used the 8 gauge wire because I had it and I thought the thicker wire would provide lower resistance for RF because of the skin affect. The 8 gauge wire also allowed me to more easily attach my tap hardware. The 8 gauge wire also will be more resistant to heating. The other difference is that my tickler coil needed to be farther away from the primary than Steve Ward's. My tickler is about 3 inches above the primary coil. If I set it much lower, I get a distorted shortened spark.
Since tuning doesn't seem to be the problem can anyone venture a guess as to why I can't exceed 19" in spark length? My construction quality is high with short lead lengths and thick wire in the RF sections (copper tubing and strap in parts of the primary tank circuit). My only remaining conjecture was MOT saturation above 120 volts. I know that I should be satisfied with 19" but the "just a little bit more" bug has bit me and I have also seen Steve Ward's single-tube 833A coil putting out out 24" sparks so that is where I would like to be.
----- Original Message -----
From: "Futuret via Tesla" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Tuesday, February 6, 2018 6:11:52 AM
Subject: Re: [TCML] VTTC MOT question
I'm not sure what you mean about the tuning being within 1/2 turn accuracy?
Generally speaking as you apply a higher voltage using the variac, you have
to retune the primary tank frequency lower to compensate for the longer
sparks which have more capacitive loading. This would tend to explain
why your sparks are not increasing when you go from 120 volts to140 volts.
This is to be expected, unless you retune. I've always seen this effect in
my work. I don't think MOT saturation has anything to do with the problem.
I've never seen MOT saturation limit my spark length in any MOT that I've
used. Yes the MOT will saturate, but you'll still get longer sparks.
If you retune, then I think you should obtain 23" sparks.
John
-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil List <tesla@xxxxxxxxxx>
Sent: Mon, Feb 5, 2018 9:30 pm
Subject: [TCML] VTTC MOT question
Having recently completed my 833A VTTC based on Steve Ward's schematic, I am getting 19" sparks with staccato which is satisfactory but I think there should be a little more. I have a very large MOT now installed. I see very little difference in spark length when going from 120 volts to 140 volts on the variac. The tuning is within 1/2 turn accuracy on the primary so I know that is not the problem. The resonant frequency is 361 Khz. Connections are short and thick. What I suspect is that my MOT is saturating above about 120 volts.
The MOT that I am currently using is quite large and is probably from a 1100 watt microwave oven. I have another MOT which has 50% more iron in its core that shouldn't saturate as easily. It also appears to have more windings on the primary and secondary. It is from a very old Amana Radarange circa 1975. I would like to try this MOT out which leads me to my questions. Does anyone have any information on this MOT? Here is what I know about it:
* Dimensions are 145 x 108 x 72 mm
* The are a total of 7 terminals: 2 are for the filament, 2 are for 120 volt power, and 3 are on the HV side
* There appear to be no shunts
* The core is not connected to the HV secondary
* It is made by Advance, the model is AC-180, the catalog number is 09-123-18
My question concerns how to connect the 3 HV terminals. 2 of the HV terminals are spade lugs only 1/2 inch apart. The 3rd terminal is a wire coming right out of the secondary. When I measure the resistance between the 2 spade lugs, I get about 3 ohms. The resistance between the wire coming coming from the secondary and either of the 2 spade lugs is about 67 ohms which seems correct for a HV secondary. I thought that the 3 terminals must be for a center-tapped secondary but the ohmmeter readings say no because of the very low 3 ohm resistance between the 2 spade lugs. It almost seems as if the 2 spade lugs are connected but I would think the resistance between them would be essentially zero and not 3 ohms. I think the 2 HV leads to use are the wire coming from the secondary and one of the spade lugs. I don't know what the other spade lug is for. I assume that since the core isn't connected to the secondary that I must ground the core and one end of the HV output myself.
So, in summary, here are my questions:
* How do I use the 3 terminals connected to the HV secondary?
* Has anyone ever seen a MOT without shunts? Maybe they did things differently back in 1975 when cost wasn't as much of an issue.
* Does anyone have any data on this MOT?
I have searched extensively on the internet for info about this MOT but I can't find anything. Any information about this mystery MOT would be appreciated.
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