Fender Twin possible output transformer fault?

[Mick Bailey]

This fender twin blew the 2A mains fuse. Customer replaced it with a 3A (only one available under gig conditions), which also blew.

Upon inspection, the internal HT fuse had also blown. The speaker lead had become detached at one of the speaker terminals and this may have caused the original fault. After checking through the amp for cap/rectifier shorts it was powered up without tubes and all voltages read fine. Tube sockets are clean and show no signs of tracking under magnification. Socket contacts are firm on the tube pins. Tubes and screen resistors are 4 months old. Solder joints are good.

Output transformer reads OK under static conditions.

I reinstalled the output tubes and bought the supply voltage up, continually monitoring the output tube voltages. All is fine below a b+ of 300v. When the B+ reaches 330v or so, the output tube connected directly to the blue secondary lead arcs violently inside the envelope and the HT fuse blows. I've tried this with another tube (used but good) and it does the same.

My thought on this is that one half of the OPT may be going OC under load and generating a flyback voltage of some KV.

I'm somewhat resistant to ordering a new OPT as they seem to be very robust in Fenders and failures are fairly unusual. Does anyone have any thoughts on this problem?

[52 Bill]

What happens to the other output tube that is paralleled to the one that internally arcs?

Which model Twin is this?

[Jazz P Bass]

Do you have any other transformer that you can, briefly, throw in there.
It sure sounds like a toasted OT to me.

[Prairie Dawg]

Get your hands on an insulation tester like an IRT2 that the refrigeration boys use and check the windings for high resistance shorts.

[Mick Bailey]

What happens to the other output tube that is paralleled to the one that internally arcs?

Which model Twin is this?

The parallel tube momentarily flashes bright purple internally - I guess the closest tube to the OPT takes the hit and opens up a low impedance path which gives the parallel tube some protection.

The amp is a 1990 model.

[Enzo]

So is it a "Twin Amp" ? (Pro tube?)

[Mick Bailey]

Somewhere I have a long-forgotten hand cranked 500v Megger insulation tester - I'd have to say I'd ruled out shorts, but thinking it through anything that intermittently causes the field to collapse will cause serious high voltage spikes. I've only used the Megger to test for inter-winding shorts or a short to core, though. Don't know how this would test for an intermittent OC under stress, or high resistance shorted turns.


I've got a few anonymous OPTs that may be near enough to substitute for a no-load test.

[Mick Bailey]

Sorry,

Model is a 65 Twin Reverb.

[MWJB]

"Output transformer reads OK under static conditions." can you elaborate on this please?

[Prairie Dawg]

That oughtta work. The issue here is to make absolutely sure that the transformer is/is not functional. Your megger will detect any high resistance short in the windings to frame or interwinding short.

[Mick Bailey]

"Output transformer reads OK under static conditions." can you elaborate on this please?

I'm referring to DC resistance checks across the windings and measurements from windings to ground with the OPT disconnected - just the basic stuff to eliminate any obvious shorts or OCs.

I've located my Megger and will do some additional checks later this afternoon.

[MWJB]

You should have read around 70ohms from each primary end to the CT?

[Mick Bailey]

Primaries read 34 & 36 ohms respectively.

The insulation test didn't show anything up - tested primary to secondary and primary to ground & secondary to ground. All read in excess of 50M at 500v test. Guess the next stage is to substitute and bring the amp back up.

[MWJB]

DCR of the primary seems a little low, but the evenness from each end to the CT might be reassuring. Do you have a low voltage AC signal source? If so, power down, unplug from the wall, remove the 6L6s and apply (ideally) 0.5 to 1VAC to the OT secondary, whilst measuring the AC developed accross the primary and from each end of the primary to the CT.

[Mick Bailey]

The amp words perfectly up to a B+ of 330v with the power tubes fitted, and 460v with them removed - not sure what a low-voltage test would show unless the transformer is loaded - can you expand on this?

It did give me the idea to set up a test rig to put a variable AC source on the secondary and monitor the primary through a load resistor/spark gap/HV probe and see at what point the transformer fails.