Re: Hmm!



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 ***, 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.
.

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