Ceriatone JTM45 blowing mains fuse

[Heliotricity]

Today my ceriatone JTM45 quit on me. After running it for an hour it just shut off. When I opened it up I saw the 2A 250v mains fuse had blown. HT fuse is still fine. No components looked burned. I checked the output tube screen resistors and both read fine.

I removed all tubes and the fuse blew immediately upon powering on (standby was still off). I checked the power transformer windings to ground with my multimeter and the primaries showed no reading at all. The 3.15 secondaries pair going to the pins 2 and 7 on V5 read .4 ohms, and the 350v pair going to the tube rectifier read 48 ohms. The transformers on this amp are made by Promitheus audio. Photo of PT attached.



I have a feeling the initial cause of all this is that one of the output tubes blew. The one suspect tube which I just swapped in a couple days ago has two large metallic ovals on the inside of the glass on opposite sides (see photos), is that a symptom of a KT66 which has overheated? I had another KT66 blow a ways back and it looked just like this.

Did a shorted tube take out my PT? What would be the next logical step in troubleshooting this?

[olddawg]

I would float the secondaries and see if it still draws current. If it's bad you are going to have to disconnect it anyway.

[Enzo]

The primary has no connection to ground, so a resistance reading there will show open. Do what OD suggested, disconnect the secondary wires and power up the transformer, if it still wants to blow fuses, the transformer is probably bad.

[pdf64]

..The one suspect tube which I just swapped in a couple days ago has two large metallic ovals on the inside of the glass on opposite sides (see photos), is that a symptom of a KT66 which has overheated? I had another KT66 blow a ways back and it looked just like this.

That may just be the gettering, like that from new https://en.wikipedia.org/wiki/Getter

[DRH1958]

Look behind those marks on the sides and see if there is some kind of getter structure right next to the plates. I also think it's getter flash. There are getters on top but could also be some on each side too. Also, if the tube fried, the HT 500mA fuse should have blown instead of the mains fuse. At least theoretically.
If you want to save a fuse, use a light bulb limiter when testing the primary. There are many places to look up how to do this. Here's one.

[Tone Meister]

Completely unrelated to the original question, but I have had an inordinate number of failures with Gold Lion tubes as compared to other currently manufactured tube brands. That includes the highly regarded Gold Lion KT66 that were burned in and matched before shipping.

Good luck with the repair, you're in very good hands here.

[Mick Bailey]

Just for clarification - did you also remove the rectifier?

[Heliotricity]

Just for clarification - did you also remove the rectifier?

Yes, all tubes are removed.

I would float the secondaries and see if it still draws current. If it's bad you are going to have to disconnect it anyway.

I just unsoldered the secondary wires and put in a new fuse. Standby is off. Plugged it in and when I flipped the power nothing happened. The indicator light didn't go on, the fuse is fine. Just to be clear, I unsoldered all six wires on the secondary side, the 3.15v pair, the 350v and the two "0"s (see diagram).



So, if the amp had powered up and the fuse had blown then it would have been an indication the PT was bad, does this mean the problem may lie elsewhere? Or maybe the issue is on the primary side? I'll start checking component values throughout the amp to see if anything is reading out of spec.

[Heliotricity]

One other thing worth mentioning, the pair of unused wires taped off on the PT primary (220v and 240v) the other day were dangling low, so much that when I was putting the chassy in the headbox one of them caught under it. I noticed and tucked it under and then screwed in the amp. Could one of them shorted to ground and caused the problem?

After the amp quit and I was troubleshooting I saw them and coiled them up so they were not loose, that is why they look fine in the photo from my first post.

[Mick Bailey]

If one of the unused taps shorts to ground this would most likely blow the fuse, as the unused tappings are on the 'hot' side of the transformer winding. Your test with the secondaries disconnected suggests the transformer is OK. A shorted turn - either on the primary or secondary - will cause excessive current draw and the mains fuse would blow if this was the case.

I'm not clear on when the fault occurred - had you previously removed the chassis and reinstalled it, and the amp failed afterwards?

[Heliotricity]

...had you previously removed the chassis and reinstalled it, and the amp failed afterwards?

Yes, but I used the amp for a couple hours the day before, and again yesterday for another hour or so before it failed.

All my tests since then have been with it out of the chassy and the mains still blow (with the exception of today when I disconnected the secondaries.) Those unused tappings also are rubber shrink wrapped at the tips too, so it is not just bare wire.

I just checked components throughout the amp and have not found anything out of tolerance, I'll keep looking though. What next?

[Heliotricity]

UPDATE: It turns out with the secondaries unattached the mains fuse IS blowing when I power up. I didn't leave it on long enough when I tested it earlier.

When I tried it a couple hours ago I thought no power was going through because the indicator lights didn't turn on so I turned it off again pretty quickly. It hadn't occurred to me at the time that the indicators not lighting up was normal when the secondaries are disconnected.

So it looks like the PT is bad after all? Is there anything I should do before removing it to test the primary side to be certain it is bad?

[Diablo]

UPDATE:

So it looks like the PT is bad after all? Is there anything I should do before removing it to test the primary side to be certain it is bad?

As long as the primary side is wired correctly without a short, then the PT is toast because there is an internal short in the PT that is drawing lots of current. With the secondaries unconnected, the primary should draw very little current.

[drewl]

I had a new Weber PT from a Marshall kit go bad years ago.

They sent me a free replacement.

Is yours old? They might replace it or maybe give you a discount on a replacement.

[Mick Bailey]

Before scrapping off the transformer I would do one final test just to be certain, and that is to disconnect everything and use the 'neon test' to discover whether you have a shorted turn anywhere.

A shorted turn either in the primary or secondary will cause excessive current draw and blow the mains fuse. Don't think that because the primary fuse blows then the fault has to lie in the primary. Most likely if there's a short it's in the HT winding.