Amp problem

[TomW]

I just turned on my marshall dsl100 this morning and after letting it warm up i hit the standby switch and the whole amp just switched off with a loud pop, and now it won't turn on. So I'm guessing a fuse has blown. But I'm confused as to why, as all day yesturday it was working fine with no sign of the tubes being dodgy. However I did have the channel switcher plugged in this morning which I didn't have plugged in yesturday, and that seems to be abit dodgy with a loose connection. Could that be what has caused the fuse to blow or is it likely something else.
Any help would be greatly appreciated.
Cheers

[Dmoney]

id change the fuse and try again, see what happens.
my dual rec had a day where it blew 5 fuses one after the other. it was fine on standby but not powered one fully.
then all of a sudden it stopped. never done it since.

[hunter]

Wich fuse?

Probably a power tube. When have you last retubed/biased the amp?

Or maybe a tube has rattled lose on transport?

Replace fuse, warm up, switch on. If it doesn't blow, check the output tubes, is one of them red at the bottom? That is called redplating and it means you should replace the set.

You can also take a pen and tick all tubes and see if there is ringing noise through the amp, also an indicator.

[Dmoney]

i thought tapping tubes was a way of finding preamp tube microphonics and tube rattle?
does it work with power tubes too?

[TomW]

Wich fuse?

Probably a power tube. When have you last retubed/biased the amp?

I've just had a look and it looks to be the mains fuse that has blown. It was quite a while ago that it was retubed, but I havn't used it all that much since getting it retubed due to getting a jcm800. Therefore I don't think its the tubes that are the problem. Anyway I'll get some replacement fuses and see what happens.

Cheers for the help

[jpfamps]

Wich fuse?

Probably a power tube. When have you last retubed/biased the amp?

I've just had a look and it looks to be the mains fuse that has blown. It was quite a while ago that it was retubed, but I havn't used it all that much since getting it retubed due to getting a jcm800. Therefore I don't think its the tubes that are the problem. Anyway I'll get some replacement fuses and see what happens.

Cheers for the help

Just because you've got fairly new valves in your amp doesn't mean that they won't short out. I've had new tested valve short out on application of the B+.

All fuses will eventually fail, so you may get away with just replacing the fuse. Try replacing the fuse (with the correct type) and see if that one blow.

If it does, the fact that the fuse blew when the standby was thrown would suggest a power valve has failed (although there could be other faults).

Put a new fuse in the amp of the correct rating and remove the power valves. Fire the amp amp again and see if the fuse blows. If it doesn't then you have got a shorted valve. If the fuse does blow, then you have got a short elsewhere in your amp and you will need to take it to a tech.

[Dmoney]

do you mean fire the amp up with no power tubes in? like fully fire it up or just onto stand by?

[HTH AMPS]

do you mean fire the amp up with no power tubes in? like fully fire it up or just onto stand by?

wtf are 'tubes'? (this is a UK forum damnitt!!!, ha ha).

ok, turn the amp on fully with no output valves installed.  this takes dodgy valves out of the equation as the source of blowing the fuse.  if the fuse then blows with the output valves removed, it's a short elsewhere - typically the PT in every case I've had so far, though a bad filter cap could cause this problem too (the case of the exploded cap in a Selmer T&B comes to mind).

[Dmoney]

can you turn the amp on to a state where it would be playable (if it had valves) without causing an issue with the Output Transformer?
I know you can pull out a pair valves in a 4 valve 100watt head to drop the power, but that effects the impedance on the output right? is having it on with no valves not risky?
maybe it isnt and im misinformed or worried about nothing.

[hamfist]

ok, turn the amp on fully with no output valves installed.  this takes dodgy valves out of the equation as the source of blowing the fuse.  if the fuse then blows with the output valves removed, it's a short elsewhere - typically the PT in every case I've had so far, though a bad filter cap could cause this problem too (the case of the exploded cap in a Selmer T&B comes to mind).

I've also had a bad rectifier diode cause main fuse blowing with no power tubes installed.

[jpfamps]

can you turn the amp on to a state where it would be playable (if it had valves) without causing an issue with the Output Transformer?
I know you can pull out a pair valves in a 4 valve 100watt head to drop the power, but that effects the impedance on the output right? is having it on with no valves not risky?
maybe it isnt and im misinformed or worried about nothing.

The only issue with turning an amp on with no power valves is that you are reducing the loading on the power supply and the B+ will rise, which could stress other components eg filter caps (although by no more than not using the standby switch in an amp with a SS rectifier). This is potentially more of an issue with amps with a valve rectifier than a SS rectifier, however I've not had a problem using this method.

For a really "pro" approach you can use a highly sophisticated current limiter (OK, OK, OK its a light bulb in a box in series with the amp), and this will prevent any damage occuring to the amp in the event of a short.

I've seen diodes, valve rectifiers and main filter caps causing blown fuses when the HT is applied, although is it usually the transformer espcially in Marshalls. I've had two in with this fault in the last week!

[Dmoney]

aaah!
cheers for that!

[hunter]

Ah learned something here.

Has actually anyone mentioned yet that this is the best forum EVAR?

[HTH AMPS]

Ah learned something here.

Has actually anyone mentioned yet that this is the best forum EVAR?

they have now - group hug!!!  :lol:

[TomW]

Haha cheers for adding all this info everyone, learn so much off this forum. However I've got abit of a n00b question now, as I'm struggling to find which fuse to get. The fuse that has blown says Lf.T2AE250V on it, therefore would the 2amp time delay fuse on this website: http://www.maplin.co.uk/Module.aspx?ModuleNo=452 be the correct one to get? Or is it a quickblow fuse I need?