Fried power transformer strange symptoms and ramblings ...

4114Effects

So this evening I decided to do some "clever" power scaling mods to my tweed deluxe build. I bought a new power transformer with a 190v and 275v taps on the secondary, so theoretically you can run it at half power using the 190v. It's been working perfectly up until now. Anyway, seems like I messed up the wiring on the selector switch and ended up connecting ov directly to 275v on the secondary ... Doh. The transformer duly released a little puff of magic smoke and smelled pretty foul. I was convinced that it would be blown, but unconnected to the amp it measures 288v across the secondary, just as it should. So cautiously I hooked it back up to the amp, with the power tubes pulled - voltage at the first filter cap was a nice healthy 400v (solid state rectified), so it's exactly where it should be. However when I put the power tubes back in, strange popping and crackling noises from the speaker, and the voltage at the first filter cap fluctuates rapidly between 120 and 160v. I guess I already know that I need a new power transformer, I'm just curious if anyone has experienced this before - I thought if a pt was damaged the windings just wouldn't work, or put out a much reduced voltage? Having thought about it a little, I guess the transformer could still give a decent voltage, but fail when a certain amount of current is drawn through the circuit? I just find it interesting, and wonder if any of you techies have a theory? Anyway, there are my ramblings, maybe this is one for @ICBM and his vast technical skills ... Cheers!

ICBM

Not really a lot I can add! Yes it is probably blown, and yes they can sometimes work, sort-of - but not under load.

It's also possible you have damaged a power valve though - an intermittent short in a valve could also make the HT voltage fluctuate like that.

4114Effects

Thanks, I did think about the power valves, so tried both of them in my single 6v6 champ build and they appeared to work fine. Time to get out the card and buy a new PT, nice expensive lesson learned. Unless you have a spare transformer of any description for sale? Would rather give my cash to a forum member :-)

MtB

What type / make of transformer is it? 

If it's a Danbury DB1249-DT with 0-190-275 taps, I have one spare.

4114Effects

That's the exact one! Will PM you

MtB

Are you sure that you're using the the correct "break before make" type switch - as those rotary ones come in different configurations.  

4114Effects

It was the correct switch. I would always turn the amp off before switching voltage anyway. I'd just stupidly wired it... Sigh. Anyway, new PT on the way, thanks mate!