Bugera 333xl blowing F4 fuse

[Fuete]

Hello everyone it`s Bugera time again !!! I have a 333xl infinium (killer amp) it has EL34 JJ`s installed with not much hours of use. Well the thing is that in a rehersal the sound just fade away and the standby light started blinking, tubes were not firing up, anyway I took apart the the chasis and found a blown fuse in the F4 position but noticed that the board has a burn mark (refer to photo). After replacing the fuse the amp just started working again but after like 20 min of playing I saw a little bit of smoke coming from the amp, while the amp still on I`ve looked in the board to see were the smoke was coming but I didn`t saw anything. Few minutes after that same thign sound fade away and fuse blown. Since the led lights (tube monitoring) at the back of the amp were on for tubes 2, 3, and 4 I put 4 6L6`s from my other amp and again replaced the fuse, the amp worked fine for 30 minutes no smoke smell but I`ve noticed that the sound was the same but the gain was not the same it has less gain.

You guys think this could be a tube problem? let me add that after a thorough inspection of the board everything seems to be fine.
Any advise?

Thanks

Photo: https://farm3.staticflickr.com/2859/...69b7d277_z.jpg

[Mick Bailey]

Can you post a link to a schematic - we need to see what circuit F4 is protecting.

It isn't a good idea to repeatedly replace a fuse without knowing why it blew and fixing the problem, otherwise the circuit it's intended to protect is getting repeatedly stressed and can result in more damage. Anyhow, the end cap and holder appear to have got pretty hot. That can happen if the fuseholder doesn't make good contact - you need to check this.

[Fuete]

Agree Mick, I`ve noticed that one of the fuse holders was a bit open. That`s why I`m asking here I don`t want to change tubes and keep trying, like I said last time the amp worked properly with a fairly new set of tubes and I also adjusted the fuse holder but since I`m not sure whats going on I`m trying to be careful. Ihaven`t seen schematics for this amp but I`ll try to find one.

Thanks

[Mick Bailey]

There was a link to the schematics in another post, but it no longer works. Perhaps someone could re-post?

With fuse-blowing amps it's worthwhile making up a bulb limiter - could save a lot of fuses, helps to protect the amp and is a good indicator of the problem being fixed (or not).