Sorry about the spelling error below. EXIST was spelled incorrectly
as exit.
Leon W 5 N A
----- Original Message -----
From: "J. Leon Pringle, Jr and Audrey S. Pringle" <w5na@megagate.com>
To: "Richard" <2@mail.vcnet.com>; "George T. Daughters"
<gdaught6@leland.stanford.edu>; " AMPS" <amps@contesting.com>
Cc: " AMPS" <amps@contesting.com>
Sent: Thursday, March 21, 2002 9:15 AM
Subject: Re: [Amps] SB220 Question... fuses
> 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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>
>
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