Re: Toob coil questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steven Ward" <srward16-at-hotmail-dot-com> 

Hi Terry!

I will answer as many of these questions as i can ;)


>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Toob coil questions
>Date: Thu, 18 Sep 2003 19:12:46 -0600
>
>Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
>
>Hi All,
>
>I may be an expert on some Tesla coil things, but not toobs 0:-p
>
>So I got this tube coil off E-bay that is really cool!!  Makes 16+ inch 
>sparks!

16" is a LOT from a pair of 811As... the coil may be designed to drive the 
tubes far beyond their ratings... You better watch that the pates DO NOT 
get even a bit of color on them.  If they turn red at all, you are pushing 
things hard!


>http://hot-streamer-dot-com/temp/P9180017.jpg
>
>http://hot-streamer-dot-com/temp/P9180018.jpg
>
>Far easier just to buy one in my case rather than trying to figure it all 
>out from scratch.  I have been trying to figure it out, but I have some 
>questions:
>
>1.  Grid current - Can one just use any DC ammeter in series with the 
>grid(s) (maybe need a bypass cap?) to measure this current?

I never used a meter to measure these things... always did it by the way 
the coil sounded and acted.  After awhile you develop a sense of if there 
is 'too much of this and not enough of that'


>Apparently it is easy, but I just wanted to be sure of the real method 
>used.  I had to replace to original tubes since a blew one up and one broke...

It broke? in operation?  If so, i think the coil is somewhat poorly 
designed to allow for long sparks but severly reduce the life of the tubes

  I got two new Svetlana 811A tubes that are working
>great (http://rfparts-dot-com/ - super great place!!), but I was wondering if 
>I needed to recheck this grid current thing.  Svetlana tubes are said to 
>be "different" than "American" or "Chinese" versions of the 811A.  An RF 
>guru at works says to always get Svetlana since they are the only ones 
>that "know" what they are doing anymore...  I was going to ask him about 
>this but he just had to fly out to Germany.   When I explained it shot 16 
>inch arcs off the top, his eyes sort of glazed over with that "deer in the 
>headlights" look anyway...  The plates barely get hot enough to glow but I 
>really have not turned it up yet till I am sort of sure I will just not 
>blow it up due to something not being adjusted right.  I think the plate 
>current is probably right on since the place metal in the tubes just 
>starts to glow hot now, but I am unsure if the grid current needs to be 
>adjusted with the new tubes.
>
>1a  What should this current be for Svetlana 811A ?  I guess there is a 
>quiet DC current and an operating DC current?

I would think you could leave things alone.  Really, there should not be 
much difference when working with VTTCs... then again someone with more 
knowledge might be able to comment here



>2.  Grid resistance.  There is a resistance to ground in series with the 
>feedback coil and the grid of the tubes.  Like R1 in this very similar 
>circuit (no SCR stuff) at the bottom of John's page (note sure if these 
>things have "official" names?):

The resistor and cap make up a 'grid leak' system.  Its a self-biasing deal 
that controls just how long the tube is on for each cycle, the voltage on 
the grid etc.  VTTCs run in class C, so the tube is only on maybe 20% of 
each cycle... you change that percentage with the grid leak values.

>http://members.aol-dot-com/futuret/page3.html
>
>Right now that resistance gets really hot and I note a number of people 
>are using big variable power resistors like Digikey sells.  That allows 
>for a lot more heat dissipation and I guess grid current adjustment.  So I 
>was just going to replace it with the big variable power type.  I was just 
>wondering if the inherent inductance in the big power resistors is a concern?

Let them run hot its fine.


>I would like to add an ammeter and make this adjustable.  I will get 
>Svetlana 572Bs if (when ;-)) I blow these up...  The 811A is out of 
>production now and the 572B is supposed to be even better...  But the 
>811A's are cheaper ;-))

572Bs will require some changes to be made in the circuit, most likely 
changes in the grid leak system.



>3.  How long do the tubes have to heat up before plate power and all is 
>turned on?

I let them heat for about 2 minutes or so at least... longer is better

>I think I blew the one tube up by not letting them heat up enough before 
>turning on power.  I also note that the heaters are very bright but I 
>think that is normal for this type of tube...  I can check the heater 
>voltage I guess ;-))

My 811As were not 'very bright'.  You better check that you are in fact at 
6.3v!



>4.  Do I need the plate chokes and resistors like the schematic above has 
>in series with the plate connectors?

They are worth the extra effort and will help incase something disasterous 
happens like a tube arc!


>"Plate chokes:  10 turns 14awg magnet wire wound on 30 ohm 20W resistor."
>Right now, it does not have these.

add one to each tube.



>5.  I note the "primary" has a big heavy inductor and big heavy tank cap.
>Are these "tuned"?

YOU BET THEY ARE!  In a tube coil, the tank is the determining part of the 
circuit.  The tuned tank sends pulses to the grid which then controls the 
tube.  With an untuned tank, the coil wont work very well ;)  And tuning is 
rather sharp with VTTCs!


>It does not seem like they are tuned to the right frequency now (not 
>sure...).  Maybe they just have to be "sort of close" since these coils 
>tend to self tune?

No, it does not self tune really.  If its out of tune, it wont make very 
long sparks.



>6.  I imagine the feed back coil produces enough voltage on the grid to 
>make the thing turn on and off (oscillate) within the tubes operating 
>curves and all?  Any insight into that?

That feedback coil looks a bit large... it might be pushing things.  The 
coil should just hummm. If you get any funny noises, its likely due to the 
grid feedback being excessive.



>7.  Sometimes it seems like the thing has a little trouble getting 
>oscillation started.  This may just be because I don't hear it breaking 
>out yet or something too...  But if it has trouble getting the oscillation 
>started, is there something I can play with to help that?

Hmm, thats strange.  Well, my dual 833A VTTC takes about 90V input before i 
get any real sparks.  Check it with a florescent tube.  It should light the 
tube at maybe 10V input or less.  It should have no trouble starting 
oscillation.



>When the tube blew up, it tripped out my $4000 tek scope, but the scope 
>protected itself (thank goodness!!).  But I have beefed up the input stuff 
>with some MOVs and all to help protect other stuff on the AC line.  I 
>think these coils could use a little study into the fault protection and 
>optimization areas.  If anyone has any thoughts as to how tube coils could 
>be improved or studied, maybe I can help?  I am definitely no expert on 
>tube stuff, but I do have the equipment to checkout just about anything here...

You better watch it! Make sure the tube coil is reliable first.



>As you can see, "I know nothing"  0:o))  so any advice, tips, insights, 
>links, and answers are welcome!!

Im in the midst of writting a very in depth VTTC FAQ.  I can send you what 
i have done so far.  Other than that, my website already has lots of VTTC 
stuff to look at.  It takes awhile before you truly know the tricks of the 
trade with these things.

Good luck,

Steve Ward


>Cheers,
>
>         Terry
>
>
>
>
>

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