Replacing a fuse in a marshall clone

[Catalyst77]

One of the fuses has gone on my plexi ceriatone. (hopefully nothing ominous has caused it)

I ordered some new fuses to match the old ones (the voltage and ampage are the same) but they are slightly larger and wider.  They still fit without any problem, does anyone know if there is any reason why i can't use them?

Cheers

Ben

[Tellboy]

Should be OK as long as it is the same 'type' of fuse as well as the same rating - i.e. slow blow or quick blow

[Catalyst77]

Should be OK as long as it is the same 'type' of fuse as well as the same rating - i.e. slow blow or quick blow

Ah

Whats the difference, as i bought slow blow, but i have no idea what the one i was replacing was :? 

[jpfamps]

Slo-blo, aka time delay or time lag fuses, are designed to withstand a larger current than their rating for a short period of time.

Fast blo are design to blow more or less as soon as the current rating is exceeded.

For example when an amp is turned on there is a high initial inrush current, so a fast blow fuse would almost certainly blow on switch on. Slo blow fuses are used here.

The HT supply to the power valves (if after the first filter cap) is usually fast blow to protect the output transformer from valve shorts.

The fuse will have a T on it to signify time delay.

[HTH AMPS]

I would also add that there is nearly always a reason why fuses blow (unless the wrong type has been put in).

Try replacing with the correct fuses, if it still blows (assuming its the HT fuse) then its most likely a dodgy output valve.  Trying running the amp without the output valves and if the fuses don't blow you're 90% certain to have a dodgy output valve.

[jpfamps]

I would also add that there is nearly always a reason why fuses blow (unless the wrong type has been put in).

Try replacing with the correct fuses, if it still blows (assuming its the HT fuse) then its most likely a dodgy output valve.  Trying running the amp without the output valves and if the fuses don't blow you're 90% certain to have a dodgy output valve.

I would agreed with all the above, although I would add I've had more than a few amps come in that "have been blowing fuses" where the owner has been using a Fast blow instead of a Slo Blo!!

[FELINEGUITARS]

Slo-blo, aka time delay or time lag fuses, are designed to withstand a larger current than their rating for a short period of time.

Fast blo are design to blow more or less as soon as the current rating is exceeded.

For example when an amp is turned on there is a high initial inrush current, so a fast blow fuse would almost certainly blow on switch on. Slo blow fuses are used here.

The HT supply to the power valves (if after the first filter cap) is usually fast blow to protect the output transformer from valve shorts.

The fuse will have a T on it to signify time delay.

Thankyou for that - very useful

[Catalyst77]

Thanks all,

The fuse that came out did indeed have a T on it, although strangely the slow blow ones i bought didn't.

Will give it a go and see what happens.

Cheers

Ben

[HTH AMPS]

I would also add that there is nearly always a reason why fuses blow (unless the wrong type has been put in).

Try replacing with the correct fuses, if it still blows (assuming its the HT fuse) then its most likely a dodgy output valve.  Trying running the amp without the output valves and if the fuses don't blow you're 90% certain to have a dodgy output valve.

I would agreed with all the above, although I would add I've had more than a few amps come in that "have been blowing fuses" where the owner has been using a Fast blow instead of a Slo Blo!!

thats what I meant by "try replacing with the correct fuses" - correct type and correct rating.

[Catalyst77]

Just put the new fuse in and it blew straight away, was def a slow blow and the same specs as the last one
(2amp)

 :(

What sort of thing would cause the fuse to keep blowing? was fine until i had it biased! :?

[hamfist]

Just put the new fuse in and it blew straight away, was def a slow blow and the same specs as the last one
(2amp)

 :(

What sort of thing would cause the fuse to keep blowing? was fine until i had it biased! :?

Try taking out the output valves, put a new fuse in, switch on and see if it blows again.

My moneys on a dodgy EL34.  if the fuse is fine with the output tubes pulled out, then you've identified the culprit.  Time for new El34's.
 The re-biasing was probably just co-incidental. 

[Catalyst77]

Just put the new fuse in and it blew straight away, was def a slow blow and the same specs as the last one
(2amp)

 :(

What sort of thing would cause the fuse to keep blowing? was fine until i had it biased! :?

Try taking out the output valves, put a new fuse in, switch on and see if it blows again.

My moneys on a dodgy EL34.  if the fuse is fine with the output tubes pulled out, then you've identified the culprit.  Time for new El34's.
 The re-biasing was probably just co-incidental. 

Ok so I took the output valves out and the fuse still blew the second i turned the amp on :(

[HTH AMPS]

Just put the new fuse in and it blew straight away, was def a slow blow and the same specs as the last one
(2amp)

 :(

What sort of thing would cause the fuse to keep blowing? was fine until i had it biased! :?

Try taking out the output valves, put a new fuse in, switch on and see if it blows again.

My moneys on a dodgy EL34.  if the fuse is fine with the output tubes pulled out, then you've identified the culprit.  Time for new El34's.
 The re-biasing was probably just co-incidental. 

Ok so I took the output valves out and the fuse still blew the second i turned the amp on :(

which fuse? - HT or mains ???

with the EL34s out, there should be no current getting pulled so the HT fuse 'shouldnt' blow.  If it does, I'd be looking at something shorting out or (more likely) a dodgy filter cap.

if it's the mains fuse blowing, could be that the PT has an internal short - you won't know that until you run the PT unloaded.

[hamfist]

Another possible option is a faulty rectifier diode. I had identical symptoms in an amp, and it ended up to be simply one faulty diode.