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Valve amp blowing fuses

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[size=4][font="Calibri"][color="#000000"]Ive been enjoying my YB200 valve amp for a while now. [/color][color="#000000"]Had it upgraded to the YBA200-2 specs a few months ago including a new quad of JJ KT88’s in the power section.[/color] [color="#000000"]It gives a lovely valvey, bouncy, thick, gritty sound that I love with my 4x10 and flats loaded P bass.[/color][/font][/size]

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[size=4][font="Calibri"][color="#000000"]Anyway, last week I decided to do a bit of tube rolling with the 2 x 12AX7’s in the pre. [/color][color="#000000"]It was sporting some Tung Sols but I wanted to give a JJ a try in V2 to take a bit of top away.[/color] [color="#000000"]Relatively easy process and put the amp back in its case and went to power on.[/color] [color="#000000"]The power light blinked then went off – the amp was now dead.[/color][/font][/size]

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[size=4][font="Calibri"][color="#000000"]I checked various things and it turned out to be the fuse in the sled under where the kettle lead goes in. [/color][color="#000000"]No biggie, I got a 10 pack from Maplins for about £2 – got home, fitted, fired up fine and played for two hours problem free.[/color] [color="#000000"]Then – last night at rehearsal I set the amp it, flicked it on to warm up and same thing happened – light blinked, fuse blown.[/color] [/font][/size]

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[size=4][font="Calibri"][color="#000000"]Luckily I had the spares on me so quickly replaced the fuse and enjoyed a problem free 4hr rehearsal. [/color][color="#000000"]We have 2 weeks off now so id like to look properly into why the fuse has popped twice but then been problem free upon replacing.[/color][/font][/size]

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[size=4][font="Calibri"][color="#000000"]Could it just be bad luck and the first replacement was a dud? [/color][color="#000000"]Could it be something more sinister inside the amp?[/color] [color="#000000"]One thing I was told at the time of the upgrade was he couldn’t get the KT88’s to within the Traynor recommended bias – they recommend 75mV with a + or – 10mV tolerance but the closest he could get was 90mV.[/color][/font][/size]

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[size=4][font="Calibri"][color="#000000"]One last thing – I noticed the replacement ones I got from Maplins in a hurry were normal ‘fast blow’ fuses – could it be I need the slow blow version?[/color][/font][/size]

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It may just be that you need a slow blow fuse - do you have a manual for the amp that would confirm this? I spent a while on an amp I built recently trying to figure out why the fuse was blowing at switch-on about 1 time in 10 with no obvious fault, before I realised that slow-blow are usually used in that particular application.
It's unlikely that changing the preamp valves would cause the fuse to blow, as most amps use plate resistors of at least 100K in line with them, so even a dead short across a preamp valve wouldn't cause enough current flow to blow the fuse.

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Yup, always use the original rating and type for fuse replacements. Don't trust that whatever was fitted is original unless you bought it new, the type and rating should be on the pcb nearby. HTH!

LD

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Just had a look at the parts list.

It shows a T6.3A fuse on the mains input.

Get a pack of those instead of the fast blow ones.

It doesn't explain why the original one blew, but it might have been 'just one of those things'. :)

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In my limited experience with valve amps (one Hiwatt 200 with KT88s) when I had fuses blow it turned out to be a sign that a pre-amp valve was about to go - it did, I replaced it and no more blown fuses. Could have been coincidence though.

If it persists check out the local recommended amp techs. David Pye of Preston was/is great - but he's moved near Blackpool now.

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