Power Surge Blows PT

[SilverFox]

I've completed an amp build and it's worked fine up to the other day. I turned the amp on and upon hearing the buzz of an unconnected input cable, shut it off, quickly plugged the cable in and then turned it right back on. It blew the fuse. I should add the amp is plugged into a Variac that was cranked up to the max setting to get the line voltage up to 120 VAC. I have low line voltage here. Be advised the meter reading on the Variac output voltage was 120VAC. My best recollection is I used the power button to cycle the power and did not put the amp in stand by first.

I finally got around to diagnosing the problem and both the filament and HV secondaries are dead. Starting from zero and cranking the voltage up with a Variac does not raise the output voltage of either secondary and at about 30% of the dial on the Variac, the mains fuse on the primary blows again.

I've checked the tubes to be sure there are no shorts and the power supply caps and diodes test out okay- No shorts. Filament run checks out fine with filament secondary disconnected.

My question is: Is it unusual for the PT to just suddenly blow or were the circumstances associated with the Variac and quick power cycling related to the failure? The power tubes would have been close to or right at the conduction stage.

What should I check to be sure there isn't a catastrophic failure laying in wait for the replacement PT?

Regards,

silverfox.

[trobbins]

Have you checked PT winding resistances? If all are near normal then you need to check for a shorted turn on primary. All tests with secondaries disconnected. Increase primary voltage and monitor primary current and secondary voltages.

A primary shorted turn may be a child mortality fault due to manufacture, where you have provided just enough stress to cause the inherent fault to fail. Or your amp design and hot switching could have caused stress. There could be many plausible reasons.

[larsvictor]

Starting from zero and cranking the voltage up with a Variac does not raise the output voltage of either secondary and at about 30% of the dial on the Variac, the mains fuse on the primary blows again.

Does this happen with all secondaries disconnected ?

[Stevem]

If as posted you disconnect the secondary wires and power it up with a fuse of half the normal needed amperage and it blows, then yes your PT is shorted!

If the PT current rating was good for the build and had not been running hot by means of having a small short in it that got bigger , or shorted filters outside, it then it was not your fault and using or not using the standby switch would have made zero difference !

[R.G.]

I agree, there is a fault in the primary circuit.

I say "primary circuit" because an unsuspected fault in the wiring leading to the primary of the transformer could cause the same thing. For instance, an internal short in the AC power switch ( been there ) could cause this issue. So could a shorted "death cap" if you used a death cap. So could a stray wire strand that escaped from the bundle when soldering.

If it were mine, I would carefully remove the primary wires of the transformer from the points where they receive the AC power. Then try powering just the AC wires. They might blow the fuse.

Or maybe not. To clinch that the PT is at fault, go take at the inductive transformer tester circuit at geofex.com:

http://www.geofex.com/FX_images/xform_test.gif

or my article for Premier Guitar on the same subject:

http://www.premierguitar.com/articles/t ... r-tester-1

[R.G.]

The site is kicking me off and I have to re-log in every few seconds, so this ought to be appended to the note above.

If the transformer fails the inductive kickback test, that's definitive that there is a solid internal short. If the wiring fails the power up test, there's a short in the wiring.

The difference between the two may save you an argument with the transformer supplier and some time and shipping money.

[Stevem]

I can't stay logged in either !

[SilverFox]

I'll make some more tests this weekend. I will say I was monitoring the HV secondary and it began to come up with the Variac then the fuse blew out at 231 volts DC. After that it wouldn't even do that any longer. Nothing. The filament was flat dead.

As I recall I measured the filament taps and they were 51 ohms. The HV was switched off when I made the tests. The HV switch is prior to the rectifier.


As for the log on issues: I was on a school system today and tried to get to TAG. They use some sort of Google server or service, and when I connected to TAG I got the Red screen indicating TAG was an untrusted site. Microsoft Explorer gave the same sort of message. The date of the last determination was, 8-10-16. I'm at home and use FireFox and Linux so I haven't had any problems here.

Thanks for the assistance. I'll check back with results later. And I'll check RG's site for the inductance tester.

silverfox.

[tubeswell]

Also another thing to check on the primary side, is if you have a Primary-side Mains voltage indicator lamp which is shorted. That could be killing your fuse.

[Stevem]

Another help I find when either when doing builds or repairs is I keep a note book of what a amps PT and OT tested at resistance wise when all was well and or new, this can save a lot of head scratching down the road if a issue crops up!