>
> > 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%3aTec...
Rather than wondering, measuring might work better.
Nevertheless, the high voltage low current paradigm of the tube
amplifier makes this resistance pretty irrelevant. That's the appeal
of tubes: insulation is cheaper than copper and moreover one needn't
be as obsessed with current paths.
>
> > 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.
> > 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 distortion 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.
Fuses used properly are not a serious problem, and neither is the
distortion good tube amps make at low frequencies.
> When you say active protection are you alluding to a current crowbar
circuit?
Often although that isn't the best way to go in most cases. One of
the best protections for tube output stages is a fuse resistor in the
cathode circuit. In fixed bias amplifiers they serve two functions:
they meter quiescent standing current and open if the bias or a tube
fails.
|
