Fuse differences

[Mick Bailey]

Looking through a selection of 20mm T500ma fuses it struck me how different their internal construction is; some have tight spirals, some loose, some wound around an element, some not.

When you get a random HT fuse blow with no other symptoms and the ususal suspects eliminated, you naturally suspect it's just the fuse ageing and the repeated toll of the rapid inrush current when charging the filter caps. But are some brands/construction more durable than others?

[Jazz P Bass]

The other rating is the 'withstand voltage'
That is the maximum voltage that the fuse can handle & not jump across the open element.
Look at that rating on the fuse & then compare the elements.
It may make more sense.
Also, if you look up the datasheet, there are different 'delay before blow' ratings.
All fuses are not created equal.

[Enzo]

What usual suspects?

Yes, an old fuse can fatigue and open when nothing is wrong, but stick a new one in there, and that solves that. SO if it still blows the occasional fuse, then I'd say you probably have a failing powr tube.

Yes, there are nuances in slow blow fuses, but this is not rocket surgery, any slow blow half amp fuse will be good enough.

[Mick Bailey]

By usual suspects I mean failing tubes, failing filter caps, shorted sockets, arcing tracks, shorted turns, excessive current draw etc.

An unbranded bag of fuses (from China - you guessed) have a straight link in them but are market T500ma 250v. Just tested a selection with a couple of light bulbs (100w + 20w) @ 240v to see how many on-offs they'll stand and they blow at switch-on. The branded German FSP fuses hold on for 7 repeated on-offs. Maplin branded fuses with a 'blob' do 3 to 4.

Could be that the unbranded fuses are wrongly marked quick-blow, or sub-standard. But my unscientific test shows a variation in what they'll stand and for how long. I guess it comes down to branding, spec sheets and traceability.

[pdf64]

Is that test (ie using an ac supply) consistent? Doesn't it depend at which part of the cycle power is applied as to the initial surge current, eg if switch on at zero crossing, then current will rise to peak over the next 5mS, whereas if switch on at peak then the surge current may be much higher.
Of course it should average out over many on/off cycles, but over fewer than 10, that variable may impact and cause a misleading result.
Pete

[Mick Bailey]

It is inconsistent, and the thermal-resistance hysteresis in the lamp filament would have an effect on current as well, when taking into account intervals and duration of on-off cycles. A DC source and constant current sink would be a better test.

However, when you switch the standby in an amp you don't know which part of the cycle you're switching, or what state of charge the filter caps are in. My quick & dirty check was just to see if there was a difference in fuse performance for a given type and rating and there is to some extent. You'd probably have to eliminate the extreme case of the ones that always blow instantly as they aren't representative of that type of fuse. Given the broad range of counterfeit and substandard parts that the market's awash with, it wouldn't surprise me if fuses were being mis-labeled as well, especially when you look at the cost of 10 time delay fuses in some places. It's not difficult to find a pack of 10 for £2.99.

[Steve Conner]

I find a little piece of 5mm or 1/4" copper brake pipe makes a very durable fuse.

[Alex R]

Helically-wound fuses have the best tone. Chinese ones are complete rubbish of course. I have some NOS UK-made half-amp ones like they used to fit in the Bluesbreaker combo, fifty quid each if anyone's interested.

[Mick Bailey]

I just re-read that in Harry Enfield's 'I saw you coming' sketch voice.

Are they the ones Eric used to use? I'd like to pay more if they are.

For those not familiar with 'I saw you coming';

Harry Enfield: I Saw You Coming - The Eco Friendly Lamp - YouTube

[Enzo]

Hah!!


I used to work in another industry, and we had a showroom in the Detroit area on a major road. We had arcade video games for sale. PacMan, Asteroids, whatever. Folks would come in looking for a game for their family room. But one day a fellow pulled up out front in his Jaguar, silk scarf about his neck, and sauntered in. Believe me, he heard an entirely personalized price structure.