"Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
news:481352EF.E36EF12D@[EMAIL PROTECTED]
>
>
> 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
>
> Fuses between the B+ and a CT on an OPT are not a bad idea.
> 90% of music from tube amps is from PP,
> and while the amp remains fully in class A.
> So the non-linarity of the fuse impedance is utterly negligible.
>
> Traditionally, ppl said fuses before speakers were bad because there the
> fuse value
> might be 4 amps, and the distortions caused by the fuse could be greater
> than the those from the amp,
> which might only measure 0.001% at a watt or two which is all most ppl
> ever use.
> A 4A fuse may have 0.01% thd, 10 times more. It doesn't make much
> difference though eh.
> A piece of small dia copper wire soldered in would be ok except that its
> temp varies and hence its resistance.
> Maybe not to much though.
>
> Active protection against excessive cathode current works a lot better
> though.
>
> The problem with a fuse is at low levels when a speaker wire has frayed
> strands causing a short
> that isn't noticed, and load becomes less than 0.3 ohms, so the output
> tubes or tyransistors cook themselves
> even at low levels even though there isn't enough current to blow a
> fuse.
>
> Active protection is better.
>
> Patrick Turner.
When you say active protection are you alluding to a current crowbar
circuit?
Cordially,
west
>
>
>
> Patrick Turner.
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