Dead OT?

[EFK]

I just picked up another dirty old Supro that wasn't working (filter caps all bad etc.), so I've gone through the amp and done all the usual cleaning and maintenance. Power on and not a hint of sound at all. Good tubes, good speaker. All voltages everywhere, preamp and power amp, are exactly where they ought to be. So I wonder about the ancient-looking OT: I've never had a bad OT before (I must be very lucky!). I notice if I pull all the tubes, I have NO resistance between the primary terminals and the center (B+ line in) of the secondary. I mean no resistance at all: this does not seem right, so can I assume this OT is overly-toasted?

[52 Bill]

There should be some sort of continuity between the center tap and each of windings that go to the plates of the outputs and from one plate connection to the other.

It would be unusual for all three terminal to go open, but possible. Try reading all combinations of wires.

[EFK]

Yes, I measured from the secondary center B+ to each side/plate and got about 75 ohms of resistance per side (actually 75 and 82, but I think a little imbalance is normal). I'm talking about that same center B+ of the secondary side over to the primary leads/terminals: I have no resistance at all between, which doesn't seem right. Shouldn't the primary be insulated from the secondary?

[52 Bill]

OK, first off let's get the phrasing correct for this. The side of the output transformer that connects to the tubes is the primary winding. The side that connects to the speaker is the secondary.

Your center tap to plate resistance readings are correct for this transformer. And yes the mismatch is ok as there is more wire on one side of the center tap than the other.

I'm talking about that same center B+ of the secondary side over to the primary leads/terminals: I have no resistance at all between, which doesn't seem right. Shouldn't the primary be insulated from the secondary?

Yes, the primary and the secondary must be insulated from each other. So there should not be any kind of continuity from primary winding to the secondary winding. Infinite ohms, no reading or over scale on your meter.

If you are saying "no resistance" meaning zero ohms or a dead short from primary to secondary, then you have a problem. If this was the case, your B+ supply would be placed across the speaker terminals and would not only damage the speaker, but would blow the fuse as well.

[EFK]

Oops, yes I was incorrectly calling the primary the secondary and vice versa. I am reading complete continuity from primary to secondary, so it appears the OT has burned through the insulation. However, the speaker is still fine and no fuse blew. This does seem odd, but then I didn't run it for long like this other than the measure voltages and run some checks. Fortunately, I have a spare OT which is a bit bigger but has identical mounting centers and is brand new so I'm going to detach the original and mount up the new one; that will prove what the problem is pretty definitively! Thanks for the help!