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Measurement help please

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Kristian

Tue Aug 05 2008, 09:05PM

Registered Member #611 Joined: Wed Mar 28 2007, 11:40PM
Location: Hudsonville, Michigan
Posts: 79

I have spent the last week tweaking on my 833C Vttc. As of right now ,it's outputting about 14" or 15" arcs with around 85V in from the variac before the plate starts to glow. I have tweaked everything I could think of and then asked the Pupman list for advice. I followed their suggestions and some suggestions I picked up at Roger's Teslathon. The one thing I have not been able to figure out is two measurements that were suggested to me from the list. Those are the grid voltage and drive current measurements. I tried hooking my multimeter up to the feedback coil to take a voltage reading and all I accomplished was turning my meter into a paperweight. I asked the list how these measurements are taken and received no reply, so I'm bringing my question here in hope that someone will be kind enough to describe to me how these measurements are taken.

...

Tue Aug 05 2008, 09:26PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

'very carefully' would be how I would describe taking these measurnments...

For the grid voltage I would assume that they mean the peak grid current, which would best be measured by using a full wave rectifier of high voltage fast diodes (uf4007 should work, if you haven >1000v on your grid you know where your problem is....) from ground to the grid. You would also probably want to add a little bit of capacitance (few nF) to the output to smooth the signal off, and possibly a voltage divider to keep from blowing up more meters...

For the grid current I am unsure if they want the peak current or the rms current, or the average current... In any case I would put a resistor in series with the grid (100-1000 ohms would probably work, make sure to use a low inductance--not wirewound--resistor) and if you can a scope across it (although make dang sure that your primary circuit is 'floating' and isn't grounded anywhere, and that you don't touch anything while the coil is running...)

As always, never get near the circuit while the coil is on, or if there is any voltage left in the caps... A mistake with this stuff could be your last!

Steve Ward

Tue Aug 05 2008, 10:50PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

You might wanna just get some old analog current meter.

I have never attempted to make such measurements as they are quite difficult to do. The reason is that you are dealing with high voltage AND radio-frequencies, where to get truly useful information you would ideally have an oscilloscope measuring it. But, who wants to risk frying a good scope (or just the probes) just to make their tesla coil work better? I sure wouldnt.

And even after you get your measurements, is anyone gonna know what the heck to tell you about them? Probably not, because no one really measures this stuff anyway!

Kristian

Tue Aug 05 2008, 11:05PM

Registered Member #611 Joined: Wed Mar 28 2007, 11:40PM
Location: Hudsonville, Michigan
Posts: 79

Hey thanks guys,

I kind of figured these measurements to be a little too elaborate for what I'm trying to accomplish. I was hoping to get a quick measurement of the voltage from the feedback coil with my meter. I knew this was a bad idea from the start, but I had to fail to learn, I guess.

My problem is the plate starts glowing at relatively low voltage input and poor spark length. I have run out of ideas. I won't go into all the things I've tried. The list is long and, short of trying another tube, I know I've tried just about everything at least twice. If either of you guys are interested in helping me trouble shoot this thing, I could PM you a detailed list of what I have set up and what I have tried to do to get this thing working.

Myke

Wed Aug 06 2008, 01:19AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I haven't built a real VTTC yet but what I can think of is increasing the grid leak resistor and increasing the impedance of the tank circuit. More L and less C.

Steve Conner

Wed Aug 06 2008, 09:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'd suggest measuring grid current, which is done by lifting the end of the grid leak resistor from ground, and inserting an (analog!) milliamp meter. Compare this to the value recommended for a self-excited oscillator in the 833 datasheet.

This method gives the average grid current, but since the grid only conducts in one direction like a diode, the average is non-zero and a useful measurement of the amount of grid drive.

If your plate glows at low input voltage, you probably have plenty of grid drive, but the impedance of the primary circuit is too low. Try using 50% more primary turns and cutting your tank capacitance in half. And remember a certain amount of plate redness is acceptable for 833s.

The destruction of your meter also suggests you're not short of grid drive

Finally, what power supply are you using? If the voltage is low, like an un-doubled MOT, 15" might be as much as you're going to get. Everyone overdrives tubes way beyond their voltage ratings in VTTCs: I'm sure I remember some guy with six 833s running off a pole pig.

Kristian

Wed Aug 06 2008, 10:32AM

Registered Member #611 Joined: Wed Mar 28 2007, 11:40PM
Location: Hudsonville, Michigan
Posts: 79

Dr. Conner wrote ...

I'd suggest measuring grid current, which is done by lifting the end of the grid leak resistor from ground, and inserting an (analog!) milliamp meter. Compare this to the value recommended for a self-excited oscillator in the 833 datasheet.

Thanks, I will try this.

Dr. Conner wrote ...

If your plate glows at low input voltage, you probably have plenty of grid drive, but the impedance of the primary circuit is too low. Try using 50% more primary turns and cutting your tank capacitance in half. And remember a certain amount of plate redness is acceptable for 833s.

I've copied Steve's single 833A design. The primary is 29 turns of 12 gauge, tapped at 25, and the cap is 2nf. The tube is an 833C.

Dr. Conner wrote ...

Finally, what power supply are you using? If the voltage is low, like an un-doubled MOT, 15" might be as much as you're going to get.

The power supply is a large doubled mot with the shunts removed. I have tried a few different mots for this. I have also tried a few different capacitance values in the leveler circuit. I thought it might have been the diode but it checked out fine. I switched it out twice anyway, just to be sure.

The only differences between Steve's and mine are the feedback coil is 26 gauge, not 24, I have a slightly larger toroid at 2"x7", the plate filter is 12 turns of 12 gauge on a 50 ohm resistor, and I have a 4kohm resistor for the grid. I have not tried more resistance, but I plan to as soon as I pick up another resistor to use.

Things that have helped performance so far are I moved the feedback coil to about 9" up from the bottom of the secondary, and experimented with the pri/sec coupling. When the feedback coil is down where Steve has his, I would only get about 6" arcs. With the bottom of the primary level with the bottom of the secondary, I could not find the tap point. The output looked about the same for turns 22 - 29. Raising the secondary .5" gave me a definite tap point and increased the output by about 4". Raising it more than that, the primary tap point disappeared and the performance diminished.

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