> A good friend and famous multi-multi contest station owner, Phil
> Ashcraft, was killed while adjusting the tuned input on his 4X1 amp.
> All HV is lethal, but when you get past 5kv, one little mistake will
> probably be your last. The HV contacted his watch band, and "welded"
> his arm to the B+.
>
> Things that would never break down at 4kv such a components,
> insulation, and even arcs in open air are not unusual. Take all the
> precautions you can, especially when the covers are off. Amp builders
> are a dying breed...lets hope we will all go SK by natural causes.
I was almost killed by using the resonant choke circuit in Bill Orr's
handbook.
I used the formula he suggested and build a resonant choke filter,
not knowing you really must have resonance above the ripple
frequency.
The supply was a small 5kva pole pig with a bridge, driving a choke
input filter..for a pair of 3-400's. I used big oil filled caps that had
very high voltage ratings, they were from the utility company.
When I powered the thing up, the 3-400Z's would arc from their
anodes to the grid support. When I disconnected the anodes of the
tubes, the blocking capacitor exploded on power up. When I
disconnected it, the bypass capacitor exploded.
I placed a 5kV meter IN the ps cabinet, and attached it through a
HV insulated test lead (40kv rated) to the HV supply positive rail.
I stood back (on a rubber HV blanket), kept one hand behind me,
and flipped a panel mounted toggle switch on. There was a flash
and the HV lead blew in two. It snaked OUT of the cabinet and
landed on my wrist (about two feet from the leads original position)
on the hand that was holding the grounded toggle switch. The only
thing that shut it off was the main fuses eventually opened.
It burned a charred half-dollar sized hole in my wrist, and caused
second degree burns up to my elbow. I still have the imprint of the
toggle switch between my thumb and index finger!
So what crossed my mind? I thought "Shit, how long will my brain
work now that my heart is stopped?" So I watched the clock on the
wall. When I saw the second hand make two complete sweeps
around the dial, I thought "darned, it didn't kill me".
If I was not working on a rubber HV mat, and did not have the habit
of working with one hand...I'd have been toast.
BTW I wrote Bill Orr a nice letter about this, and suggested he
might warn people about it in the area where his handbook
suggests using resonant filters. I have no idea if he ever changed
his book.
73, Tom W8JI
W8JI@contesting.com
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