sound but no sound

[samwheat]

i ran into a problem today ..... i heard a pop and now there is no sound coming from the speaker ..... i can hear distortion coming from the OT but nothing from the speaker

the ot is 5k se and i have 58 ohms on the primary and almost nothing on the secondary ...... there is no continuity between the primary and secondary

a long time ago it would cut out sometimes when moving the volume pot, its been fine until now

the amp is a se jcm type with switchable power tubes (ax84 variation)

i tried different cables and plugged another amp into the same cab, so it can't be them

any ideas?

[John Phillips]

If you can hear distortion coming from the OT, you know that the rest of the amp is working OK at least.

What it means is that the problem in the circuit is either in the OT itself, the wiring between it and the speaker jack, or the speaker jack (if using a different cable and cab rules out those). It could be either an open circuit or a short, either will make the OT sound stressed.

Unfortunately you can't tell whether the OT is OK from the DC resistance. Just one shorted winding will cause that sort of fault, and you won't be able to measure the difference, even if you knew what the correct resistance was before.

I would disconnect the OT secondary from the jack, and temporarily hook up the speaker directly to the wires with alligator clips. If you still get no output, you almost certainly have a blown OT.

[samwheat]

i turned it up and hit the ot once with a tape measure (maybe something is loose) ...... one of the other sockets arced (there are 3 of them) and the mains fuse blew

[John Phillips]

That sounds like a blown OT.

It's very worrying that one of the speaker jacks arced... that would indicate very high voltages present on the secondary side - and potential speaker damage.

[samwheat]

John,

It was flyback arcing in one of the other tube sockets.

The mains fuse blew ....... the B+ fuse was ok

Did something else meltdown by chance? PT?

[John Phillips]

Ah, tube sockets not speaker sockets! :)

That's much less serious. Still not good, but actually there is a possible good side to it.

Yes, it's interesting that the mains fuse blew instead of the B+ fuse. Sound from the OT but no sound at the speakers would tend to indicate a fault in the output circuit, but (assuming the values are well chosen) an arc anywhere near the OT would probably blow the B+ fuse first. So it may be a slightly more complicated chain of events...

An arc on the primary side of the OT could definitely be caused by a faulty speaker connection (wiring, or jack) - that's why bad speaker cables kill amps. The effect is exactly like a car ignition circuit - the breaking connection (the "points") on the low voltage side causes the OT (the "coil") to dump its stored magnetic energy, and since the current that can be drawn is very small, the induced voltage on the high voltage side is very large, and it will find the weakest spot to arc (the "spark gap"). Often this is inside the OT windings, in which case it's fried. The good news is that if it arced at a tube socket instead, that was the weakest spot, and there's a good chance that the OT insulation was stronger.

If it blew the mains fuse, it could be because the arc then went to the filament wiring and caused a short there - pin 3 (plate) to pin 2 (filament) is the most common arc on an octal tube socket. What material are the tubes sockets? If they're resin, it's quite likely that the material is permanently carbonised where the arc occured and the socket will need replacing. If they're ceramic, even if there is a flash burn it will just be on the surface and can be cleaned off (this is why ceramic power tube sockets are better, BTW).

But before you run the amp again, you need to find the possible cause of the arc. It does sound like it could be an open speaker connection, to me. The problem is if it's inside the OT (not impossible) - then it's still dead. But its less likely - the most likely is a bad solder joint, faulty jack or impedance selector switch, if it has one.

That's my best guess, anyway.