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Re: Microwave Xformer



>> Here are some spark lengths that can be expected from various tubes:  Two
 >>  graphite plate 811A's give an 11" spark, one 833A gives a 16" spark, two
>>  833As give a 22" spark, one 4-250A gives a 12" spark, one 4-125A gives a
>> 7"
>>  spark,   one 3-500 Z gives about a 16" spark. 
 
 >The only tubz I ever got to produce a spark was a pair of 811A's which
>produced a 3in brush (max) with the plaes over heating like crazy....What
are
>the speks of the system that one needs to use to get 11" out of them?.....
 I
> also have currently, a 4-500z... which I have yet to get to produce
> *anything* in the way of a spark (although it lights a neon tube up just
fine
>from a food away).  Perhaps its my secondary and/or primary design... about
 >1000 turns of #30 on a 2in form (tall candlestick type) and a primary diam
of
> 4" with 26 turns of #12 insulated house electrical wire (there are two of
> these primary's, one is the real one and one is the "grid tickler" )  any
>suggestions?
 
>Thanks
 -DavidF-
  >>

David,
The fine tube coil that gave the 11" sparks using two 811As was built by
Buster Knight, a member of the TCBOR, Richard Hull's Tesla group.   Richard
can probably give you the exact specs, (Richard's presently off-line--moving
his internet hardware--he'll be back soon), but I can give a rough idea from
memory:   Sec, about 4" dia. by 14" high?,  # 27 magnet wire.,  Pri, about 7
1/2 to 8" dia.?, by 2 1/2" high,? # 12 pvc ins. wire,?  grid coil--unknown
dimensions.  tank cap--unknown size.  Powered by a microwave oven
transformer.   It  is essential  that the tubes have a graphite (this is
thick and massive) plate rather than the sheet metal plate.   The coil was
built in a custom wooden cabinet with meters, variac, and tube observation
window on the front panel, and the pri. and sec was installed on top of the
cabinet.  all the other parts were inside.  The coil could be left running
continuously--the tubes did not overheat.

I am not familiar with the 4-500z tube, but I suppose it is capable of a 16"
spark like the 3-500z.  The 4-500z will probably work best with 3 or 4KV on
the plate. (I seem to remember that the 4-500z is rated for 3 kV max.?)  The
4-500z will need a voltage applied to the screen grid,  Someone who is
familiar with the tube can probably suggest the proper dropping resistor
value to use to supply this voltage.   On some tetrodes, the screen can be
tied to the control grid, but I don't know if this will work well on this
tube.

I prefer to use secondary forms that are wider than 2".  On one of my early
coils, I replaced the 2" diameter sec with a 3" dia sec and the spark
increased from 6" to 9".
Using the 2" dia. sec and a 3-500z, I never got more than a 7" spark, a
larger (7" by 11") secondary gave 12" sparks using a microwave oven
transformer.  The use of a microwave oven transformer will limit the spark to
about 11 or 12".

Obtaining the best results from a tube coil requires a careful adjustment of;
grid feedback (adjust the number of grid coil turns, or move the grid coil
closer or farther from the pri),  tuning (careful tuning will maintain spark
length while allowing for cooler tube operation), grid leak resistance (more
resistance will tend to reduce spark and cool the tubes).  Tank load
impedance must be suitable for the tubes:  Large cap with few pri turns will
heat up the tube plates, or demand  larger (or more) tubes.  Small cap with
more pri turns will draw less current and tubes will run cooler.  Some of the
adjustments tend to interact also.   It takes time to tune up a tube coil and
obtain maximum efficiency.   But the results are worth the effort.  It's
often helpful to design the coil to easily allow for these adjustments, clip
leads, sliding grid coils, etc.  When all is working fine, then you can
neaten it up if desired.

John Freau