Re: valve Tesla coil questions

Tesla List wrote:
> >From mconway-at-deepthought-dot-netgate.co.nz Mon Dec  9 20:09:24 1996
> Date: 10 Dec 1996 05:20:44 GMT
> From: Mark Conway <mconway-at-deepthought-dot-netgate.co.nz>
> To: tesla-at-pupman-dot-com
> Subject: valve Tesla coil questions
> Hi Everyone,
> I'm in the process at the moment of designing and building a valve (tube)
> Tesla coil to go with my semiconductor and my spark gap coils. I will be
> using two 811A's (thanks to the person who posted about Svetlana tubes a few
> months back- I didnt think anybody still made valves! Isn't the net a great
> thing - not only did I find out about the existence of recently made tubes
> but a quick web  search soon found a store in Oz that sold these valves!
> >From Brent Turners excellent book I see that the filament power supply to
> these tubes must be centre tapped in order to prevent the filament burning if
> the electron current across the filament (cathode element) is not balanced.
> Since 811A's need 4 amps at 6.3 volts each I was unable to find such a
> transformer anywhere. What I've done is buy a 6 volt transformer rated at 10
> amps. I will unwind the secondary and put a centre tap half way along the
> wire.
> A plate transformer rated at 1500 volts is also impossible to find over here.
> What I plan on doing is removing wire off a microwave transformer until I
> reach the required voltage ( I'll feed 12 volts ac into the primary and start
> unwinding the secondary until I reach a voltage of 75 volts. This should (I
> think) give me a secondary voltage of 1500 volts when the primary is supplied
> with 240 volts. All of the microwave transformers that I have, seem to have
> magnetic shunts in them. Can I remove all of these shunts to boost the output
> current or should I keep one of them in to stop too much current flowing?

The shunts will cause the plate voltage to collapse as you begin loading
down the transformer. You could remove some or all of them, but you'd
then want to monitor the plate current drawn with a milliammeter
(probably not a bad idea anyway). Running the plate transformer off a
variac would allow you to use the existing transformer. Another option
would be to connect another transformer's secondary  in series with the
HV transformer's primary to "buck" part of the incoming voltage. 

> Tank capacitors are also hard to come by here. Brent Turner and Duane Byland
> both recommend mica radio transmitting capacitors but I have not been able to
> find any of these beasts (anyone know where they can be found in NZ?). Brent
> Turner recommends that a high quality cap be used in this application to keep
> the primary Q high enough to sustain a pure sine wave. Looking in the Farnell
> catalogue I see some disc ceramic caps that are rated up to 15000 volts dc.
> They dont give an ac rating. Would these caps be ok to use here or are
> ceramic caps no good for radio frequency voltages? 

Disk ceramics are not optimal (low Q) for tank circuit use unless
specially constructed. The circulating current in the parallel LC tank
circuit will be fairly high (of the order of amperes RMS), and the
self-heating will tend to change the cap's value unless NPO material is
used, and the heating will cause them to fail. Mica or
polypropylene/polyethylene are better choices.

> I could also use some
> metalised polypropylene caps rated at 2000 volts dc (ac rating for these caps
> is given as 600 volts) in a series parallel arrangement to give the required
> capacitance and voltage rating. Would these caps be ok in this type of
> service? 

These would be a better choice. However, unless they can withstand the
heavy RF circulating current, self-heating of the metal
plates/interconnects will destroy these caps as well - not from
dielectric heating, but from reisitive/skin effect heating of the

> Could I build a rolled polyethylene cap for this application? I
> build 4 Richards type caps for my spark gap coil and they work great! Would
> homemade caps be too low Q in this application?

A home-built roll or plate cap should work quite well, as long as you
use aluminum flashing to minimize skin-effect and cap plate resistance

> Cheers!
> Mark

Good luck, and safe tube coilin' to ya!!

-- Bert --