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> Gu81m Vttc
2ft-sstc
  Posted: August 04, 2011 02:33 am
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here was the first light http://www.youtube.com/watch?v=t7qAixBNQxM

and some minor tuning that I did http://www.youtube.com/watch?v=YDKPu2U8Ato

I'm gonna do some more tuning tomorrow, I'm gonna swap the bulb out for 150watt (instead of a 200watt) and I'm gonna add some more turns on the feed back and take some off of the primary.

here the circuit I used http://www.davekearley.co.uk/TeslaCoil/VTT...%20circuit.html

I used the top schematic, any tuning suggestions?
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CWB
Posted: August 04, 2011 02:48 pm
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from the directions on the schematic :
"L2 - Feedback coil, 20 turns insulated wire on a 150mm former,
mounted 1cm below the primary coil, moveable for tuning."

taps are a good idea but the amount of feedback (aka : grid drive) is determined in part by the physical placement of the coil . the closer to the cold end of the primary , the less drive there will be . too much drive will not allow the tube to operate in the class C mode .
from your video it does not seem to be movable .
the best thing to do is wind this coil on a slightly larger form and use small plastic or wooden wedges (in from the bottom) to hold it in place once the best spot for drive is found .
as you have no method for determining grid current i suspect that the amount of drive is held to a maximum value by virtue of "fail safe" design .

how much B+ are you running ?
good well lit still pictures of the unit would be nice for determining good hf wiring procedures .

i do not think the placement of the primary coil is correct in the schematic .
normal convention is to place the primary as close to the bottom (cold end) of the secondary as possible .

the plate current is normally monitored for a dip in the current ... this indicates resonance .
the grid current is peaked to determine resonance .
you have no easy way to do this safely .

i also question the number of turns on the primary . as a rough guess it is not close to the plate impedance of the tube . this is where a tapped primary would help ... but you also have to be able to determine resonance ...
have you ever built a grid-dip meter ?


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2ft-sstc
Posted: August 04, 2011 05:12 pm
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I will try Moving the feed back, its not that hard to make the wire is just tacked to the pvc with hot glue, i'll try moving it, My B+ is 4200 witch is probably a little high, but really its not heating too bad, When i get some time i'll do some more work, i just had 5 minutes to right now, tonight i will have time more time.
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tekwiz
Posted: August 04, 2011 07:03 pm
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If you want to draw arcs with a screwdriver, try holding firmly to the metal part. It's when the actual arc plasma touches your skin that the pain occurs...mostly due to the plasma's 20,000 temperature.
I've drawn 24" arcs from a SGTC many times this way, with no more than a slight tingle. wink.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: August 05, 2011 12:15 am
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I'm at 5 inch sparks now biggrin.gif cool.gif I'll make a video soon, as soon as i take care of the plate heating problem now, it heats up in like 10 secs
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CWB
Posted: August 05, 2011 02:17 am
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plate running hot ...
too much B+
too much drive
suppressor screen "swinging" too much
not enough negative bias on the grid
tank circuit out of resonance
excessive coupling between primary and secondary (or not enough) .

the first thing to do is drop the plate supply down to the recommended voltage (or less preferably) .


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R. Feynman '88
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2ft-sstc
Posted: August 05, 2011 03:59 am
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lots of people built this circuit and with enough tune they can run the circuit with that b+ but i'm gonna run it as just a rectified mot, then when its tuned I will crank er up again thumbsup.gif

I'm up to like six or seven inches... but my camera is dead, rest in peice...es doh.gif

I'll buy a new one tomorrow
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GPG
Posted: August 05, 2011 06:12 am
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Perhaps a dip meter will help. Haven't tried one on a tesla though.
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CWB
Posted: August 05, 2011 01:38 pm
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good idea on dropping the B+ during tune-up .
thoriated tungsten filaments can be ruined in short order by excessive plate/cathode current ... not to mention either wiping out the grid or having thorium deposited on it so that it goes into grid emission .

you are still faced with the prospect of not having any metering in order to help you determine if things are operating correctly (within parameters) .
i do not recommend blind tuning for the inexperienced person ... especially around HV .

you may be able to use something like two turns of wire with a #47 lamp (or similar) connected to it (short leads !) as an aid in determining resonance and maximum power and transfer efficiency .
this would be held near the coil (and above the primary/tickler coils) on the end of an insulated rod ... starting at the bottom and moving up until the lamp glows at half brilliance . fix the coil into this position and then start tweaking the tuning . you must keep the same spacing away from the coil .
you will have to move the lamp in order to keep it around half brilliance ... moving toward the cold end is what you are striving for .
an option/adjunct to moving the sniffer coil is to keep it at the same vertical level and move it away from the coil in the horizontal plane .
if the sniffer coil is placed near the primary it too can be used to determine what is going on with the driven side of the TC .
the amount of coupling/loading is small and should not have a substantial effect on the tuning when removed . one can always leave the coil in position (glowing dimly) to aid in determining if any changes happen during operation .

