Re: Toob coil questions

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

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Steve,

At 10:26 PM 9/18/2003 -0500, you wrote:
>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!


Oh!!  I did note that the red hot area was sort of a concentrated hot 
spot...  Thanks!!  Now I know what I am seeing is real and scarry...



>>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'

Hmmm...  Us nubies don't know a hot tube from a lit match...  Not too good 
at the "sense" stuff ;-))



>>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 think the  #1 tube blew from not enough heat up time.  The other was poor 
glue attach stuff...  The 1988 tube's glue had all detached from the 
glass...  The new Svetlanas are rock solid there...


>  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

It seems to work well.  I just read that grid current was a big deal.  It 
might very well be perfectly fine...




>>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.

I see...  Right now the resistors are thermally insulated by teflon from 
finished wood...  Seems like a big thermal weak spot.  Need to go for 100C 
all day duty cycle...



>>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.

"Hot" means fire in this case....  I need to go to big variable resistors 
for sure...  Need to get that 350C against wood to 100 C in free air...



>>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.


Yeah!! ;-))  Simple with variable resistors in the grid leak...




>>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

OOOPPs!!! :-)))  The one I blew up had like 5 seconds of heating on it 
:o))))  Message received ;-))



>>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!


HP 34401 DVM on ACrms...

Tube #1 base = 6.018Vrms

Tube #2 base = 6.063Vrms

Prolly just fine there...





>>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!

"Been there, done that" ;-))



>>"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.

OK...




>>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!

Ok... I'll check real deep into that!!!  Probably real close now but I am 
sure that can be tweak real exact...



>>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.

Ok,  I will try an reconcile that with the tube charts...




>>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.

Ok, this may very well be a false impression on my part...



>>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.

Yeah!! ;-))

Cheers,

         Terry

>Good luck,
>
>Steve Ward