blown fuse problem with a Marhsall Super Lead

[RhoadsRocks82]

hey everyone last night i blew the mains fuse in my Marshall super lead and i was wondering why that would of happened like if there something wrong with my amp or not or just a fluke or what. Okay so i just got a new pedal a MXR M-108 10 band Eq and i was playing with it and thats when it blew this is exactly my setup and what was happening.
i was playing my Gibson SG standard into a MXR Distortion+ dialed in at volume all the way up and distortion at 9 o'clock into my MXR 10 band Eq that was boosting my mids and my presence a lot into a volume pedal then into my Marshall Super Lead witch i had cranked to 7 then into a Marshall Power Break. now that is the hottest signal that have ever ran into my amp and it was all going fine and sounded great and my dad was liking the way it sounded so he went to the power break to turn it up more and he went and clicked the volume nob on the power break up once and then it buzzed and then it the amp died and it smelled a little and the mains fuse was blown. do i simply just replace the fuse or is there more behind this problem?? please explain to me what went wrong and why thanks a lot

[John Phillips]

Did he turn the dial on the Powerbrake while you were actually playing at that volume...? :(

It's a bad idea to change the setting on any attenuator while playing, apart from continuous controls like the low-volume knob on the Hotplate.

Sounds like it caused a nasty spike on the amp's output - which could have blown anything from a power tube to a transformer, unfortunately.

It's not impossible that the Powerbrake is damaged too - where did the buzz and smell come from, could you tell?

[RhoadsRocks82]

yes he turned the dial while i was playing. and i know for a fact that all the tubes are fine and we even bought new fuses but they were not the right ones i am supposed to have a 4 or 5 amp fuse i believe and this one was only a 2 amp fuse so when i put it in everything powered on the tubes were glowing and everything and then when i switched the standby to on it buzzed and died and blew that fuse to i think because it was only a 2 amp fuse but no smell this time. and the smell was coming from right were the fuses go but also right above were the fuses go is the output transformer so not quite sure but it was something around there

[John Phillips]

Just a question - how do you know the tubes are fine? If they're a brand new set that wasn't in the amp when it blew, they probably are (though it's still not quite a certainty). If they're the set that was in when it blew, it's doubtful.

A 2A fuse isn't right for the mains fuse and may well blow on start-up just because the value is too small, but if there was a nasty buzz noise as well, there probably is something wrong with the amp. It's OK to use too small a fuse for testing but not too large a one.

If it didn't blow until you flipped the standby to on, the good news is that the power transformer (which is the one directly above the fuseholders) is OK.

Pull all the power tubes (not the preamp tubes) and power up again. If the fuse now blows, the problem most likely that the rectifier diodes are blown (or possibly one or two other parts, but not likely). That would also cause the PT to buzz.

If the fuse holds, put two power tubes in, in either the inner or outer sockets. If the fuse now holds, one of the other tubes is faulty. If it blows, pull them and try the other two tubes. If it blows no matter which pair of tubes is in, it's most likely a blown OT.

This is a definite possibility from switching the Powerbrake at high power unfortunately :(.

[RhoadsRocks82]

how much do you think is will cost to have fixed worse case scenario? the output transformer?

[John Phillips]

Is it an old Super Lead? If so, and you want to keep as much of its value as possible, the worst case scenario is $500+ for an authentic original transformer (maybe more, if it's a really old one), plus about $100 in labor to fit it... :(

If it's a more modern amp or you don't mind a repro tranformer, less - I don't know exact prices in the US, but check out companies like Mercury Magnetics.

[RhoadsRocks82]

um it is an original one i think it's a 74' or somewhere around there. Umm how horrible would a re pro transformer be compared to an old one would it even be noticeable if so whats the difference like any lack of tone or volume or what? would it even be enough to notice though??

[John Phillips]

IMO the difference is small enough that a good repro will sound close to, or exactly, the same; using an original will keep the long-term value of the amp as high as possible though.

Don't assume the worst just yet - it still could be several other components. From your description of the buzz from the PT and where the smell came from, I'm still inclined to put a small bet on either a shorted power tube (or possibly a socket, these can arc under that sort of stress too) or rectifier diodes. A blown OT usually doesn't blow fuses as soon as you flip the standby, because the most common fault is a short inside one of the windings, not between the windings - this type of short doesn't directly connect the high-voltage supply to ground, so the fuse doesn't blow until you play, and some output power is produced.

I also forgot to say (sorry!): first test of all - pull the HT fuse and power the amp up. If the mains fuse now blows when you turn the standby on, it's either the rectifier diodes or a blown first-stage filter cap. Has it had a cap job?

[RhoadsRocks82]

well i already took it to tech to fix it i was just trying to figure how much it should cost so it is getting fixed i just wanted some more info you know thanks a lot though when i find out what is wrong i will post it on here thanks a lot

[RhoadsRocks82]

turns out it was the tubes they are being changed and will have it back on monday so everything worked out thanks for all your help!

Dan