Is a dead power transformer always a dead PT?

[JBasham]

Hello Gents (and Ladies)

Well I think I know what my problem is, but I'm bummed at the prospect of fixing it.  So to put that off, I'm just checking with the Hive to make sure I'm stuck replacing my power transformer.  The amp in question is a homebrew 300B parafeed monoblock that I cooked up about ten years ago. 

According to my notes (I have really forgotten the details of putting this thing together) I used a center-tapped new stock Hammond PT with a full wave SS rectifier and a CLC filter for the B+.  All filaments are AC but the 300B has the old Steve Bench circuit.  The B+ for the driver tube is voltage and current-regulated (C4S sandwiched with the 2009 Tube Cad current reg).  Driver and power tubes are cap-coupled and I used one of Mikey's grid chokes.

So a couple weeks ago I fired up the system and this amp didn't power up.  I turned everything off and went away for a while, came back to start everything up again, and it powered up fine.

Couple days ago though, it just would not power up.

I tested it, and it's delivering AC from the mains to the PT primary but there's no voltage at any of the secondaries.  The primaries and the secondaries are individually measuring resistance at appropriate levels that indicate they aren't cooked open.  I'm not measuring any obvious shorts between the primaries and the secondaries, but of course the filament stuff is still tied into the circuits so I'm not getting true measurements there - I'm getting resistances consistent with the circuit components.

So I'm pretty much assuming it's time to order a new Hammond and do the work to swap out the old one.  Unless anybody has a better idea!

Thanks

Jamey

[Doc B.]

I guess the remaining question is if you are getting the specified resistance at the primary and the secondary. If, for example, the secondary resistance is lower than spec you may have shorted turns. And are you actually measuring the incoming AC right on the transformer primary terminals themselves? What I mean is, probes not touching the the power inlet wires attached to the primary terminals, but the metal of the terminals themselves. If resistances are right and you see voltage at the primary wiring a bad connection between a wire and a primary terminal would seem to be the only possible scenario.

[JBasham]

Thanks, I need to investigate those two issues later.  I will flip the other amp and measure its PT for a comparison. 

Being as it's a Hammond 300 series with pigtail leads, I'm not sure it even has terminals to measure?  But I'll remove the bell and see what I can see.

[Paul Birkeland]

If the primary measures open, it's possible that one of the primary leads simply detached where it was soldered :/

[Paul Joppa]

All transformer failures are quite rare, but a fairly common one is an internal short of some but not all of a winding. So when you say the resistances are "appropriate" do you mean plausible or within 10% of the design value? Some Hammond transformers have specifications that include winding resistances - these are a fairly recent development so look for them on their web site.