Marshall power transformer filament winding shorted - twice

[Pavezer]

Hi!

I'm new here and thought that maybe you could have some ideas with my problem.

I have a 50W Jubilee Marshall tube head and around a year ago amp started to blow the fuse. A thorough checking proved that the power trasformed had broken down. Measurements showed that there was a short to ground in the filament winding. Ok, I invested a new PT, threw it in and eveything was ok and amp worked well. But occasionally, amp made weird noises, not humming, but like chirping and chirring sounds, also sort of ringing at the same time. Also, noise frequency went sometimes up and down. But these noises were really quiet and random, so I didn't bother to worry. I just thought that it might be a lousy or bad preamp tube or something like that. It was quite random and rare and didn't affect the sound and playing at all. So i just didn't bother. It was so silent and all.

Couple days ago, the amp died again, started to blow the fuse and I opened it up, did some checking and measurements and found out that the PT had a shorted filament winding. Again, to the ground.

What do you think:
Have I just been unlucky and the same flaw just.. happned again? Maybe the PT I inserted wasn't that great and noise came out from that and then it just blew?
Or could there be something wrong in somewhere else that might have affected or loaded badly my PT filament for a year and caused the flaw?

Any ideas before I get a new tranny and start to wait for the break-down

Thanks a lot in advance!

- P

[Valvehead]

did you buy the PT from Antique Electronic supply ?

[Pavezer]

did you buy the PT from Antique Electronic supply ?

Nope, I live in Europe, Finland, and I ordered the PT from TubeTown, Germany. It was fairly cheap - that was all that mattered at that time. I was in a hurry and short of money
Darn, I just can't remember the brand of the PT right now.. (nothing r e a l l y famous though..) Anyway it was a straight replacement and they recommended it.

[Don Moose]

Well, the first question that occurs to me is whether you're running the specified tube complement (3x 12AX7 + 2 or 4 EL34)?

If you've gone to 6550, KT88 or some other output tube with much higher heater current demands, that extra current and the extra heat that makes could slowly degrade the insulation on that winding.

Is the pilot lamp the correct unit?

Then again, it could just be a crummy tranny.

Anyway, hope this helps!

[Mars Amp Repair]

well,
You probably didn't notice or I suppose you would have mentioned it....did the transformer run hot? It would seem a breakdown in the fil windings twice could indicate too much current draw....just what I believe don was alluding to. A breakdown in winding insulation seems to indicate this kind of condition over time, rather than an instantaneous short.

Looking for any appreciable DC on the fil windings might yield something...still pretty unlikely.

Weird one for sure. glen

[Enzo]

Um, you do realize the transformer heater winding is center tapped and that the tap is grounded? So there will always be continuity to ground. And the low turns number and thick wire have next to no resistance, so any resistance reading you take will look like a short. I'd be hard pressed to determine there was a short in that transformer using only my meter.

Disconnect the heater winding wires INCLUDING the center tap. Now measure resustance to ground from the winding. I bet the short is gone. Now power up the transformer. is there 6VAC between the wires? Is there 3v from each wire to the center tap wire?

Pilot lamp is neon and inside the powr switch.

WHICH fuse is blowing?

How old are the power tubes? The noises and stuff you describe are consistent with failing power tubes. Failing power tubes often will pop the fuse now and then but they still work. Until they fail completely anyway.

[Pavezer]

well, You probably didn't notice or I suppose you would have mentioned it....did the transformer run hot?

I didn't notice anything special while it was working, besides that extra noise.
Anyway, I didn't check the heat of transformer that closely, so it might have occured without me noticing it. At least it wasn't running burning hot, I think I wouldn't have missed that.

[Pavezer]

Um, you do realize the transformer heater winding is center tapped and that the tap is grounded? So there will always be continuity to ground. And the low turns number and thick wire have next to no resistance, so any resistance reading you take will look like a short. I'd be hard pressed to determine there was a short in that transformer using only my meter.

Thanks for the info! I'm quite novice with these things and on my way of learning stuff. I do realize that heater winding is centertapped. My (good quality) multimeter shows 0,00 Ohms and I have been (so far) confident that it shouldn't be that much of a zero. So this could be possible and I have been missing a lots of reality..

Anyway, when i traced down the flow in the first place, the fuse blowed even if I disconnected all the primary voltage leads. And the fuse didn't blow when I disconnected the primary windings from power switch. I still have the old transformer and maybe I have to measure it again to make it sure it was filaments in the first place. Doh.. It might just have been my own flaw.. Anyway, the transformer was bad and amp started working with a new one.

Disconnect the heater winding wires INCLUDING the center tap. Now measure resustance to ground from the winding. I bet the short is gone. Now power up the transformer. is there 6VAC between the wires? Is there 3v from each wire to the center tap wire?

I will do that when I get my hands back to the amp.

Pilot lamp is neon and inside the powr switch.

Indeed

WHICH fuse is blowing?

Oops, sorry, my bad for not telling that one... It's mains fuse

How old are the power tubes? The noises and stuff you describe are consistent with failing power tubes. Failing power tubes often will pop the fuse now and then but they still work. Until they fail completely anyway.

Power tubes are ok, noise has been there even if swapping the tubes. And fuse haven't popped after I installed the new transformer. Now it's (naturally) popping even if power tubes aren't installed.

Anyway, I narrowed the flaw to PT again by unsoldering secondary windings off from rest of the amp -> still popped the mains fuse. And when unsoldering the PT from power switch circuit, the fuse doesn't blow. Secondary high voltage measures some tens of ohms and filament pure zero ohms. I haven't marked these up as exact numbers, because in my mind the filament was the obvious problem, but as said, I might have been (totally) wrong, so I will do all the measurements again.

[Enzo]

I use a pertty good FLuke meter myself, and I expect to see just about zero ohms on the heater winding. heavy wire, low turn count, low resistance.

When checking a transformer, always also check resistance between the windings and the frame of the transformer. If winding wire shorts to the iron, it will ground that voltage and blow fuses.

If none of the secondaries are connected to anything and the thing still blows the mains fuse, then it is indeed bad.

[Pavezer]

If none of the secondaries are connected to anything and the thing still blows the mains fuse, then it is indeed bad.

Yep. That's one thing that's sure.
I just have to find out why. In another words: if there's something that's drawing too much current and loading the PT too much and broke thge tranformer - twice. Any ideas?

I will make all the measurements again to locate it it wasn't the filament side in the first place. That will help to (possibly) tell the reason or give some sort of hint why this might have happened. Or.. i just had a bad transformer right from the beginning. Do you think it could be possible that a bad PT made this kind of noises?

[Pavezer]

A quick update for a thread i started more than year ago.

Amp has been doing fine with the latest transformer.
The extra noise was as simple as bad effects loop (surprise - not) I was quite confident that there was a major connection with the noise and blown transformers, but it seems that it wasn't that obvious.

So.. at moment the noise is gone and the amp is doing well. I think i just had a bit of bad luck and dealed with not so fine transformers.

Thanks to all repliers.

[drewl]

Wow, year old thread....I was going to suggest troubleshooting the switching power supply since it's taken from the filament supply right?