[AMPS] HV PS topics

[Dick Green]

> KM1H never suggested a Globar in the HV path, only as an alternative to a
> parasitic suppressor resistor.

I think you misunderstood what John said. He was talking about the value of
the resistor KM1H suggested (15-ohm 25W), not the type.

> Non stop and repetitive maybe but not tiresome....

Try using the DELETE key.

> Why? Are you designing a new amp or just refurbing something like a
> LK-550??

I'm simply trying to do a good job fixing the LK550. After what the tube arc
did to the plate choke, I'm not in the mood for more HV mishaps. There's no
harm in trying to determine the right value and mounting technique for the
glitch resistor, and to understand why (there's plenty of time to ponder the
glitch resistor -- the new plate choke hasn't arrived yet.) I'm not an EE
and have only worked inside three amps prior to the LK550. I like to avoid
mistakes, especially with dangerous high power devices. Some of the
experienced people who hang out on this reflector have offered useful advice
in a courteous manner. Others get off on being obnoxious.

> Now you used the correct word  "limit current".  If sized properly it
> will survive and so should all the parts it is designed to protect.

I don't really think it's as obvious as you seem to believe, at least not to
me. What I was looking for was a real explanation, not an insult. So tell
me, when there's an arc or a short in an LK550 with a 15-ohm 25W glitch
resistor, just what value is the current limited to? How much current will
flow in the plate choke windings and parasitic resistors, and for how long?

> Figure it out yourself; all it takes is Ohms Law. The answer varies by
> the application.

Are you telling me to figure it out myself because you really don't know
how? Sure, Ohm's Law can tell you how to size the resistor for normal
operation -- at 1.3A plate current, a 15-ohm resistor dissipates 25W. But
Ohm's Law doesn't tell you how to size the resistor to survive arcs and
long-duration shorts (if that's even possible.) When there's an arc, the
entire 3200V of the HV ps appears across the 15-ohm glitch resistor, and the
instantaneous current jumps to 213A. Obviously, there's not 682 kilowatts of
energy stored in the filter caps, and the current isn't really 213A (the
ampere is defined as 1 coulomb of energy passing a given point in one
second, and the arc and/or capacitor dump don't last that long.) Seems to me
that the reason the glitch resistor survives is that the shorting event
isn't long enough to heat the resistor to the point of failure. Either the
arc ends, the filter caps discharge, or the AC fuses blow before the
resistor pops. So, the question is, when one of these events happens, just
how much energy does the 15-ohm resistor handle, and for how long? How much
is dissipated in the resistor and how much flows through the plate choke and
parasitic resistors? I've gotta believe that this is a function of the ps
voltage, resistor value, amount of energy stored in the filter caps, and the
duration of the shorting event. Can you tell me those numbers? How exactly
did you decide that a 50W resistor would be adequate?

> You remind me of someone else who spends all his time asking questions,
> second guessing the replies, and spends most of his time going around in
> circles.
> Exercise your Extra Class License and have at it.

This is uncalled for. You have absolutely no basis for making such a remark,
and I resent it. Some of us come to this reflector to learn. Others
unselfishly educate us and earn our respect. The rest waste our time by
posing as experts and making unkind remarks. I'll be happy to compare life
accomplishments with you anytime -- then we'll see who has been going around
in circles.

Dick, WC1M



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