Flat Primary
This is the latest
flat primary. It was built for the Model1B. Note the lack of strike
rail. I am experimenting to see if
(1) I can decrease primary strikes through removing extra turns and strike rails
(2) if MMCs can live through the occasional primary strike
Results after the 1st 2 test runs are promising!
I recently rebuilt a flat primary instead of the 45 degree primary. The 45 was drastically overcoupled and forced me to raise the secondary up so that the 1st turn of the secondary wire was half-way up the primary spiral. Having to raise the secondary made the coil too high and hard to handle. Here is a pic of the new flat primary at the SoCal Teslathon. The circular base is one of those white laminate covered particle board circles from Home Depot. The primary supports are made from HDPE and are attached to the circular base with Nylon bolts. The primary is 1/4" copper tube and the strike rail is 1/2".
Don't build one like this! IT WILL BE OVER COUPLED!
The top and bottom decks are made from 3/4" thick particle board and are 24" square. Home Depot will make several cuts for you when you buy wood. Have them cut one of the 2'x4' sheets in half. The legs are made from an 8' 2x4 cut into 4 equal lengths. I made the legs this tall to accommodate the 20" tall capacitors. All legs have have triangular gussets on top and bottom to add strength. I had Home Depot cut the other sheet of particle board into (1) 8" strip and (2) 6" strips. The 8" strip becomes primary coil supports and the 6" strips are used to make the gussets.
I found a really neat trick to make the primary support. First, cut (4) 8" squares. Now draw a line from corner to corner. On the first square, measure up 1", drill a a 5/16" hole, and then drill a hole every 3/8" for 14 more holes. Once all holes are drilled, carefully cut along the hypotenuse. Now you can use a round file and sandpaper to clean up any 'bad" holes. On the next square, measure out 1 5/32" and drill the 1st hole. Continue as before. For each new square, advance the 1st hole by 5/32". This way the tube will form a coil and will naturally wind upwards.
UPDATE (12/03): There have been lots of messages on the mailing list recently that say a cone primary really doesn't buy you anything. Most experts currently reccomend using a flat spiral primary. Too late for this project... The flat spiral keeps the Torroid (and HV) farther away from the strike rail. Shapes other than flat may also lead to overcoupling problems.
The primary is unwound onto the support straight
from the box. Excessive bending leads to work hardening ( a neat science demo in
itself), so get a helper and do it right the first time. It is not difficult at
all. I used some epoxy to secure the tubing to the particle board.
HINT: Actually calculate how much copper tubing that you need before you go to
the store.... 50 ft doesn't spiral that far.