Cause of power transformer failure

lepeu66 · Post by **lepeu66** » Sun Oct 14, 2007 4:21 pm

I have managed to fry my JTM 45 PT. Long story short, smoke involved! My question is what kind of things will cause this to happen. Could a bad rectifier tube be the culprit? Well my first mistake was having the wrong mains fuse in. Any thoughts would be appriciated so it doesn't happen again. But, on the up side, I have the ultimate paperweight.

Blast · Post by **Blast** » Mon Oct 15, 2007 9:52 pm

A short across any of the secondary taps could cause that problem. Yes, bad rectifier or a shorted filter cap, also. How big was the fuse?

lepeu66 · Post by **lepeu66** » Tue Oct 16, 2007 12:15 pm

Fuse was 4amp. I am willing to bet it was the rectifier tube, but who knows. It could have been bad from the get go. Just wish I knew. Well, I guess I have a wile to research the issue till I come up with funds to replace. Does anyone know anyone who might want a dead pt?

Blast · Post by **Blast** » Wed Oct 17, 2007 9:37 pm

I would like to have believed that a 4 amp would have prevented that damage. But then I live in a "perfect world" where nothing goes w

lepeu66 · Post by **lepeu66** » Sat Oct 20, 2007 10:24 am

Could a reversed diode cause this pt meltdown....yes I have already punished myself!!!

Blast · Post by **Blast** » Sun Oct 21, 2007 11:52 am

If you mean a reversed bias voltage diode (I think that's the only diode if your using a rectifier tube) that would inject positive voltage rather than negative voltage to the bias circuit. I would think that would immediately pop the bias voltage filter cap(s) immediately after the diode. Or open the resistor in front of the diode (the one between the PT and diode). In standby nothing else MAY occur to the rest of the circuit. But out of standby the positive voltage should effectively fry your output tubes. I can't see any of that smoking your PT but I'm not an expert by any means and hopefully others will chime in here.

What I AM an expert in is curiousity. I can remember "smoking" a PT back in my youth by accidentially shorting a secondary while building an amp. But then I had no mains fuse on it, either. I remember the white smoke pouring out of it until I pulled the plug. What a downer- I had to ride my bike down to the local repair shop and get another TV chassis out of their trash. That was the early '70's and 50's TV transformers were excellent for big amplifiers. But then I digress...

My understanding of fuses is that they are supposed to protect the consumer more than the circuit itself. The 4 amp fuse is probably geared more towards the wire utilized in the circuit rather than to the PT itself, right? When you design a circuit (and a fuse into one) you take into consideration what COULD possibly go wrong under NORMAL operation. I'm trying to figure out what would have happened if this occurred during normal operation. And really the only way I could see it happening would be a short to the transformer from a tube- either filament of any, or plates of a rectifier tube (or bridge diodes for SS rectification). Although the possibility is there, is the probability? Probably not, right? Filaments are more prone to opening rather than shorting. Plates? I don't know.

I guess what would have happened if the amp was allowed to continue smoking is that either a direct short in the PT would have EVENTUALLY occurred that would have EVENTUALLY blown the 4 amp fuse, OR, the afflicted winding in the PT would have EVENTUALLY opened leaving only a room full of smoke. I suppose there really is no chance of fire since most PT's are heavily encased (well, how about those open bottom laydown types?).

If you can remember back to the fault, how long did the PT actually smoke before you saw what was happening and pulled the plug? Did the house breaker/fuse ever trip? Had your amp EVER worked in normal operation until the smokeage, or were you in the process of building it? Are you sure you had a 4 amp fuse in place? Isn't 4 amp correct for that amp?

lepeu66 · Post by **lepeu66** » Sun Oct 21, 2007 12:12 pm

Yep, She was working perfectly for about 8 months. However, being also prone to curiosity, I had to tweak a bit. I replaced a few resistors in the preamp section and the diode. I however installed the diode in the wrong direction. This I believe would effectivley halted any voltage getting into the bias section? The fuse is supposed to be 2amp. I have since tested the power tube and they appear to be fine. The rectifier tube however did not make it. I am hoping when I eventually replace the transformer, the reversed diode was the culprit, and since I have already fixed that, all should be well. But I'll have to wait and see.

lepeu66 · Post by **lepeu66** » Sun Nov 11, 2007 9:34 pm

Answer: I installed the new PT, seems wrong diode orientation and wrong fuse is equal to $125 ERROR. All is well now

Blast · Post by **Blast** » Mon Nov 12, 2007 10:52 pm

Time to rock!

lepeu66 · Post by **lepeu66** » Thu Nov 15, 2007 9:51 pm

Damn I missed this sound!!!!!!