Heater Winding Short - Bad Design or Coincidence?

[abbottamps]

Hey guys,

I've got a strange issue in an amp I built. The amp in question has two 6L6's, a 12AT7, and two 12AX7's... the thing that's sort of unorthodox is how I wired up two light bulbs that power off the heater lines. I would call it T-tapped (that's the term they use in the fire alarm field when you wire in a smoke detector without sending return lines back from it), but regardless, I tapped into the heater wires and sent an independent run to another light bulb that I'm using to light behind a plexiglass faceplate on my amp.

The amp was used a handful of times, probably a half dozen band practices while my friend was borrowing it. Before that, the transformers have been used on different variations of the circuit as experiments. When this happened, the fuse blew, and the power transformer was very hot. The filament wires with the transformer out of the amp have a dead short to one another.

It's a Mercury Magnetics power transformer that has a 6.3V line @ 6A, which I assumed was enough for what I'm doing. I'm not sure how much current those bulbs draw though... Could the second light bulb draw that much extra current to ruin the transformer? Is it because of how I wired it? (not sure if it matters, but the center tap for the heater winding is grounded... no 100ohm resistors)

If anyone has any insight, I'd greatly appreciate it. I'm going to send the transformer out to be fixed, don't want to fry the next one now that I have the amp where I want it in every other aspect.

Here's a picture to show exactly how I have it wired up.

Thanks!

[abbottamps]

I also want to add that I ran through the heater wiring, everything is good, no shorts, nice and neat solders, nothing that could even be an intermittent short.

[Tom Phillips]

I'd say that the transformer just went bad. You didn't describe the previous circuits it was used for so I suppose it's possible that it had been damaged before you installed it in the existing amp. Seems like you are only using about half the capacity of the 6.3V / 6A winding in the existing application. I don't suspect that the light bulbs would cause an overload. If you are using common #47 bulbs they only draw 150mA each.

[Steve Conner]

The filament winding is wound with very thick wire, it's normal for it to measure a dead short. Run the transformer with all the secondaries disconnected from the circuit, does the fuse still blow?

[Tom Phillips]

The filament winding is wound with very thick wire, it's normal for it to measure a dead short. Run the transformer with all the secondaries disconnected from the circuit, does the fuse still blow?

+1
Re-reading the original post I think that further investigation is in order to find the root cause of the fuse blowing.
The most common problem is usually found to be a shorted component in the HV supply or a bad power tube.

[Enzo]

My first suspect would be a crappy bulb socket. I bet the transformer is OK.

[R.G.]

My first suspect would be a crappy bulb socket. I bet the transformer is OK.

+1!

Fortunately, there is a simple test for whether the transformer has an internal short. The neon-bulb shorted-transformer tester will tell you directly.

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

There are some simpler ways, too.
(1) hook up a light bulb tester to the amp and open **all** of the transformer secondary windings. Apply power to the primary. If there's an internal short that would heat up the trannie, you'll get a bright bulb.
(2) If you get a dim bulb, measure (... carefully...) the primary and secondary voltages. If all windings are in the proper ratios, there is not an internal short.

When you repair it, I'd advise putting a fuse in the heater winding, one in each side if it's center tapped. Make the fuses the full rated current of the winding, slow blow, and then an overload big enough to hurt the winding will break the fuses.

This is the first good illustration I've come across since Gary Moore refused to believe an overload on a heater winding would not necessarily pop the primary fuse, and could not be convinced otherwise.

[soundguruman]

You probably have an unrecognized short somewhere,
Like a tube with a plate to heater short.
Very rare, but it happens.