My amp started blowing fuses

[Tdale]

Yesterday, after playing for a while, turning it off, and then turning it on again later, fuses started blowing. No tube damage to be seen.

Any idea where I should start looking to find out what causes this?

Tommy

[Bob-I]

Yesterday, after playing for a while, turning it off, and then turning it on again later, fuses started blowing. No tube damage to be seen.

Any idea where I should start looking to find out what causes this?

Tommy

Start by removing the tubes and testing again. If the fuse doesn't blow, check the voltages for anything way outta wack.

If it still blows, start removing things. Disconnect the relay supply, then the bias supply, then disconnect the main B+ line from the standby switch.

I just had a brain fart and caused my own amp to blow fuses. I connected the relay supply to the 5V AC lines from the PT. Sudden;y the fuse was blowing. Dummy me forgot that I wired for a tube recitifier so I was dumping 420VDC into the relay supply rectifier. I used the removal procedure to find the problem.

[Tdale]

OK..good idea to start with the tubes. I don't think it could be a bad wiring, since the amp worked fine one minute, and then blew the fuse 5 min later. But it could be a tube I guess...or maybe a soldering that broke..

When I start it up without tubes, I don't just leave it in standby mode..I power it up all the way?

Tommy

[Bob-I]

OK..good idea to start with the tubes. I don't think it could be a bad wiring, since the amp worked fine one minute, and then blew the fuse 5 min later. But it could be a tube I guess...or maybe a soldering that broke..

When I start it up without tubes, I don't just leave it in standby mode..I power it up all the way?

Tommy

Power it up as you normally would, power first, then standby 30 seconds later.

[Tdale]

Tried with no tubes..the fuse blew in standby mode... can't find anything wrong..

Tommy

[Tdale]

I brought my amp to a amp-meeting yesterday, that's where it was played. We changed cabinets all the time, so it might have been on full power, with no speakers connected, but not for more than a few secons...or half a minute. But it was never played withouth a speaker connected.

What does it sound like if a OT burns? Does it make a sound?

What will happen if the amp is on without a cabinet connected?

Tommy

[Tdale]

I tested the primaries on the OT, and there is no connection between the two leads going to the plate of each output tubes.. is that a bad sign?

Tommy

[Bob-I]

I tested the primaries on the OT, and there is no connection between the two leads going to the plate of each output tubes.. is that a bad sign?

Tommy

Yes, that's a bad sign. Too bad, I've made mistakes like that and never blown an OT. I hope that's not your problem.

To test disconnect the RED lead from the power supply to the OT primary CT. If the fuse still blows, that's not the issue.

[Tdale]

Will do.

Tommy

[Tdale]

Turns out the OT is ok.

I took out the tubes, disconnected the 5V secondari that feeds the relay power supply. Disconnected the 6.3V. Took out the power switch, and soldered the leads together. Disconnected the high voltage secondaries and the bias tap.

Still blows the fuse.. It must be the power transformer.. I can't understand what else it could be?

Tommy

[smzinno]

Tdale, I had this happen when there was a bad diode in the rectifier bridge on a Carvin X60. Few cents for new diodes and no more blown fuses. Just something else to look at , hope it helps.

[Tdale]

I measured the diodes, and they seem to be fine..

Tommy

[krash]

I have had diodes that looked ok with a meter but once they get a little bit hot (very fast) will allow reverse current and blow the fuses.

in fact I have some like that right now in a Revelation 72 at the house that needs to be fixed. I still don't know exactly what causes them to go bad like this. Maybe a bad lot of diodes?

So you might try removing the diodes and re-test. if it blows fuses with NO diodes or nothing connected but the PT to the switch/fuse, then it's the PT.

[Bob-I]

I measured the diodes, and they seem to be fine..

Tommy

I thought you told me that you disconnected the PT secondaries. That alone would eleminate the diodes from the circuit.

[Tdale]

Yes, I did disconnect the secondaries, but before that, I tested the diodes too.

In fact, today I disconnected everything on the secondary side, the center taps too, so only the primary side was connected. Still blows the fuse, but it took a few seconds before it did. There is a humming sound when I plug in....

Tommy