Shorted Power Transformer Primary - Help!

[SkaakS]

Hey All,

I have a tube guitar amp that the power transformer just broke on. The primary winding has a short (I determined this by using the 'light bulb' in series trick. When putting a 60W light bulb in series w/ the power transformer the light bulb lights up since there is no load across the primary windings). I'm trying to determine the cause of the problem, and I thought some of you might have some ideas. Here's what I know so far:

1) The transformer has one primary winding (115V) and three secondary windings (320-80-0-320, 17-0-17, and 6.3V).
2) There was a 3.15A fuse on the primary winding and a 1A fuse on the HT when it blew.
3) The amp had previously had a screen resistor blown out on one of the output tubes. I replaced the broken resistor with one of the same value/wattage and checked all of the leads on the PCB near the blowout to make sure no traces were damaged . It was my assumption that this blow out was caused by a faulty power tube, so I replace all of the tubes as well.
4) After replacing the broken screen resistor and the tubes the amp worked great for about 6 hrs total playing time.
5) The primary winding broke while in standby mode, after playing it for about an hour at moderate volumes. (Actually it was the primary fuse which blew, leading me to discover that the primary winding had a short).
6) The amp is configured in such a was that when in standby, there is nothing connected between the high voltage secondary windings (no caps, resistors, fuses, etc). Therefore, the only things drawing current while the amp was in standby were the tube heaters, the channel switching power supply, and the output tube bias power supply.
7) I've pulled the PT out of the amp and checked most of the connections in the output section (primarily the tube heaters, bias circuit, etc).

I can replace the PT, no problem, I'm just trying to figure out how/why the PT might have broken in the first place. I would hate to spend the money to buy a new transformer, only to have it break too.

What confounds me is that it blew while in standby, not during the hour long high volume session before it. Even so, shouldn't the fuse on the primary side have protected the wire from conducting too much current? Could something else have caused the problem (Power surge, overheating, bad tube, etc)?

Thanks everyone!

[Enzo]

The fuses in the amps are not there to protect the transformer, really. They are there to prevent failures in the amp from starting your house on fire.

Did you disconnect ALL the SECONDARY wires before doing the bulb test? Any short across a secondary will make your bulb light or fuses blow. Have you applied power to the transformer since removing it from the amp?

[SkaakS]

The fuses in the amps are not there to protect the transformer, really. They are there to prevent failures in the amp from starting your house on fire.

Did you disconnect ALL the SECONDARY wires before doing the bulb test? Any short across a secondary will make your bulb light or fuses blow. Have you applied power to the transformer since removing it from the amp?

There wasn't anything connected to any of the secondary windings when I did the lightbulb test. I haven't tested to see if any of the other winding are shorted, but I definitely know that the primary one is.

[Enzo]

OK< if the secondaries were all floating,then the thing is shot.

Not to belabor it, but any winding that is shorted will make your bulb come on. It is not testing only the primary.

[SkaakS]

Interesting... Is there any way to determine exactly which one is shot? It might help narrow down where (if anywhere) the problem occured.

[guitician]

Not if the primary is shorted. You can take ohms readings on the secondary circuits to see if anything is shorted.

[SkaakS]

Is resistance a valid test? I was under the impression that winding could give an acceptable resistance reading but still be shorted. in any case, here are the resistance readings of the various windings:

Primary: 2.3 ohms
Secondary (320V): 14 ohms
Secondary (17V): 5.3 ohms
Secondary (6.3V): 1.3 ohms

One thing I did notice that for all of the secondary windings, the resistance reading came up immediately on my meter and didn't change whereas the primary reading started at around 5 ohms then slowly dropped to 2.3 ohms.

[guitician]

I meant the circuit that the secoundary supplies current to. Anyhow, that 14 ohms for a 320V winding seems low, and may be the shorted circuit. Check the rectifiers and capacitors for that circuit.

[Steve Conner]

As I see it, there are two possibilities. Either the PT was bad from the factory and limped along for a while, or it died from overheating caused by overloading.

Did you bias the amp yourself, or swap in a different power tube type to the type it came with? Could it have been running a higher idle current than recommended?

Is this a factory-built amp or some home-made thing? If the latter, are you sure you sized the PT correctly? If you salvaged the PT from something, did you even know its current ratings?

[SkaakS]

I meant the circuit that the secoundary supplies current to. Anyhow, that 14 ohms for a 320V winding seems low, and may be the shorted circuit. Check the rectifiers and capacitors for that circuit.

I've checked the caps and diodes for the HV circuit, and there are no shorts. I need to pull the caps out to test if they're still good, but they definitely are not shorting.

[SkaakS]

As I see it, there are two possibilities. Either the PT was bad from the factory and limped along for a while, or it died from overheating caused by overloading.

Did you bias the amp yourself, or swap in a different power tube type to the type it came with? Could it have been running a higher idle current than recommended?

Is this a factory-built amp or some home-made thing? If the latter, are you sure you sized the PT correctly? If you salvaged the PT from something, did you even know its current ratings?

The amp is a 2005 Framus and it worked great up until the screen resistor blowout. I did replace the tubes after the resistor blew, but I replaced them with the same type of tubes (Ruby STR EL34s). I biased it to exactly 32mA, which is what they recommmend. It still baffles me that the PT blew when in standby. I mean, in this amp the output tubes are completely un-powered (besides the heaters) when in standby. Wouldn't that mean that the biasing has nothing to do with it?

[52 Bill]

You've read each winding with your ohmmeter, but have you checked for either primary to secondary shorts or secondary to secondary shorts or any winding to frame shorts?

Your transformer may have died because of some earlier problem. Maybe it had been stressed by heat or vibration that started a breakdown of the winding insulation. Maybe there was a problem with the winding machine the day it was made, and the winding tension was too high. It could be that this problem could have just continued to get worse until it finally gave up.

[Enzo]

And an ohm meter will spot gross shorts, but if a few turns are shorted together, it won;t affect the resistance reading much, but it will draw a ton of current when powered.

[SkaakS]

I just checked, and there are no shorts between any windings or to the frame. I'm thinking it must have just been the transformer's time to go or possibly the screen resistor blowout may have weakend it to the point where it was on the verge of breaking.

You've read each winding with your ohmmeter, but have you checked for either primary to secondary shorts or secondary to secondary shorts or any winding to frame shorts?

Your transformer may have died because of some earlier problem. Maybe it had been stressed by heat or vibration that started a breakdown of the winding insulation. Maybe there was a problem with the winding machine the day it was made, and the winding tension was too high. It could be that this problem could have just continued to get worse until it finally gave up.

[SkaakS]

And an ohm meter will spot gross shorts, but if a few turns are shorted together, it won;t affect the resistance reading much, but it will draw a ton of current when powered.

Good to know, thanks!