Toob coil questions

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

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!

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?

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...  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?


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?):

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?

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 ;-))


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

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 ;-))


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

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


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

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?


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?


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?


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...


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

Cheers,

         Terry