Fuse Pops - Transformer Dead

[Diablo]

My year old 5F6A clone blew the fuse yesterday while playing, so I started troubleshooting. Stuck in a new 3A fuse and it blew as soon as I turned the power on - before I powered up the B+ (standby). That limits the problem to either the 6.3V heaters, the 5V heater for the tube rectifier, or the PT. I desoldered the PT from the heater wiring, and pulled all the tubes. I tested the heater wiring and found no shorts to ground. I checked the resistance of the PT windings and from each winding to ground and all checked out OK according to this guide:

Fuse Blows
Power Transformer

However, with no wires connected on the secondary side of the transformer, I plugged it into my light bulb limiter. The 75W bulb glows brightly. So my PT is toast. What I don't understand is why did the static measurements for the PT all test OK? Each winding was less than 1 ohm (including primary and secondary), and no windings were shorted to ground (greater than 1M ohm)? The transformer doesn't smell burnt, and there are no obvious signs of distress.

[Jazz P Bass]

75 Watts

If the power tranny is pulling that kind of current , unloaded, it has an internal short.
I would think that the primary should read a little more than 1 ohm.
I just checked a '62 Bassman. The primary reads 3.8 ohms.

[Diablo]

OK, a little more careful measurements of the PT. The primary winding is 1.7 ohms. The 6.3V heater and 5V rectifier winding are .4 ohm. The secondary HT winding red/red is 27.1 ohm. The HT windings appear to have an internal short though. One side to center tap is 28.9 ohms, the other side to center tap is 19.9 ohms. I feel better now...although my amp isn't....

[Diablo]

And now the rest of the story. The manufacturer sent me a new PT this morning. I wired it into the amp, added a 500 mA slow-blow fuse on the PT secondary center tap to ground, and added a 4 amp fuse on the filament heater winding. Fired it up with the light bulb limiter and no shorts. Then fired it up without the limiter and checked that plate voltage was still the same as before (463 volts) and the bias was OK (34 mA) . Played it for a couple of hours...good as new.

[Diablo]

Now I have the same model of PT in a 5E8A, and it too has died after about a year of use...hmm. This one popped the fuse when I flipped it off standby. Installed a new fuse and it popped as soon as I turned on the main switch. Back to the light bulb limiter, and the transformer makes the bulb glow brightly with nothing connected on the secondary side of the PT. So, I have another shorted transformer. I'll see if the manufacurer takes care of me again.

[Chuck H]

Who is the manufacturer? Inquiring minds want to know. You can just rhyme it or give obvious clues if you don't want to kick anyone under the buss.

Chuck

[Diablo]

Chuck, I sent you a PM. I don't want the manufacturer to get heaved under the bus.

[Bruce / Mission Amps]

Chuck, I sent you a PM. I don't want the manufacturer to get heaved under the bus.

Why not? I've only had 7 bad power transformers in over 1,300 units.
Some of those were actually pilot error... not the tranny's fault.
If it was a repeat of one of mine, I'd expect to hear about it.

[Diablo]

I did let the manufacturer know about it, and they are sending out a replacement at a discount. I unwound the first one that shorted to see if I could find the meltdown. All I found was one spot on one of the windings that the insulation appeared fused. Perhaps the coating on the magnet wire isn't high enough quality or consistent? If I fry one more transformer, then I'll likely switch to a different manufacturer.

[Diablo]

I received the new transformer today and just finished hooking it up in the 5E8A, testing with light bulb limiter without tubes and then with, checked plate voltage and tube bias, and it's rockin' again.