Charred OT Primary at Tube Base

[fiftynine]

I've got an AC30 style amp that was fine, then I lost output and the fuse blew. There's a bit of burning/charring around one of the OT primary wires and one of the EL84 bases. I've tried the same set of tubes in another amp and got no output so I switched it off quick.


I'm not that clued up on this. Is it something gone wrong in the amp that's caused a tube failure or just a tube failure that's charred up my valve base? What can I do to limit the damage if it happens again and is there any sure fire test to see if my OT is still OK?


Cheers.

[Paul1453]

Not a sure fire method to check the OT, but a reasonable starting point.

Your OTs primary windings should have a reasonable amount of resistance.
Depending on the OT, the ones I have are between say 100-800 ohms of resistance.
This is a PP OT, correct?  You should have around 1/2 your resistance from each side to the center tap.
If either of these sides is short, or a lead reads short to the frame, you have a problem.

The secondary side will read much lower on resistance.
But if you have say 400 ohms across your primary inputs and 200 ohms on each side, with no shorts to ground.
I'd try that OT with another pair of known good tubes, after closely checking the socket and associated components for problems.

[sluckey]

Could we see a hi rez pic of the charred socket?

[fiftynine]

Doing it now, Paul.


Hi rez enough, Sluckey?

[Paul1453]

That is charred alright.

Looks like it was arcing to the chassis.
Looks a lot like pattern seen when a cap blows it's top, but there is no cap there.

Now thinking that OT might have been the cause of this problem.   

[silverfox]

I'm not that clued up on this. Is it something gone wrong in the amp that's caused a tube failure or just a tube failure that's charred up my valve base? What can I do to limit the damage if it happens again and is there any sure fire test to see if my OT is still OK?

Look around for some more burned components related to the cathode bias, if this version used that like the schematic on file here. Beyond that I don't know how much experience you have with working around high voltage and that can be hazardous if you're not aware of the dangers. The wire may have melted due to a tube failure and then arc-ed to the chassis. To test the output transformer you should have some sort of current limiting device like a Variac or Light Bulb limiter otherwise you'll most likely have to start de-soldering wires to test the OT.

If you have an ohm meter you could check the tube in question for shorts but since it seemed to have failed in a different amp, most likely it shorted.

Something else that will need to be done is, the area where the power arced to the chassis will have to be cleaned up and checked and perhaps the socket replaced or it could happen again due to the carbon tracks left by the arc and  possibly melted insulation.

Main concern here is, how much experience do you have?

silverfox.

[fiftynine]

Silverfox, I have enough experience to notice one of the spade connectors to the speakers is hanging off (loose as hell) but it's not sunk in to check this sh!t every time. The failure was no load. I'm going to solder those bastards on and put a new tube base in. The amp is fine.

But, I have switched amps on with no load in the past and not had this. It was literally 10 seconds. Has the no load condition created a build up of charge and arced through pin 7 to the chassis? Is that how no load failures work? Is there any circuit tweaks or other safety measures I could integrate to keep damage to a minimum if this happens again?

[sluckey]

But, I have switched amps on with no load in the past and not had this.

This is safe as long as no signal (or squealing noise) is passing through the OT.

Is there any circuit tweaks or other safety measures I could integrate to keep damage to a minimum if this happens again?

Yes. Look at this schematic...

     http://el34world.com/charts/Schematics/files/fender/Fender_65_super_reverb.pdf

Diodes D2 and D3 (on the OT primary) provide some protection against the high voltage 'flyback' signal that can result from having no speaker load connected to the OT. They are 3000v diodes. You can substitute 3 series connected 1N4007 gor each diode.

[fiftynine]

Nice one.