What makes a power transformer hum loudly? (audible, acoustic buzz, actually)

[rascalseven]

I been building for many years, but have never experienced this before now.  I would be tremendously grateful for any light you guys could shed on this:

I built a dual 312 for a friend of mine, and it worked great, sounded fabulous, he used it for about 30 minutes to an hour.  He then switched it off to try another preamp, and when he switched it back on it hummed/buzzed pretty loudly in the rack, the power LED flickered, and then it died.

I'm using Peter's green psu (which I've used several times in the past with no problems, btw), and two 16vac transformers with their primaries in parallel and secondaries connected in series to create 16-0-16 that I need for the psu.  The CT of the secondary is grounded.

I had a slo-blo fuse in the thing (all I had when I built it) though the power trannies are EI core.  When I put a fast acting fuse in it died instantly (the LED barely lit at all).

I've checked the psu board well, disconnected the psu from the audio circuit, replaced regulators, checked all the wiring, and it still blows fast acting fuses before the LED can even fully light.  For the fun of it (isn't this fun?  : ) I put a slo-blo back in, and powered it momentarily.  Any time I flick the power switch it buzzes loudly, and the chassis is vibrating.  When I measure the windings on the psu iron everything checks out, it seems.  There is no short, no break, and no direct connection to ground except the CT of the secondary.

This is a new experience for me.  Anyone know what's going on here?  Thanks so much for any help you can send!

JC

 

[owel]

Sounds like there's a short somewhere.

Is the trafo getting hot?

The secondaries that you wired in series, are they in the same phase and not opposing polarity? (same with the primaries in parallel)

 

[pucho812]

current? do the transformers get hot?

 

[rascalseven]

It was the bridge rectifier (the only active component on the psu that I didn't replace earlier).  Swapped in a new on, and the voltages are perfect.  I guess it was just a bad component.  First time for everything!

And no more buzzing transformer!

Thanks, guys.  Carry on.

JC

 

[matta]

Joel,

I feel you! I had to repair to repair a Valley People 2 slot rack, similar deal, kept blowing fuses... replaced the regs, all polarized caps etc, same issue... dead short in the power transformer, a first for me as well... had to have a new one wound, and not it is happy as Larry, like you diode issue, I really didn't think it could/would be the power transformer. RE the buzzing, interestingly I asked the said transformer manufacture why they dipped the trano in varnish, he said to stop the lams from vibrating, never thought of that before  :-\

Cheers

Matt

 

[riggler]

Are you sure that the rectifier is operating within it's limits?
Are the power transformers toroids? If so, make sure the center bolt only touches one side of the chassis!!!!

And you're sure the amp isn't oscillating or something, causing excess current draw, blowing the diodes over some time? I'd be curious to know how much current is really flowing through the diodes.

 

[Mike Cleaver]

Transformer laminations "can" vibrate if they are loose.
Back in the day of high powered tube radio transmitters, there were huge power and modulation transformers in vaults in the basement.
Bass notes could cause the building to vibrate in sympathy and a loose lam would be very obvious, often corrected by an engineer with a giant socket wrench twisting the bolts a little tighter (with the power off, of course!!)