@ GPG :
yep , i suspect that the dip may not be pronounced and relatively broad ... but should be ok for determining the self-resonance of the secondary and the primary tank circuit .
also , as the op has no real good way to change the coupling/loading he should really be looking at monitoring the cathode current (the safest way) by means of a (say) .5 or 1 ohm resistor in series with the cathode and metering the voltage across it . from there , conversion to mA can be done .
the best way would be to use a genuine analog mA meter in series (a cheapo digital will go wonky around rf) but these are hard to come by today ...
unless you have them in your bone-pile . wink.gif

ps ... i found a data sheet on the tube :
http://www.mif.pg.gda.pl/homepages/frank/s...018/g/GU81M.pdf
according to this , 3 KV on the plate and 450W dissipation (in this instance) is the max .
forget that instantaneous or short term crap ... this is only good for safety factor in the "oh hell !" mode .
usually the tube manufacturers ratings are a good idea to follow ...
if you want higher output voltage from the coil , increase the secondary .


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2ft-sstc
Posted: August 05, 2011 04:20 pm
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Here is some 6 inch steamers http://www.youtube.com/watch?v=VpYOTQEDMfU

tuning getting better, it ran for like 30 and the hadn't changed colour yet, i shut it off because the hot glue tethering the grid bias resister to the cabinet started to burn, one of these times my ceramic resisters are just gonna crack
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tekwiz
Posted: August 05, 2011 06:51 pm
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A dull red plate is normal for many higher power tubes at near full power operation, but you have to have fan cooling to prevent the tube envelope from getting too hot, especially in the seal area where the plate supports go through the glass.
Many tubes are fitted with a glass chimney around the tube itself to facilitate airflow. You can make one by cutting the bottom out of a suitable glass jar. There are also finned heatsinks for the plate supports available, to help keep the seal temps down.


BTW: Silicone RTV is vastly superior to hot melt glue for HV work...it's good for a few hundred degrees. wink.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: August 05, 2011 07:48 pm
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lol i just need a proper bias resister, i thought the resister said 50 watts but it says 20 doh.gif thats what i get for using parts bin stuff eh?!
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2ft-sstc
Posted: August 30, 2011 02:28 am
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I need some help tuning now, I keep tuning, but instead of the sparks getting longer, they get hotter. i had to switch to a thicker breakout point! Jan Martis suggested that make the toroid bigger and a just the primary tuning, all that aloud me to do was make the arcs hotter yet! it looks like other people have had this problem too! Its deceiving how big the sparks are on the camera, but there pretty big in real life, but people have gotton 18 inch sparks from this setup, here is the video http://www.youtube.com/watch?v=rdADuMjXJUk I think my frequency is too high, ideas?


thanks
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CWB
Posted: August 30, 2011 02:37 pm
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a "hotter" spark would indicate that you are able to supply more current over voltage .
it is the voltage that determines the length of a spark ... not the current .
i would increase the secondary turns .

also ...
usually , a lower frequency will produce a "hotter" spark .
with the type of circuit you are using there are many factors that determine the actual operating frequency .
a possible problem is that the circuit may actually be oscillating due to parasitics and not the actual design frequency . parasitics can cause some strange behaviors .
one way to use a gdo is to remove the B+ and bias , fire up the filaments and check the resonant frequency . there will be a slight difference between the cold and hot resonance .

one could do the theoretical calculations but the fastest way to take into account the actual variables (for the most part) is to use a "grid dip" meter or "wave meter" .
a frequency counter could be used but ... by the nature of the circuit it is rich in harmonics , this would confuse a normal frequency counter .
a sure-fire method would be to use a spectrum analyzer ... this would graphically show you what is actually being produced .

to really do a good job of tuning , one needs information about what is really happening .
this is why broadcast/commercial (and amateur) transmitters have various means for metering/monitoring certain important parameters .


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2ft-sstc
Posted: August 30, 2011 03:27 pm
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next time i see some of the ham radio guys i will ask them some queston, I knew i had alot of paresitic oscilations before, i made some ajustments and the tc got WAY quieter!


thanks
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tekwiz
Posted: August 30, 2011 08:09 pm
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Sounds like the Q of your coil circuit is too low. This results in a broad, shallow tuning peak with lower output voltage.
Raising the Q will raise your output voltage for any given power level.
Do a little research into "Q factor" to find out what you can do to increase your coil's Q. More Q will mean a narrower tuning peak with higher output voltages.
If you don't already have one, invest a few dollars in a cheap frequency counter. This is an invaluable aid for anyone dealing with RF resonant circuits. It can also be used to calibrate a simple dip meter, which is an instrument to test the resonant frequency of RF resonant circuits, like your coil.
Counter modules are quite cheap these days.
Like this one: http://www.ebay.com/itm/FREQUENCY-COUNTER-...=item2307333f34
I have a similar unit & am quite pleased with it. These modules are complete instruments, needing only a power source & an enclosure. wink.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: August 31, 2011 05:52 pm
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I looked up the Q factor wink.gif how do I increase this in tc? longer secondary? fatter wire on the seconday? (my secondary uses 28 gauge wire) every says how to caulculate it, but not increase it unsure.gif wacko.gif
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tekwiz
Posted: August 31, 2011 07:38 pm
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--------------------
Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: September 01, 2011 03:38 am
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the equations figure in a resister, also... i'm not a math professor dry.gif so i don't... well, I'm in 9th grade I've never seen math like this before. wink.gif dah goot news is I WANT TO LEARN smile.gif and also tommorow I will talk to a guy that used to work at tv station.he will be at ROLLAG cool.gif



