valve Tesla coil questions
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
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?
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? 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? 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?