Is This Power Transformer Dead?

[Matt D]

A friend gave me a power transformer from an old Baldwin organ. I lucked out and found a schematic online and it looks perfect for a cathode-biased 6l6 amp. I hooked it up to my variac and found all the taps indicated on the schematic: 720v, 12v, 6.3v, 5v, and a center tap.

Measured with an Ohmmeter, there is no continuity between any of the taps, except between the 720 and the center tap.

However, after applying 118 to the mains, when I measure the voltage between the different taps I get all kinds of different voltages. For instance, between one leg of the HT and and one leg of the 5v tap I get 280v. I get 370 between one leg of the HT and one leg of the 12v tap. All of the taps show at least a little continuity in voltage. Between the heater taps it is mostly less that one volt but between the HT and the heaters it is significant. Is that normal or is something awry with the transformer?

[Phil_S]

What you report is not normal. Normally, windings are fully isolated unless multi-tapped, for example if there is a bias tap on the HT winding. There may be some sort of induction effect at work here?

Aside from the ohms test, does your meter have a continuity indicator? Use that to check between windings and see if you get any beeps. If you do, it's a pretty good bet there is an internal short. If you don't then it's just something weird I think I wouldn't worry about.

[martin manning]

I think this is normal. Each of the secondary windings has AC voltage at a particular phase on it when the primary is energized, but with no common potential (ground) reference between them they are floating, voltage-wise. They are magnetically coupled though, so I guess I wouldn't be surprised to see a potential difference measuring between them.

I don't have any multi-winding PT's laying around, but in a related experiment I can measure a potential from the primary to the secondary when I energize a 6.3-0-6.3 filament transformer. With all secondary leads unconnected voltages are around 7 or 14 VAC, depending on which leads are measured.

[Matt D]

Thanks guys,
I get no beeps on the continuity tester so I guess I'm probably fine.

[LOUDthud]

Sometimes there is a shield between primay and secondary but very rarely between multiple secondaries. The heater windings are wound last on top of the HV secondary so stray capacitance couples some of the HV into the heater windings as a common mode voltage. This is why you get a huge hum if you don't ground (or connect to a voltage) the centertap. The common mode voltage can be hundreds of volts. Hummmmmmm!

[Koop]

Resonant transformer is a sort of leakage transformer. It utilizes the leakage inductance of its secondary windings in blend with external capacitors, to form one or more resonant circuits.The polarity of a transformer is either additive or subtractive. In order to find out the polarity of a transformer, a voltage is applied between the primary bushings.

Hmmm...is this another attack of the SpamBot?

[Matt D]

Hmmmm.... Well regardless, I got the heater strand and B+ up and running yesterday. All voltages are correct on the heaters and I have a B+ of 370 with a 5U4. I'm going to turn it into a 5e5. Thanks for the help.