Nah, before, just inverted polarity. Push-pull, one group of tubes with
the plate at +900V, the other group with the cathodes at -900VDC. The
grid of that 2nd group had to be below the negative rail. I built one of
those myself, when I was a kid and blissfully unaware that doing two
Cockroft-Waltons directly off 230VAC mains was, ahem, a tad on the
brazen side. Woe to those who would not heed the dot on the power plug
(they aren't polarized in that part of the world).
This project was also the time I learned that electrolytics have a
finite amount of ripple tolerance. The physics teacher at school never
told us that, it was all ideal capacitors there so I assumed that would
be the real world. One of the 470uF caps in the upper cascade decided it
had enough of it and became a spacecraft. Fluorescent lights in the room
dimmed. Now wait, there is no dimmer, and why does it all hum where
there is no transformer ... phssooosh ... *BANG*. Crater in the ceiling
plaster, burn hole in the carpet, missed my head (and right eye ...) by
just a few inches.
That reminds me of when I experimented a bit with using capacitor
ballasts for high pressure sodium lamps around 1980 or in the veary early
1980's.
There are a few problems:
1. Capacitor alone in series with a "discharge lamp" (arc and glow lamps
in general) favors spiky current waveform, hard on electrodes of "hot
cathode" ones (keep in mind "current crest factor"), and disfavoring
efficiency of fluorescent lamps due to nonlinearities of those.
A remedy that has been used is inductor in series with the capacitor,
with series resonant frequency well above power line frequency but
preferably below the 3rd harmonic.
2. High pressure sodium lamps that are only rated for "inductive"
ballasts have a millisecond or two of lag of current vs. voltage, making
them "slightly inductive" (though there is no current-vs-voltage lag at
zero crossings), and "more inductive at worst time with capacitive
ballasts".
Despite lamp voltage and lamp current zero crossings coinciding, I have
had "HPS" (high pressure sodium) lamps with capacitive ballasts become
"sufficiently inductive" to get both the lamp voltage and the ballast
voltage exceeding the line voltage.
3. More important still - AC usage of capacitors even of non-polarized
types!
Without an AC voltage rating, "You're On Your Own"!!! At Your Own Risk!
70% or even 45% of the DC voltage rating can still have AC usage at 60 Hz
making the capacitor get "awfully warm" to "egg-frying hot" and then fail
spectacularly with a loud "bang"!
With an AC rating, keep in mind frequency. If frequency is unstated in
the AC voltage rating, do not assume safety at same voltage with
frequencies above 60 Hz. However, I do still like to assume like to
assume safety of rated AC voltage at 60 Hz, and at higher frequencies
or with audio waveforms that RMS current same as achieved by max voltage
60 Hz sinewave is safe as long as DC voltage rating has no peaks
exceeding across the capacitor and RMS voltage across the capacitor does
not exceed the AC voltage rating. (Keep in mind that most AC voltmeters
read 1.11 times average voltage, and with most waveforms other than
sinewave and squarewave RMS is more than 1.11 times average!)
- Don Klipstein (
[email protected])
