PhattyMo wrote:
>
> Iain Churches wrote:
> > I wonder what the DC resistance of a 250mA fuse might
> > be?
> >
> > I have just been speaking with a guy would told me that he
> > has replaced the fuse from the CT of the OPT of his tube
> > amp with a wire link, "because the fuse adds to the power
> > supply impedance and does not have a linear response"
> >
> > He could not quote any reference to substantiate this.
> >
> > I wonder what the audio bandwidth impedance of a
> > 250mA fuse might be? -)
> >
> > Compared to the cost of an OPT, the fuse seems to be a
> > good component for such a small investment.
> >
> > Iain
> >
> >
> >
>
> Put a second fuse in the NFB path to 'cancel out' the first fuse! :o)
That won't work unless the NFB path had equal current as the output has.
The output fuse can be placed within the NFB loop.
But when the fuse blows, there is no FB aplied, not so good.
The trouble with an output fuse is that it doesn't
protect an amp against low level failure when a shorted output is
present, typical
for when frayed speaker cabling allows frayed ends to touch.
High current then flows through the short, but not high enough to blow a
fuse.
But this high current also fows in the output tubes or transistors,
and the SOA is exceeded. Tubes will begin to glow red hot after a minute
and SS devices can fuse to a solid short within seconds, causing PSU
diodes to fail etc,
and fuses remain intact, and many active protection circuits fitted to
SS amps don't save them either.
In just about all amplifiers, there is never any circuit fitted which
automatically
disconnects the speakers if the load goes below 2 0hms, or turns off the
amp
for the same reason, and all regardless of the level used.
Patrick Turner.
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