Trouble: Main transfo short?

[gary f]

I had a problem yesterday. All of a sudden, my tube amp stopped working. The fuse blow.

I changed fuse and the problem came again. I checked with a amp meter and I blow the 10 amp fuse on that one also. I disconnected the heaters, and started again, same thing.

The transfo is a hammond 207HX with B+ and heater voltagte on the same unit. The B+ and heaters are on separate switches. All of the time, the B+ was off.

I'm thinking I may have a short on the primary side of the transfo. How can I test that? Is that a common problem?

Thanks
F

[kevinkr]

Shorted primaries are fairly uncommon in my experience, but not unheard of, shorted high voltage secondaries are not at all uncommon particularly if the transformer is sized a little on the small side for the load current.

Measure the primary winding resistance with a multi-meter. Hopefully the Hammond Spec sheet includes this information.

The other possibility is that there is a filament wiring short. Check this carefully.

For trouble shooting purposes a lamp in series with the power to the amplifier is a good idea. It will prevent damage to the transformer if the transformer is not the culprit, although from your description it most likely is. For cheap and cheerful, not to mention really quick just use a 100W bulb or more and a cheap extension cord, cut just one of the wires in the cord and connect a lamp socket in series.

If the transformer failed it is quite likely that it is slightly to significantly overloaded, and incidentally most shorts in modern power transformers seem to occur in the high voltage secondary which has the highest voltage stress and is also often running hot due to the large non-sinusoidal currents in this winding. Even though the B+ was turned off if the transformer is energized a short in the winding will result in large currents flowing in that winding.

It might not be a bad idea to replace the existing transformer with one having a higher current rating if one of the windings shorted.

[gary f]

Thanks Kevin

A secondary short is also possible, I will check resistance of windings and post, but the Hammond specsheets do not specify the resistance of windings.

I doubt that I have a filament short (defect tubes) because I did test with filaments unconnected and it was the same problem.

I will completely remove the transfo and test alone. Thanks for the Light bulb tip.

It is possible that the transfo was a little bit too small. I had some slightly audible hum from it and it was getting hot. Is that a symptom?

Anyway, thanks for all the information.
I may replace with a bigger unit, if it fits...

F

[Makofoed-6sn7]

Hi,

I'll gues you already done it, but you did pull the tubes and then check? Not so uncommon that the output tubes devolpe a short, or the rectifier tube might do it. I know that EZ35(6X5), EZ90(6X4), EZ80(6V4) and EZ81 are famous for doing that...

Good luck, let's hope that the mains tranny is not shot.


Best regards,

Magnus

[gary f]

I did not try without tubes. But I ran out of fuses.

I will buy some more fuses and try without the tubes. Good idea.

I didn't think it was the tubes since I did not apply any B+ power to the tubes nor any heaters voltages, they were disconnected.
Only when I put power to the main transfo, fuse blow.

But still, I will test evevrything. Thanks

F

[kevinkr]

If the transformer buzzes loudly and gets hot enough that you can't touch it, this is usually a good indication of either a undersized power transformer or a circuit defect. Depending on the insulation class if the core reaches 60C this may be too hot as well. (Class A insulation)

The symptoms you describe are what you would expect in varying degrees from an undersized transformer, but might also be the result of poor quality control. If the transformer is bad, I seriously recommend trying to find one in the Hammond range with the right voltages and a significantly higher current rating.

[ethermion]

Gary,

Hold off on the fuses. Perhaps some more testing without applying power.

I don't know the 207hx, perhaps 270hx? If so, there is a 5V winding that you might not be using. What is it doing? If you turn off B+ and heaters (6.3 only, I am guessing), a shorted 5V winding could easily blow fuses.

I don't know how you wired up the B+ switch. You could have >500v on that switch, which over time will give you trouble.

I am sure your amp is not spec'd for 10A fuses. Do not put in larger fuses. Between 1 and 2 amp slo-blo is probably all you need.

[gary f]

I don't know the 207hx, perhaps 270hx? If so, there is a 5V winding that you might not be using. What is it doing? If you turn off B+ and heaters (6.3 only, I am guessing), a shorted 5V winding could easily blow fuses.

good guess, It is a 270HX. Sorry, i made a typo.
The 5 volt winding is curently not used and wires are insulated.

I am sure your amp is not spec'd for 10A fuses. Do not put in larger fuses. Between 1 and 2 amp slo-blo is probably all you need.

I have 2 amp slo-blo. My voltmeter has a 10 amp fuse for current measurements and I blew that one when trying to measure current draw from the amplifier.

This is the B+ power supply schematic. I have 275-0-275 secondary. The switch seems OK right now. A short in the Hi-voltage secondary or any secondary can cause this problem.

F

[ethermion]

With switch 2 open (B+), ohm out B+ to ground before the diodes. Should be pretty high if not infinite.

The orange drop before the diodes could be shorted. I doubt it, but

With the unit unplugged from the wall, ohm out the primary to ground. Should be pretty high, if not infinite.

Ohm out the heater taps to ground before the switch. Should be high if not infinite.

Double check the 5V are insulated and tied off.

Somewhere, juice is getting to ground and you need to find it.

[DigitalJunkie]

I had a similar fuse-blowing episode with an amp,and it took me a little while to track it down and figure it out.
One of the heater wires had rubbed-through the insulation right where it came out of the transformers end-bell... a few wraps of black-tape fixed it right up.. (I was too lazy to take apart the tranny and put new wires on it.. ) It was tricky to find,cause you couldn't really see it,unless you looked close.