"Doug Bannard" <ve3spf@[EMAIL PROTECTED]
> wrote in message
news:Sf-dnZWbQ9c3s47VnZ2dnUVZ_ommnZ2d@[EMAIL PROTECTED]
>
> "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.
>>
> 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.
Hi Doug.
It seems to me to be such a good idea, that I wondered
why it is not more common. If an output tube shorts, then the
replacement of that tube and the fuse is a much more palatable
proposition than the replacement of both tube and OPT
>
> 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.
I have a HP distortion analyzer, so I could plot distortion across the
audio
bandwidth at various power levels with the fuse or with a link in its
place.
But then again, this is one of those inexplicable "audiophile" things:-)
People can hear things than none of the rest of us can measure!
Regards
Iain
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