LSS throbbing sound/fuse blow

[roundaboutmusic]

Hi Guys,

I've looked through some of the other threads re: LSS and tubes/fuses going, but I thoght I'd check with you all before I retubed mine.

The problem is that the amp makes a low throbbing sound when turned on and starts to smell smoky, then the fuse blows and the whole thing dies. However, this only happens in 30w mode, in 15w or 5w it's fine (with a new fuse).

I've inspected the power tubes, they all light up, one seems a little brighter than the others (no matter what position it's in) and the rectifier tube glows normally. Unfortunately I don't have any spare tubes to play with at present.

Is it likely to be a power tube or the rectifier tube?

Also, on an LSS which tubes are paired, is it the two outside and two inside? And which one is the single one used for 5w mode?

Cheers,
-tom

 

[plan-x]

Could also be a pre-amp tube. Flick the individual tubes with your finger while they're on and look for any arcing inside that tube. Outside pair together and inside make the other pair.

 

[roundaboutmusic]

Hi, thanks for the reply.

I tried swapping out the power tubes for some new ones, but sadly the problem is the same. On further inspection the two outside tubes glow quite blue while the inside two are fine, I'm guessing that seeing as I only have a problem in 30w mode then I probably need to get the rectifier diodes checked?

From what I understand, the amp uses the tube rectifier in 15w and 5w modes, so seeing as it works fine in those modes, it's most likely te diode rectifier. Does this sound likely?

Cheers
-tom

 

[aaland_brian]

I had the same problem with an throbbing oscilation and crackling and it turned out to be a preamp tube, I didn't really hear much noise when I flicked it, but when I pulled it out it completely broke apart. I would replace the cathode follower preamp tubes and maybe the rectifier tube too if it's an earlier LSS. I put in some penta lab 12ax7's which have held up nicely.