Ampeg SB-12 blowing fuses.. I suspect the OT

I came in possession of a broken Ampeg SB-12. Problem was it was blowing fuses.

I replaced all the filter caps and powered it up with a current limiter. The bulb lit up once I took the amp off of standby.

I removed all tubes, powered up again and the bulb did not light when taken off standby.

Put in a known good rectifier tube. Powered up, took it off of standby and again, the bulb lit.

Tried to isolate the problem further by removing the leads from the filter caps and disconnected the bias supply circuit and the bulb again lit when taken off of standby.

Disconnected the B+ feed to the CT on the primary side of the OT and the bulb did NOT light up when taken off of standby.

I measure approx 30 ohms from the CT to the chassis, 170 ohms from pin 3 of the first 6L6 and infinite resistance from pin 3 of the 2nd 6L6.

I suspect the OT is done. Is there anything I am potentially missing or can check?

Here's the schematic:

http://www.prowessamplifiers.com/schematics/ampeg/SB12_Portaflex.html

Thanks!

Sounds like it has a short AND an open. Disconnect and check DCR again. 

Apologies, one of my measurements above was incorrect.

Measured with the OT primary leads disconnected.

Center tap measures  30 ohms to chassis and the other primary leads measure 180 and 220 ohms to chassis  respectively.

With the tubes pulled and the OT pri CT disconnected,  did you check your bias voltage, heaters (5v & 6.3) and the B+ at the rectifier cathode? 
I'd just scrutinize the heck out of it.  transformer failure is my very last last suspicion, unless you know a user abused it by not having a load on the secondary or something.

andyfromdenver said:

With the tubes pulled and the OT pri CT disconnected,  did you check your bias voltage, heaters (5v & 6.3) and the B+ at the rectifier cathode? 
I'd just scrutinize the heck out of it.  transformer failure is my very last last suspicion, unless you know a user abused it by not having a load on the secondary or something.

Click to expand...

I did.. With the OT primaries disconnected I get 6.4v and 5.1v on the heaters. I get approx 480v at the rectifier cathode and approx -52v on the bias supply. Yes, these readings are a little high but not unexpected since there's no tubes installed with the exception of the rectifier.

I'll be honest, I never worked on an amplifier with a blown OT. This would be a first! The amp doesn't look abused but I really don't know the history or if it was ever operated without a load. It's certainly a possibility!

Thanks for the help!

fallout said:

..... The amp doesn't look abused but I really don't know the history or if it was ever operated without a load. It's certainly a possibility!..

Click to expand...

Could you disconnect secondaries from the circuit too and test resistance between primary CT and secondaries?  Most probably there is a short between a primary and secondary layer because of high voltage (arcing if the transformer was left unloaded with signal applied), overheating and/or vibrations.

Found this
http://www.geofex.com/FX_images/xform_test.gif

Thanks for your help guys! As suspected,  she's dead.

I removed the transformer and tested resistance between the primary CT  and the secondaries. I measured  very little to no resistance.

Upon further inspection, It looks like one of the contacts in the 4 pin speaker connector was not making contact with one of the pins in the speaker jack. I'm sure the previous owner attempted to play the amp with the speaker unknowingly disconnected, taking out the OT.

Thanks again!

This will give you some idea of the DC resistances expected on an output transformer

https://www.diyaudio.com/forums/tubes-valves/119293-output-transformer-dc-resistance.html

I have measured the secondary of a Champ transformer at less than an ohm.

Gus said:

Found this
http://www.geofex.com/FX_images/xform_test.gif

Click to expand...

Gus, thanks for that link! Just for fun and since I have a few NE-2's laying around, I tried the tester mentioned above. It's simple and confirmed my suspicions. Tried it on a 'good' transformer and the NE-2 flashed and then the transformer from the SB-12 and no flash..

Thanks!