[AMPS] 3 phase HT power supplies.

[Einar Persson]

Hi David (and all)

If you already have the 3-phase supply, use it.
But I wouldn't do it without at least a small capacitor across the HV.
I tried a 3 phase 6-pulse supply without cap a few years back, with bad result.
It worked fine without a capacitor, but shutting it down was a surprise.
I had the supply and it delivered abt 3kV DC @ 0,6A for a pair of 813's. It
worked very fine with a 16uF capacitor across the output.
(16uF was what I had from an old BC tx)
One day I wanted to know if there would be any hum in the tx without the cap so
I disconnected it.
Switching on was no problem (soft start). No one heard any hum in my tx, so
everything was fine until I switched the supply off.
2 rectifier "legs" in the 6-pulser blown to pieces. Rectifiers were 10x1N4007 in

each "leg" at the time.
I had the power supply away in the basement so no way to see what happened so
when I switched it on again I also smoked the soft start resistors.
I thought the rectifiers were to tiny so I tried with 10x1N5408 in each leg with
about
the same result. Some "legs" in the rectifier in pieces.
(bought 1N5408's for 1 SEK each, about 14 for a £ at the time, so it wasn't a
very
expensive try :-)
After repairing the 6-pulser I connected the capacitor again and everything
worked
fine again. No more diodes blown to pieces.

I didn't make a serious investigation about what caused the failures, but my
guess is that a
spike in the transformer, when switching off, destroyed the diodes and that the
capacitor,
when there, takes care of the spike.
It must have been some hefty spikes to break 10kV worth of diodes so I'm quite
confident
one can also destroy a transformer.
After that experiment I built another 3-phase 6-pulse HV supply. (5.2kV DC @
1.35A CCS)
It's more or less the HV ps from a commercial PHILIPS FM 88-108MHz  transmitter
from
the late fifties or early sixties. (Philips 8FZ 705)
It has a 3-phase transformer, 60x1N5408 in the 6-pulser and a 16uF/5.5kV oil
filled capacitor
connected across the output. (That's the original HV cap from the PHILIPS tx)
In series with the +HV line to the amp I have a 50 or 60ohm 160w resistor to
hopefully save the
tube in case of an arc over in it. (Also original resistor from the PHILIPS tx)
No chokes in the supply.
The supply works very well with no problem so far.

16uF is a lot more than needed, just a few uF would do the job, and is
preferable because of
the smaller storaged energy.

Your thoughts about using 3 single phase transformers in 3-phase configuration.
A friend of mine used that concept several years, in a 4CX1000A 2m amp, with
good result.
The trannies were 220V on the primary and if I remember correct 1600V secondary.

He used the Y configuration on both primary and secondary. Each transformer
primary connected
between a phase and "zero" and a 6-pulse rectifier on the secondary.

My opinion is that it's better to split the power between all tree phases if
possible. The advantages
are better voltage regulation = less twinkling lights, less ripple and if
needed, more power than from
a single phase supply.

Hope I've answered some of your questions.

73 // Einar // SM3MXR




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