JTM45 RI blowing main fuse?!?!

[peterc52]

Hi guys,

I sold a JTM45 RI today and it has always worked flawlessly with me. He even tried it at my place before buying.

But now he says it keeps blowing the main fuse. What could cause the problem? Power tubes? Rectifier tube? Or something else?

Would of course want for him that it continue working.

Help me out!

 

[MississippiMetal]

Tell him to disconnect all the secondary wires and turn the amp on. If it blows still, the problem is likely the power transformer. If it stays intact, then start troubleshooting your way up. Tube rectifier is a good place to start. Does the reissue have an HT fuse as well?

 

[peterc52]

Yep, it has a HT fuse as well.

Oh, man, I don't think he knows how to do that.

Damn, what could cause a broken power tranny? And what are the chances of it not working 5 hours after he picked it up. It worked 100% at my place where he tested it.

 

[MississippiMetal]

It could just be a coincidence. But there could be something else to it as well.

Im a bit concerned that it blows the mains fuse and not the HT fuse. The HT fuse blowing is usually easy to explain because more often than not it's the tubes shorting that cause it to blow. The mains fuse, on the other hand, would indicate either a bad rectifier tube, or a shorted HT winding or heater winding. These kinds of shorts tend to occur internally and usually require replacing the power transformer.

I'd recommend replacing the tube rectifier first and see if that solves the problem. If it doesn't, then your issue is likely more involved and I'd get it to a competent technician for further diagnosis.

 

[glip22]

It could just be a coincidence. But there could be something else to it as well.

Im a bit concerned that it blows the mains fuse and not the HT fuse. The HT fuse blowing is usually easy to explain because more often than not it's the tubes shorting that cause it to blow. The mains fuse, on the other hand, would indicate either a bad rectifier tube, or a shorted HT winding or heater winding. These kinds of shorts tend to occur internally and usually require replacing the power transformer.

I'd recommend replacing the tube rectifier first and see if that solves the problem. If it doesn't, then your issue is likely more involved and I'd get it to a competent technician for further diagnosis.

HT fuse in my experience has mostly blown from preamp tubes. Also the power tubes blow the mains. One could be shorted. Replace the power tubes if it still does it after the rectifier tube is replaced. Before doing so with tubes removed measure across your screen resistors to see if they are ok. Look for burns on them as well.

 

[MississippiMetal]

It could just be a coincidence. But there could be something else to it as well.

Im a bit concerned that it blows the mains fuse and not the HT fuse. The HT fuse blowing is usually easy to explain because more often than not it's the tubes shorting that cause it to blow. The mains fuse, on the other hand, would indicate either a bad rectifier tube, or a shorted HT winding or heater winding. These kinds of shorts tend to occur internally and usually require replacing the power transformer.

I'd recommend replacing the tube rectifier first and see if that solves the problem. If it doesn't, then your issue is likely more involved and I'd get it to a competent technician for further diagnosis.

HT fuse in my experience has mostly blown from preamp tubes. Also the power tubes blow the mains. One could be shorted. Replace the power tubes if it still does it after the rectifier tube is replaced. Before doing so with tubes removed measure across your screen resistors to see if they are ok. Look for burns on them as well.

Power tubes have been known to blow the mains fuse, but the HT fuse in 9 out of 10 cases is usually taken out before the mains fuse. You have a smaller rated fuse directly on the high voltage rail of the devices that short, and it will theoretically blow before the higher rated fuse on the other side of a transformer. I work on 3 up to a dozen tube amps a week for the last few years and I've encountered less than 5 cases where tubes took out the mains fuse before the HT, and one of those was because some smartass threw a 10 amp slo blow fuse into the HT position. He's lucky it didn't kill his PT.

I had a quad of JJ EL34's that kept blowing the HT fuse in a Marshall DSL I worked on a few days ago. All 4 tubes measured just fine on my EMC 213 by short and emissions testing, but one would randomly short during play, or sometimes upon flipping off the standby switch, and there would go the HT fuse.

 

[Racerxrated]

I had a Fortin modded Ceriatone for 1 week and the same thing started happening. New PT unfortunately was needed. Ugh.