Slow Burn or Fast Blow in my Fender??

57Deluxe

Fuse went in my Super Reverb last nite and on inspection was fitted with a T1.6A (slow burn) Fuse, but the manual/specs says Fast Blow - F1.6A

What is the perceived wisdom here??

Fast or slow??

ICBM

Neither. If it blew a T when it should have had an F, it's even more conclusive that there's something wrong with the amp, since it takes a more sustained overload to blow a T. It will have blown the original F at some previous time, presumably before you owned it.

Most likely due to a faulty power valve or rectifier.

Once you've identified it and replaced whichever one it is, you really should put an F in since that's what's specified.

57Deluxe

...no - stupidly Fender screen print the Standby decal on the wrong side of the switch and in the heat of the moment I got confused and turned it on and off full power in quick succession... I put a Fast Blow back in and it barked up A-OK... but was curious to any reason why they fitted it Factory with a T but spec an F. My Supersonic is Fast Blow...

ICBM

57Deluxe said:

...no - stupidly Fender screen print the Standby decal on the wrong side of the switch and in the heat of the moment I got confused and turned it on and off full power in quick succession... I put a Fast Blow back in and it barked up A-OK... but was curious to any reason why they fitted it Factory with a T but spec an F. My Supersonic is Fast Blow…

It's very unlikely that either turning the amp on and off without using the standby would blow the fuse, or that the factory fitted the wrong fuse. More likely that someone else has already changed it because it blew the original fuse. Which shop did you get it from?

If there is an intermittent fault - most likely the rectifier valve arcing if it blew the fuse immediately on power-up - it may not show up every time.

57Deluxe

DV247 - was new.

ICBM

57Deluxe said:

DV247 - was new.

Or previously returned... having blown the fuse while the first owner had it. Then the shop couldn't find anything obviously faulty so they stuck the wrong type of fuse in it and sold it again.

There are shops which seem to make a habit of this sort of thing.

I very much doubt it came with the wrong fuse type from the factory. I think that would be a breach of regulations, as well as extremely unlikely to happen since the assembly line will only have the correct fuses - the worker won't have a box of different ones to choose from.

My guess is that the rectifier valve has an intermittent arc fault - this is common - and that it doesn't normally do it unless you switch on with the standby off. But it could, probably when you crank the amp up. So I would be inclined to change the rectifier valve as well, especially if it's a Sovtek - which Fender do use and which seem to have the worst failure rate like this.

57Deluxe

...had it 2 years no probs home use only...

ICBM

Well why ask then!

57Deluxe

/\ I was asking whether given the choice, others would opt for a Fast Blow or Slow Burn fuse...

ICBM

Why not just fit what it says on the amp, if you don't think there's any reason not to?

57Deluxe

/\ because it had a T fitted and over the original screenprint data for a T replacement it has a factory add-on sticker for an F replacement...

ICBM

So, why don't you fit what it says on the amp? They clearly put the sticker over the top for a reason. They would also have fitted the fuse it says!

I've given you the reasons which in my experience would usually explain why it would have the wrong fuse in and why it blew again, but feel free to ignore them.

It's Slow Blow (sometimes abbreviated to Slo-Blo in the US), not Slow Burn by the way .

jpfamps

The mains fuse is in place to go open circuit when excessive current is drawn by an amp, excessive current indicates a fault in the amp, and thus preventing the amp catching fire. Although it may also prevent further damage to the amp, this is really a secondary function of the fuse.

Ideally the fuse will blow very rapidly when the current draw exceeds the "normal" current by small margin.

However, when you turn an amp on there is a significant inrush current (the magnitude of this will vary depending on when in the mains cycle you turn the amp on). 

This current is much greater than the current draw of the amp under normal operation.

To survive the inrush current the mains fuse either needs to much larger than the normal current draw, or a time delay type designed to withstand a significant in rush current.

Fender tend to use fast blow fuses for the mains fuse where as other manufacturers tend to employ time delay, and I only think that this is because Fender don't use a fuse on the HT supply (a power valve short on the HT is the most likely cause of a fuse blowing in a valve amp), so prefer to have a fast blow mains fuse to blow quicker under fault conditions.

At the risk of repeating myself, all UK Fender amps I've seen are wired to 230 V, rather than 240 V and thus run an higher than specified voltage making valve shorts more likely.

ICBM

jpfamps said:

Fender tend to use fast blow fuses for the mains fuse where as other manufacturers tend to employ time delay, and I only think that this is because Fender don't use a fuse on the HT supply (a power valve short on the HT is the most likely cause of a fuse blowing in a valve amp), so prefer to have a fast blow mains fuse to blow quicker under fault conditions.

Agreed. My guess is that the reason they prefer Fast Blow is that they don't like HT fuses, and a fast blowing mains fuse has (slightly) more chance of saving the rather under-rated screen resistors they use. In my opinion it's poor design.

This is what can happen when you don't use an HT fuse…

http://www.theangelconversations.com/pics/Twinolux1.jpg

http://www.theangelconversations.com/pics/Twinolux2.jpg

'Slow Burn' is probably the right description there .