The AM Forum

Do any of you know how many arcs per second it takes to make a spark gap transmitter sound like a clean tone?

Define "clean tone" ?

Mike,
Your Dad's sounded pretty good what is his spark rate per second.
A friend has a new milling machine......

COOL ;)

We have a couple. The big one is a non syncronous spark gap. In other words, it  arcs at random points in the AC cycle depending on the number of "teeth" and the speed of the motor. The OM's has 8 teeth and a motor that rotates at I think 1200 rpm or 2400 rpm, I cant remember. The faster, the better.
A syncronous gap has exactly 4 teeth and is mounted on a motor that spins at 1750 rpm, and the gaps are adjustable back and forth so that you can sync up to exactly the peaks of the AC waveform. More effecient, but lousy tone and more complex to manufacture.
Quenched gap used several small gaps close together to the point you don't see any spark. Mainly used aboard ships, and much quieter. Very rare to find gaps these days. Also, fed with 60 cps AC, the note is very mushy. Many ships used 110VDC, so a 500cps generator was employed, very "sweet" high pitched note, also, rare as hens teeth.
Hope you build one!

So I suppose you have to arc as fast as possible yet quench the arc
Sounds like the larger the diameter disc  the easier it is to do.
I  saw a 5 inch disc once

I have a cool old motor that would be a nice mudulator.
A buzzard 3/8" silver plated tube coil.

never built a spark rig or an 813.

This would be a cool Hostraders rig to work Dale

I was thinking Mike, How about a gear like the input shaft from a B.W. T10
The teeth would last for a while.  ?????

You kinda need something with longer teeth, a couple of inches and insulated from the motor shaft.

Yea arc might not quench. guess it is time to machine a disc

Try searching on teslta and rotary gap. Theres still lots being built. A tesla coil is after all only a VERY short vertical antenna.  My experience is that theyre nasty and noisy, at least  compared to MOSFET driven TC . But they do add the 'Frankenstien' ambience. (Although to be honest Bride of Frankenstien has better electrcal effects)
                                                                                     Ian

Ian,
My life is incomplete unless I build a rotary spark rig. I want it to be as clean as possible. I plan to use a High Q tuned circuit.  Recently I've been into lightning and EMI testing so it has sparked new interest.
 

You're gunna need a condenser that can handle the really high peak voltage across the tank. The OM HB one using 1/4" glass plates and actual tin foil and squeezed them together with blocks of wood, threaded rod, and nuts and bolts. I think he used 7 pieces of glass and 6 pieces of tin about 12"x 12" and bolted all of the leads together at the top.
He tried using large black mica's, but would puncture every one as soon as the key was depressed.

wow Mike that must be some high voltage! I was thinking of a 1/4 inch spaced cardwell. I also have another cap with 1/2 inch spacing.

I guess a 7000 volt spark must generate 14,000 V RF  peak to peak
then damped.

yea, that initial jolt is usually what does it. After damping, it gets recharged again. How much damping is what actually controls how broad the rig is. Too heavy a loading causes little damping, and makes the sig real wide.

Just a reminder, damped emissions, designated by the letter B, are forbidden: from regs 2..201(f);

"Type B emission: As an exception to the above principles, damped
waves are symbolized in the Commission's rules and regulations as type B
emission. The use of type B emissions is forbidden."

As I recall, the rotary gaps were used to produce a higher pitch to the "tone" of the detected signal as there were no BFo's and such with TRF receivers.  Also, at its pre vacuum transmitter peak, mechanically driven transmitters were high frequency alternators capable of kiloWatts of power at frequencies up toward 100 kilocycles. 

Fun to play with or have as a display piece.

Rob,
"There you go with those negative waves"(as odd ball would say)
It is just a new configuration of spread spectrum

Just load it into the ant, have a friend listen on a rx down in the broadcash band, and send random dit dah's like it was an electric fence arcing to grass or something. ;D

We don't need no stinkin' rules ;D