Power transformer fried... why?

[odourboy]

A couple of weeks ago, a 50W non-HRM build of mine packed it in after 2 1/2 years of reliable service. I've determined that the high voltage secondary winding in the power transformer is partially shorted (I presume it overheated and melted some insulation - it does have a bit of that nasty burnt wire smell) causing excessive current draw and a blown fuse.

I went through the amp top to bottom and can not find a fault with anything downstream - HV rectifier, PS components, tubes all seem fine and having now replaced the PT, the amp is working perfectly again (didn't even have to rebias and the dropping string voltages are right where they were when I documented the amp).

So I've been trying to figure out why this happened and how to prevent it from happening again. The PT is an Allen Amps TP40 (made by Marvell) and the HV tap is rated at 200mA. Based on measurements across the PS RF mod resistor, (B+1 to B+2) the B+2 and beyond is using about 21 mA. I don't know what the power tube plates are drawing. Tubes were 2X 6L6GC and 4X 12AX7. Bias on the 6L6 is 34mA idle current.

Any thoughts as to what happened?

[David Root]

Could be a thin insulation spot or occasional insulation voids finally heat aged to the point of giving it up. Same thing can happen in pickup windings (from age, not overheating!).

The mfg process can do this with the wire, for whatever reason you'll get a thin covering of insulation, or just voids in normal thickness insulation, for a certain length of wire, it can happen.


Could also be a defective thermal overload detector I suppose, but I don't know enough about them to know if that's even a possibility

[Structo]

So you didn't have a power tube short out or other fault that may have damaged the PT?

Strange it would last 2 1/2 years then go.

The PT that Brown Note gave me for my D'lite is 300ma on the HT.
6A on the heaters.

Perhaps depending how your power supply is wired and where your first bit filter sits, such as if it is on the cold side of the standby which causes huge in rush current when the switch is thrown, could have damaged it?

What is that called.....back EMF?

Just guessing.

[fredfretter]

Good, my first post here. I finally feel like I can contribute something, even if it is just advice as to how I protect transformers.
I like to put a slow blo fuse inline with the center tap of the PT and its ground.
If I remember a .5 amp for a 2 x 6L6 amp, but this depends on tranformer rating. I also like to use a fast acting fuse on the OT center tap, usually about .1 amp for 2 x 6L6.
Sorry about your PT. I had a 67 Deluxe Reverb blow on me and since then I have been using the above safeguards. Great forum!

[David Root]

Slightly OT here but what kind of fuseholder do you use, fredfetter? I am building a 50w ODS with a set of '67 Bassman iron and this sounds like a good idea.

[stelligan]

The PT is an Allen Amps TP40 (made by Marvell) and the HV tap is rated at 200mA.

Bummer. I've had one of these on my bench for a couple years that is going into my current build. Did you replace it it with the same PT?

[Firestorm]

Odourboy --

Just trying to suss out the possible failure modes; you're quite sure it's the high voltage secondary that's shorted?

[fredfretter]

David, if you have room in the chassis, a cradle type holder works well, or use an inline fuse holder if you don't won't to drill any holes and save space.

[FUCHSAUDIO]

A properly designed and built transformer (power or output) should not just die. Unless you had a true malfunction (like a tube died and you had an oversized fuse in there), the transformer should not fail during normal use.

The reasons for a failure would be : Over loading current for an extended period of time (burning wire and/or insulation), too large a fuse which would not blow if a tube or filter cap failed. The only other possibility is a poorly made transformer which may have absorbed moisture (due to inadequate varnishing during manufacture) or undersized wire that allowed it to get too hot and eventually burn out.

I've repaired amps and antique radios from the 20's and 30's, still running on their original iron....gotta tell ya something.

[odourboy]

So you didn't have a power tube short out or other fault that may have damaged the PT?

Strange it would last 2 1/2 years then go.

The PT that Brown Note gave me for my D'lite is 300ma on the HT.
6A on the heaters.

Perhaps depending how your power supply is wired and where your first bit filter sits, such as if it is on the cold side of the standby which causes huge in rush current when the switch is thrown, could have damaged it?

What is that called.....back EMF?

Just guessing.

Now that it's up and running again, the power tubes seem to be working perfectly. And there's certainly nothing unusual about the PS implementation other than I've got a 470 ohm power resistor rather than a choke.

[odourboy]

The PT is an Allen Amps TP40 (made by Marvell) and the HV tap is rated at 200mA.

Bummer. I've had one of these on my bench for a couple years that is going into my current build. Did you replace it it with the same PT?

Yes - I had another one from Allan sitting on the shelf. It's got the same specs but is a couple of years younger and has two holes in the bottom bell for the wires rather than one. It's running 'cool as a cucumber'.

[odourboy]

Odourboy --

Just trying to suss out the possible failure modes; you're quite sure it's the high voltage secondary that's shorted?

Yes. All the windings measured appropriately with an ohm meter except the HV secondary pair (bias tap was right against one half, but not the other) and with a Variac feeding about 10VAC, I checked voltages on all the secondaries and the HV was much lower than it should have been.

[odourboy]

A properly designed and built transformer (power or output) should not just die. Unless you had a true malfunction (like a tube died and you had an oversized fuse in there), the transformer should not fail during normal use.

The reasons for a failure would be : Over loading current for an extended period of time (burning wire and/or insulation), too large a fuse which would not blow if a tube or filter cap failed. The only other possibility is a poorly made transformer which may have absorbed moisture (due to inadequate varnishing during manufacture) or undersized wire that allowed it to get too hot and eventually burn out.

I've repaired amps and antique radios from the 20's and 30's, still running on their original iron....gotta tell ya something.

I totally agree Andy! Thus my post. I'm very perplexed by this (not to mention, embarrassed... it was being used in a session when it failed).

It had a 2A slow blow on the mains. It's never blown a fuse before. There's no evidence that any of the PS caps has gone bad, but that seems to be the most likely unless the transformer itself had a defect that took time to reveal itself.

[David Root]

Odourboy, were those two transformers, the one that failed, and its replacement, made by the same mfr?

The TP40 I have is two holes in the lower bell, and EIA code 1660709.

[odourboy]

Odourboy, were those two transformers, the one that failed, and its replacement, made by the same mfr?

The TP40 I have is two holes in the lower bell, and EIA code 1660709.

I can't say for sure if they are from the same manufacturer. Between the original and the second, Allen Amps had got custom marking done on the bell versus the original which had (I presume) the manufacturer's codes. The bell holes are different but the look, form factor and wire stripes are the same, so I suspect they both came from Marvel (also sell under the name Triode I think).