[TCML] Pulsed valve coil

[Carlos Van Camp]

Thanks Dave,
I tried what you suggested, but still no luck.
I played with the size of the Cathode to ground bypass cap, and found that 
by increasing it, the popping sound in the arcs went away and increased the 
stability of the waveform somewhat.
However, when I put a small cap directly across the Transistor terminals, it 
muffled the switching so badly that I couldn't get any arcs from the coil.
The problem now, is that I can turn the plate voltage up to full no problem 
at low pulse rates (50-200pps), but at higher pulse rates the transistor 
dies every time.
If the plate voltage is left at say 50%, I can crank the pps up all the way 
to a few 1000pps with no problem.

So I have two questions.
How can I stop the transistor from dyeing?
Is it normal that the arcs get shorter with higher pps rates?

Thanks

Carlos


----- Original Message ----- 
From: "dave pierson" <dave_p at comcast.net>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Sunday, July 24, 2011 3:59 AM
Subject: Re: [TCML] Pulsed valve coil


>
>
>> So I have finally got my small valve coil working with a square wave
>> generator driving a transistor in the cathode/ground circuit.
>> I can vary the pulse rate and duty cycle, and am pretty happy with the
>> outcome...
>
>> When I connect the scope accross the cathode and ground at the
>> transistor terminals, I get a nice clean square wave.
>> However, when I start to apply the plate voltage to the valve with a
>> variac, I start to see multiple parrallel traces directly above the
>> "on" or high portion of the wave form.
>
>> The higher the voltage on the plate, the higher they go, and the
>> output of the generator seems to get unstable. (you can hear random
>> popping sounds in the arcs, instead of a nice clean tone)
>
>> When I first turned it on, it was really bad, so I put the circuit
>> inside a grounded metal box.
>
>> It has improved things greatly, but still seems to be a problem at
>> high power levels.
>
>> Can anyone shed some light on how to solve this?
>> purhaps a lower impedence ground connection?
>> Currently, I have left the bypass cap connected between cathode and
>> ground (so it is effectively accross the transistor), but I'm not sure
>> this is correct?
>
>    One possibility:
>    One aspect of shielding enclosure/Faraday Cage application
>    is that stray signals can be induced/coupled to the
>    wires in and out.  These strays 'ride thru' the shield
>    box on the wires and affect circuit operation.  Some level
>    of filtering may be useful, from simple bypass caps to
>    more elaborate filters, in each line, as close to the
>    shield wall as possible.  Ideally the filter is mounted
>    in/at the shield wall.
>
>    best
>     dwp
> _______________________________________________
> Tesla mailing list
> Tesla at pupman.com
> http://www.pupman.com/mailman/listinfo/tesla