Possible Tube Problem?

A few weeks ago I was playing my LSC when it emitted a hum and then died. I checked the fuse and it was shot. I replaced the fuse, removed the power tubes and rectifier tube and turned the amp on. It remained on which meant it was a tube related issue. I breathed a huge sigh of relief! Following the proper procedure I replaced each tube one by one. I was shocked when the amp worked perfectly with all the tubes back in and on every setting.

Earlier this evening, I was playing my LSC when it started to emit that familiar hum. This time, however, I was able to switch the amp to standby. I inspected all the tubes and none of them were hotter or glowing anymore than normal. I turned the amp back on and continued to play. A few minutes later I heard a bit of crackling so I switched the amp from 10W to 50W. The crackling disappeared which should indicate that either an outer power tube or the rectifier tube is the culprit.

I've played the amp almost everyday for the past two weeks without any issues. I even had it on for quite a few hours straight yesterday without incident. What do you think caused the humming and blown fuse?

From what you've written, I'd say that you have a bad power tube. I've seen them fail intermittently as you described. But, since it blew the fuse once, total failure is not far away. The bad part about power tubes going out is they sometimes fry the grid resistors. If there's a burnt smell at all, or if there's any red plating on any of the tubes, that tube's resistor is probably damaged and needs replacement.

Personally, I would replace the entire set of power tubes. if you never use the 100 watt setting then just replace the outer tubes. It's always cheaper to replace tubes than to pay a repair charge unless you are able to do the repairs yourself.

My experience with rectifier tubes is that they fail completely, but I've only seen a few fail (all in my LSS). For the additional $20 or so, I'd replace the rectifier too just to be safe.

Thanks for the reply, Ricardo. It does sound like a power tube. I had a rectifier tube fail in my LSS and it completely went out. Would a grid resistor be either good or bad? Would I know if it went out?

If the resistor is completely out, you'll know- smell, maybe a little smoke, red plating of the tube, bad tone.... However, the resistor could partially burn, and still function with a changed resistance. There's no way to tell for sure without inspecting and measuring the ohms of the resistor, but a burnt smell and/or any hint of red plating on a power tube after there's been a problem like you describe is a tipoff. But....a resistor could be damaged without those symptoms. If there is damage though, you'll probably hear a difference in tone.

Replacing grid resistors is not a big repair, or major damage. They are also somewhat sacrificial to avoid more major damage from a tube failure.

Thank you for your help so far. Where exactly is this resistor and what should it measure?

Sorry, I don't have a schematic or an LSC to help you out.