"Iain Churches" <IainNG@[EMAIL PROTECTED]
> wrote in message
news:Q9EQj.342739$147.94179@[EMAIL PROTECTED]
> I wonder what the DC resistance of a 250mA fuse might
> be?
Maybe an ohm or two, depending on rated thermal delay, etc.
For the details, check a fuse manufacter's web sie for a data sheet, for
example:
http://www.littelfuse.com/searchresults.html?No=20&RT=Part&NN=0%3aTechnology%3a158%3b0%3aIsObsolete%3a262%3b0%3aNon_Euro_Part%3a263%3b0%3aOpening_Characteristic%3a4294966626
> 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
Yes, but probably not signficantly.
> and does not have a linear response"
Fuses elements are bulk metal and are therefore linear, given that their
temperature is not varying.
However, the temperature of a fuse element is varying pretty strongly with
some time lag, depending on the current flow.
If you apply a sinusoidal time-varying current to a fuse, there will be a
fairly linar voltage across the fuse, particularly at low frequencies.
This
is a well-known problem with loudspeaker fuses. It is better to fuse the
drivers each with their own fuse to avoid modulation of high frequencies
by
low frequencies. Of course, the tweeter voice coil will be doing this, as
well.
> He could not quote any reference to substantiate this.
Greiner contributed an article to the JAES in the 70s or 80s that covered
the effects of fuse resistance on loudspeakers and included distortion
measurements.
> I wonder what the audio bandwidth impedance of a
> 250mA fuse might be? -)
No joke, the fuses resistance will be varying when passing a strong low
frequency tone.
> Compared to the cost of an OPT, the fuse seems to be a
> good component for such a small investment.
Given how much nonlinear distoriton tubed amps have at low frequencies,
fuses are probably not a serious problem. If you really want to protect a
power amp without side-effects, you'll probably use other means.
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