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Output Transformer Trouble

1/6/2006 7:04 PM
Jack Koochin
Output Transformer Trouble
Hi guys,  
Working on a 50W AB-165 Bassman here, and it appears that the OT has gone open on one side of the primary. This seems to have coincided with the 500mA B+ fuse blowing at one point while running the amp medium-hard, during a hard playing transient.  
Could the B+ fuse blowing cause enough back emf to fry the OT? I've never experienced this before, but is it possible?  
There are no spike-protection diodes on the primary of the OT, by the way. Do you think the diodes would've saved the OT if they were there?  
On the other hand... is it possible the OT was already on its way out? A faulty OT causing the fuse to blow? Everything in the amp measured fine current-wise, bias etc, so there is no real reason for the 500mA fuse to blow, even if the amp was pushed all out.  
1/6/2006 9:58 PM

Not likely in my book that the fuse blowing caused the OT to fry. There is not enough energy in one spike to cause enough current to flow to melt the wire. The first thing I would suspect is a carbonized path on one tube socket from pin 3 to pin 2. That would explain the fuse blowing and the open OT. Check the OT primary (disconnect the red wire) and tube socket for shorts with an ohm meter.  
High voltage spikes will usually cause arching inside the OT or at the tube socket. Eventually a carbonized path is created and, when enough current flows, poof, no sound. The diodes will prevent this but they can fail and cause just as much damage.
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1/6/2006 11:43 PM

Blowing fuse kill the tranny? Not likely. Tranny death throes blowing fuse? Probably.  
Asking if diodes would have saved the tranny requires us to know how it failed in the first place. If a plate lead shorted to ground and burnt it out, then no. If it was run without load and the windings arced, then yes.  
Last time a light bulb burnt out in your house, what caused it? It was fine until it blew. Everything works until it doesn't. The tranny picked that time to fail, maybe from external reasons like arcing sockets, or maybe from something internal we will never know about, but anything capable of burning out a transformer is certainly able to blow a fuse
1/7/2006 10:49 AM
Jack Koochin

Thanks Loud, Enzo,  
Yeah, this was a bit of a mystery... all the time I've seen this amp in use/had it in operation, it always had a correct load on it, no plate leads shorted to ground, no shorted output tubes. Can't measure anything conductive on the respective socket (if it is measurable) carbon-trace-wise either.  
The only clue is that this amp has recently blown a couple of 500mA B+ fuses, and that was during loud transients while playing thru a good speaker/resistive load. This shouldn't have happened if the OT and sockets were healthy.  
Thanks again guys,  
1/7/2006 1:26 PM

Things can fail for reasons you never become aware of.  
Unless your meter tests resistance with 500 volts, it won't necessarily detect arc paths. Your B+ will leap places that 1 volt ohm meter will never cross.  
There can be a crack in the winding enamel somewhere that is not shorted but only arcs across on loud peaks. The only way to descover that is during operation.  
Are any of the power tubes microphonic? If so they are much more likely to have intermittant internal shortts than tight tubes. Again, ohm meters and tube testers do not treat tubes like operating them in an amp. A momentary short can occur in a tube that you could not detect with the tube ouot of the amp.
1/8/2006 12:53 PM
Jack Koochin

Yep, seems like this one just packed it in for "no apparent reason".....  
The output tubes - never checked to see if they were microphonic.  
Guess I'll just replace the OT and output tube sockets to be sure, and carry on.  
Thanks again.
1/8/2006 1:23 PM
Jack Koochin
Anyone Suggest a Replacement OT
Any suggestions for a replacement OT for this '67 Bassman ? Will be used for guitar, not bass, in case you were wondering. Cost is an issue... just need a decent sounding unit for a reasonable price. So far I've looked at AES, Mojo, and Weber.  
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