[TowerTalk] Tesla's birthday

[jimlux]

Roger (K8RI) wrote:
> 
> 
> jimlux wrote:
>> Bill Aycock wrote:
>>  
>>> Andy--
>>> The Tesla coil is really two coils, with the primary usually wound as 
>>> a flat spiral as a disk at the base of the other, which is a vertical 
>>> tower. I think that what you are calling a  "Fan" is the primary.
>>> Bill-W4BSG
>>>     
> 
> First time I ever saw one using the "pancake coil" as part of the 
> transformer. All I have seen just use a larger coil form with fewer 
> turns for the primary. Most also used a few hundred watts into the 
> primary in the neighborhood of 100 KHz.  I've seen a lot of them using 
> 811s but no few using a pair of  4-400s.


You want something tough and tolerant of overloads and flashover for a 
tesla coil. Typically gain isn't an issue, because you can get as much 
grid feedback as you want, so triodes are cheap and popular.  Tube coils 
do tend to have the two helical windings because they can tolerate a lot 
more coupling.

Spark gap coils do better with flat primaries (though helicals certainly 
work). It's a combination of reasons.. you want lower coupling (k=0.15) 
and the E field shaping is better with the flat primary, so you don't 
get flashovers as often.

Also, don't neglect looks...


These days, more people are going to IGBT based designs, but not so much 
the classic power oscillator, but using the semiconductor device as a 
solid state replacement for the sparkgap.

>>>     
>>
>> Bill is right.  That's a flat spiral or inverted cone primary, and the 
>> 4 "blades" are the supports for the turns. Cardboard on a quick and 
>> dirty, some nicer insulating material for others (I use HDPE cutting 
>> boards).
>> _______________________________________________
>>   
> Clear plexiglas tube makes a really nice looking solenoid coil form.

Acrylic is nice looking but has some significant practical problems. 
and even relevant to TowerTalk.  It has to do with the different 
temperature coefficient of expansion of the form and the copper. The 
turns get loose or tight, and that causes problems.  Most coilers use 
something to glue the turns down, and that would impair the clean 
acrylic look.