Richard:
Fuses are rated both in terms of voltage and current as you point out.
The voltage rating of a fuse is that voltage that may exit across the
fuse ends after the fuse has opened without the fuse again passing
current (conduction) through it. This is a safety precaution well
worth everyone understanding. Many don't!
73,
J. Leon Pringle, Jr W 5 N A
----- Original Message -----
From: "Richard" <2@mail.vcnet.com>
To: "George T. Daughters" <gdaught6@leland.stanford.edu>; " AMPS"
<amps@contesting.com>
Sent: Wednesday, March 20, 2002 6:59 PM
Subject: Re: [Amps] SB220 Question... fuses
>
>
> >
> >It has been written:
> >
> >> >I am doing some mods from Harbach on the 220 and one of them is
the
> >> >fine wire fuse between the HV feed through and the PC board.
Has
> >> >anyone done that and could you use a small amp fuse and fuse
holder
> >> >rather than the wire and PC board mod?
> >
> >Rich replied:
> >
> >> ? A fuse will not prevent damage** to the tubes. A fine wire
will
> >> not prevent damage to the tubes because it is essentially a
fuse.. A
> >> glitch resistor - that can pass the screwdriver test - will
prevent
> >> such damage.
> >> The trouble with fuses is that they do not limit peak fault
current.
> >>
> >> Glitch resistors do.
> >>
> >> ** (filament-grid short)
> >
> >I've told this story before, but maybe it's time to repeat it.
When
> >I worked at Fairchild in the early 60's, we tested many of the
common
> >fuse types to see which would be best at protecting (then)
expensive
> >silicon transistors. To sum up a very extensive spread sheet (we
> >called them "data tables" back then) even the lowest rated fuse
would
> >carry a HUGE current for a while. A 1/8 amp fuse would carry 20
> >amperes for milliseconds... long enough to zap the transistor. I
> >just made up those figures, (I don't have the data with me) but you
> >get the idea.
> >
> >Our summary statement was "An expensive silicon transistor is an
> >effective way of protecting a cheap fuse."
> >
> ? chortle.
> - Furthermore, when a 250v fuse is used to interrupt current flow
from a
> tyoical 2500v anode supply, before the fuse can interrupt the flow
of
> current, a metal-vapour arc takes place inside the fuse as the metal
> element melts. The voltage drop across the arc is only c. 20v until
the
> fuse explodes. Hence, current limiting does not begin until after
the
> explosion. This is why fuses carry a current rating and a Voltage
> rating.
>
> >The same might be said for the grid of certain expensive tubes in
> >their ability to protect fuses.
> >
> ? 1/4w - 1/2w carbon-film (frangible) grid-fusing resistors, plus
a B+
> glitch resistor, have proven to be fairly successful at protecting
> 3-500Zs from filament-to-grid shorts.
> ,
> cheers, George
>
> - R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
> www.vcnet.com/measures.
> end
>
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