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Re: [TCML] Keying a spark gap tesla



I was wanting to either key the 120v supplied to the nst or thru a relay still to the nst and 12v from my power supply.  I can't make it play musice like a drsstc but at least I was hoping to throw out some code.


Sent from my Cricket smartphone

-------- Original message --------
From: Jim Lux <jimlux@xxxxxxxxxxxxx> 
Date: 11/21/2014  08:38  (GMT-06:00) 
To: tesla@xxxxxxxxxx 
Subject: Re: [TCML] Keying a spark gap tesla 
 
On 11/21/14, 5:25 AM, John Cooper wrote:
> Can a spark gap tesla be modulated by cw (morse key)?  Not too cause
> interferance or actually transmit rf but just to play with.  Back in
> early days of radio this was done with a spark gap transmitter.  But
> I dont think they actually had high voltage on the key itself?  If I
> broke the 120v with the key powering the nst wouldnt it weld the
> contacts eventually?  I think I could do it using a 12v relay but not
> sure bout the lag between key closure and relay activation.  Has
> anyone heard of this or has messed with spark gap tranmitters?
>

Keying 120VAC is no big deal. Tube rigs that key in the cathode circuit 
often have hundreds of volts on the key, if they're operating 
grounded-grid.  I had a tube based code practice oscillator that would 
give you a fairly healthy shock.

A typical shipboard transmitter might only have used 500 or 1000 W, so 
you're switching 5-10 Amps on the key, which isn't all that unreasonable 
with decent contacts.

If you look at the design of shipboard radios of that era (lots of 
exposed components with HV), and imagine what it would be like to be a 
radio operator in a heavy sea, with the ship pitching and rolling, I'm 
surprised that electrocution wasn't a regular occupational hazard. 
Maybe it was.

(BTW, that is the origin of the 1000W input power limitation for amateur 
licenses in the days of yore; ships and hams had comparable powers for 
spectrum management purposes)

I believe that the spark transmitter at W1AW uses a relay for keying.


Here's part of a description of a system in a museum in Australia:
High powered system - Stage 2:
High powered systems use alternating current at around 500 Hz from a 
rotary converter via a high voltage transformer to charge the main 
capacitor.

ST 021405, Rotary Converter - G. Weymouth, Radio, circa 1915

High powered system - Stage 3:
The signal then passes through a keying relay, which protects the morse 
operator (and the key) from the high voltages and currents required by 
the transmitter. The morse key could not stand up to the high energies 
involved in controlling the sparks.

ST 015472, Keying Relay - AWA, Radio Transmitter, circa 1914



http://museumvictoria.com.au/collections/themes/12667/early-radio-transmission-and-reception-1900-1914


"the history of telegraphy" by Ken Beauchamp says:

The Marconi system became available as a series of spark transmitters, 
the smallest of which was a 0.5kW set, the type T17, which was installed 
on many small and medium-sized cargo ships. A synchronous rotary 
sparkgap, which could be operated as a plain spark gap in an emergency, 
was coupled to the primary of a tuned aerial transformer, as shown in 
Figure 6.10. The power source was generally a motor alternator providing 
100V a.c. at 500 H, giving the transmitted signal a characteristic sound 
when detected at the receiver. A larger version, the type T18, providing 
1.5 kW, was available for use on board larger ocean-going ships and gave 
the operator the choice of four standard wavelengths, 450,600,700, and 
800m.  Keying these high-power spark transmitters meant breaking a 
fairly heavy current and using a wide gap setting for the key, which had 
substantial contacts.  A shipboard wireless operator of the time 
recounted the difficulties this caused:

"I recall the old key which was used ot break the heavy current for 
telegraphic signalling purposes. It was actually pounded and was 
equipped with giant platinum contacts.  The maximum speed we could 
handle was about ten words a ....
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