"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?
>
> 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
>
Hi Ian:
Here's the Littelfuse data for a standard 3AG type fast blow 250mA fuse.
The cold DCR is 2 ohms.
http://www.littelfuse.com/data/en/Data_Sheets/312_318.pdf
I know that the DCR of these things does change over a cycle, the amount
of
change being dependent on issues that determine fuse heating such as:
1) Ambient temperature.
2) Air flow over fuse.
3) How close to the fuse rating is the rms current you're passing through
it.
4) Current waveform.
I have never seen any numbers (that looked to be from a reliable source)
showing the magnitudes of the DCR change.
I always include fuses (of about 250mA) in the CT of the output
transformers
of my own amplifiers.
There's an easy way to either put the boots to or substantiate this as a
problem. Find someone with an Audio Precision System 2 with DSP option
(there is a newer model available now...see Audio Precision site) and
simply
do a THD test with and without the fuse. When you look at the spectrum
that
the AP has plotted, you will see immediately if this issue is fact or
fable.
One of the few (and believe me, they're aren't too many) things I miss now
that I am retired, is not having access to an Audio Precision any longer.
Best Regards : Doug Bannard
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