What kills power transformers?

[Doug]

replaced a dead pt in my silverface deluxe. New one sounded great, better bass response, better overall. Second day, smoke, pops fuses. All tubes out pops fuses. Disconnected all wires except the primary and pops fuses. Can't find any bad caps or resistors in the amp. Filter caps test good, although original. Should I suspect the transformer or be looking elsewhere?

[R.G.]

Well, if it pops fuses with just the primary connected and you 're sure that all other loads were removed, this transformer is history. Frankly, I would suspect a defective new one and ask for a replacement.

That leaves you with the problem of what killed the first one.

What kills power transformers is heat. It can be generalized, like working inside a hot cabinet at full power for along time with the vents blocked. It can be localized, like when a tube heater shorts and just the heater winding insulation melts and then shorts, overheating the rest of the PT.

It can even be a result of putting too much voltage into the primary for the tap. Saturating the primary of a transformer (it's impossible to saturate a transformer from the secondary) is a good way, because it's the primary inductance that's holding the full power of the AC line out of the transformer, only letting in what the secondary needs. If the volt-time on the primary saturates the core, inductance drops preciptitously, and the AC line pours through overheating the primary wire resistance.

In all cases, when the wire insulation decomposes by heating and any one turn shorts, the dance is over and destruction ensues.

[Euthymia]

Usually, trying to supply too much current kills them.

My guess is that there's something in the amp that killed both transformers.

More specifically, a guess is that one or more of your tubes has an intermittent short, but there are other things it could be. Your bias supply may have an intermittent fault.

Be sure to check that the amp isn't over-fused before you sacrifice another transformer.

After replacing the transformer, bring it up with the tubes pulled, and running through a lightbulb limiter.