Power transformer gurgling

[harry]

Hi all,

I'm building a simple preamp using a power transformer which appears to be from the 50s/60s. It has a 280-0-280 60mA winding, a 6.3V 3A winding and a 5V 2A winding.

The primary taps are 240, 220, 200 and 110V. There is a terminal labelled "SCN" which I have taken to be a screen and grounded (ungrounding this makes no difference to my problem).

The PT was sold to me with the 240V tap not working. I live in the UK, so using the 220V tap gives me a slightly higher voltage on all the windings - this is good, actually, because I wanted to rectify the 6.3V tap to provide me with about 6Vdc for two ECC83s.

The 280-0-280 goes through a simple fullwave rectifier into 47uF, then a 22k resistor and 47uF where HT is drawn for the 2nd tube, then a 56k resistor and 47uF where HT is drawn through 100k resistors for the 1st tube.

The 6.3V winding goes into a diode bridge, then a 4700uF resevoir cap, into a 6V regulator, into another 2200uF of capacitance, then to the heaters. 5V tap is unconnected.

When I apply power, the power transformer makes a quiet gurgling sound and gets hot. If I leave it for about 30 seconds, the gurgling continues after power is applied. Voltages appear stable. About 320Vdc HT and 6V out of the regulator.

Is it just a dud? Or am I somehow stressing it? It probably hasn't been used for a long time.

I'm a bit inexperienced with PT issues, I don't know what to look for. Any comments/observations greatly appreciated.




Edit: I just noticed some weird probably-PCB-containing ooze has fallen out of the bottom onto the chassis. I'd say it's a goner. Still interested as to why this might happen.

[Enzo]

It is boiling inside. Seriously.


Did you ever test the transformer by itself? A miswired circuit connected to it, or a bad part in that circuit could certainly load it down and cause trouble. The trick is to apply mains to the primary with the secondaries connected to NOTHING. If it still gets hot, the tranny is dead.

And you can go to RG's Geofex web site and look up his simple transformer tester.

[Jazz P Bass]

240Vac tap

I would be concerned about that 240V tap "not working".
I'd set it up with a Variac, an ammeter & a fuse.

[harry]

It has a 1A fuse on the primaries, which hasn't blown yet.

I'll try it with no connection on the secondaries and see what happens, thanks Enzo.

[tubeswell]

Ditch that PT and get a known good one, before something bad happens. 2CW

[harry]

That's what I was planning on doing, tubeswell.. but I thought I'd follow Enzo's advice. Even though everything looked fine, I cut the wires to the secondaries and resoldered and it works just fine.

All I can think is that one of the wires was shorting to the chassis but I couldn't see it (terminals are a bit close)

Works great now, all voltages just fine, circuit operates, PT doesn't heat up at all.

Thanks all.

[bluesfreak]

the gurgling sound could be sign of internal arcing. Have you checked if you see any spikes in secondary voltages? As an alternative you could measure current on primary side but you need a quick (probably an analog) current meter for that, digital ones are often too slow in reaction so you wont see that...
Bets would be to let the bugger go, it's time for a rewind or the recycling can...

[Enzo]

Harry, I suggested the test, but I also think it is probably history. But one point of the test is to try to make sure there is not some problem with your circuit. After all, we don;t want to burn up the next transformer, and the next, etc.

if the tranny does not gurgle on its own, then something was loading it to death. SO you need to determine if your circuit has a fault. if rewiring made the difference, so be it.

The thing might work now, but if it got hot enough to drip its innards out, then it is compromised. It won;t be up to its full specs any longer.

[Steve Conner]

It is indeed boiling its waxy guts due to an internal short. Throw it away before it burns your house down!