~Tristan
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CWB
Posted: September 01, 2011 11:54 am
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QUOTE (2ft-sstc @ August 31, 2011 09:38 pm)
the equations figure in a resister, also... i'm not a math professor dry.gif so i don't... well, I'm in 9th grade I've never seen math like this before. wink.gif dah goot news is I WANT TO LEARN smile.gif and also tommorow I will talk to a guy that used to work at tv station.he will be at ROLLAG cool.gif



~Tristan

i hope he is an older fellow ...
and i don't mean 35 !
laugh.gif laugh.gif

/me takes my dose of geritol mixed in with my first cup of coffee of the day

"ROLLAG" ??


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"Know how to solve every problem that has been solved"
R. Feynman '88
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tekwiz
Posted: September 01, 2011 07:08 pm
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QUOTE (2ft-sstc @ August 31, 2011 06:38 pm)
the equations figure in a resister, also... i'm not a math professor dry.gif so i don't... well, I'm in 9th grade I've never seen math like this before. wink.gif dah goot news is I WANT TO LEARN smile.gif and also tommorow I will talk to a guy that used to work at tv station.he will be at ROLLAG cool.gif



~Tristan

Something a bit more basic? http://www.crystal-radio.eu/enlckring.htm
You don't necessarily need heavy math to understand what affects your circuit. That's where the graphs come in, because you're more interested in general factors than in exact calculations. Graphs are great for learning what effect a given variable has.
This article indicates that using finer wire, or using Litz wire for your primary will increase it's Q.
Litz wire, BTW, is a multi stranded wire where each strand is individually enamaled, but all strands are in parallel.
The effect of this is to increase the surface area of the wire without increasing it's size. This is important because of the skin effect. The skin effect means that you aren't really using the center of the wires in the windings...the power flows mainly over the very outer skin of the conductor(s).

http://en.wikipedia.org/wiki/Skin_effect

Another very important factor is to make sure both primary & secondary resonate at the same frequency, as you will never reach maximum output unless they are. These frequencies can be calculated, but it's very helpful to have a frequency counter and/or a dip meter, as there are too many factors that interrelate in a TC circuit for easy calculation.



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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: February 27, 2012 12:16 am
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I haven't posted in while but over the past month I have had alot of sucscess with the coil and i have learned alot biggrin.gif The tube runs dull orange (which is actually colder than what it was designed to run)
I build a new cabnet for. it first really perked up a month ago when I got a ceramic ham capacitor. a couple of weeks ago I just bough a whole bunch of soviet mica capacitors and It perked up even more, I have 2 In parrelel currently and I think I need 1 more tongue.gif I have 6 more left! thumbsup.gif

here is a video of it running witch about 16 inch sparks cool.gif http://www.youtube.com/watch?v=Fj2I9M2rtMA
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CWB
Posted: February 27, 2012 02:08 am
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good deal ...
finessing (ie : resonance and coupling , etc) plus good parts are a better way to do things .
most people (wannabes) tend to "brute force" stuff . this is not good engineering by any stretch of the imagination .
as you mentioned , the plate is a dull red versus the yellow color that the other "i know best" clowns put forth .


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tekwiz
Posted: February 27, 2012 03:52 am
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Sweet!! That's a pretty impressive corona brush. Do you realize that you can safely draw arcs to yourself by holding tightly to anything metal & drawing the arcs there? The skin effect combined with the high frequency nature of the TC output keeps yopu perfectly safe from shock. In fact, the only reason you need the metal is that arcs are very hot & the ends can cause small burns. Your coil looks like it should be good for 36" arcs to ground like that.

If you don't mind, I'd like some details on your coil construction & the physical layout of your coil. Thinking of making one myself.


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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2ft-sstc
Posted: February 27, 2012 05:24 am
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Thanks guys! Tubes like the gu81m were designed to run with a red plate. This acts as the tubes getter as they have no chemical getter. Some specs are...

Primary cap: 2x 40KvaR 470pf caps in parrelel
Feedback cap: 7x .01 uf caps in series
Feed back resister: 2x 75 watt 120v in series
Power supply: huge mot with voltage doubler
Primary tap for plate: 45 turns
Primary tap for capacitor 37 turns

If I missed something let me know biggrin.gif
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