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Re: Pulsed Tube Coil Work (with updates)
> On the subject of pulsed tube coils, I wish someone would try
>building
>a self-pulsed coil. Blocking oscillator principal which was used in
>VHF
>radar transmitters about the beginning of WW2. Tubes like the
>VT-127/VT-227/VT327 (triode similar to a 100TH) would put out about
>50
>kw peak when running at around 10 kv on the anode. A rectified NST
>would be a neat way to power such an oscillator. (With a suitably
>large
>filter capacitor, of course.) The design is very simple, just using
>a
>"grid leak" in the megohm range.
>
Hi Ed,
I use a blocking oscillator for my 833 A coil to pulse it, and I will
try grid leak pulsing (John calls it sputter mode) on a larger 2-tube
833C coil that I am now working on. The grid leak resistor is not very
large for the 1 tube coil, only 40 kOHM, with a .001 microF cap in
parallel, and according to my scope, the coil pulses around 20 to 30
times per sec. The rate of pulsing depends on the value of RC and also
the voltage that the capacitor gets charged to, which is dependent on
grid coil no. of turns and coupling. The frequency of the pulses is
unfortunately not very stable for me, and as the main tuning cap heats up
and other changes occur, it will slow down and deliver as few as 2 or 3
pulses per sec. I think there is another effect besides slowing down the
repetition rate when the grid cap is made bigger, because when I use .004
microF, i get loud, crackly, more conventional bluish sparks as opposed
to a thick orangy spark. I think the only real advantage is that its
simpler than an external pulsing circuit. From my experience, it is
difficult to determine beforehand what values of R and C will deliver
pulses of a certain frequency. If you know any pointers on this, it would
be a big help to me.
-Carl