1978 Bassman 70 Output transformer issue

[twilightofthedogs]

Howdy, just picked up a 1978 Bassman 70 that is blowing fuses without power tubes in. I suspect a bad OT as the voltages seem correct and nothing blows when the amp is turned on, only when the standby is turned on. Still have a little more trouble shooting to do before I pull the trigger on a transformer, BUT , if I buy a new OT, my inclination is to go for something that is not ultralinear, as they are fairly expensive, and for the most part, undesirable in these amps. That makes me think I just go ahead and try to mod the whole thing to be more like the popular AA864. Any thoughts? Any particular issues with going from UL to non-UL? Thanks in advance!

[pontiacpete]

I suppose you checked the rectifier and the caps. Are they good?

[twilightofthedogs]

Thanks for the reply. I observed that the filters were new spragues, so I am assuming that they are at least modern, not the originals. I haven't verified that each one is functional. I haven't checked any coupling caps or anything. The rectifier seems to be be putting 450 steady DC volts on the standby switch before I engage it, and I checked the rectifier diodes with a diode checker and they seemed to check out. I guess the next step is to start disconnecting the OT leads off and seeing if that stops it.

[Enzo]

Never assume, always KNOW. You think the output transformer might be bad? Then disconnect its center tap wire from the B+. Now fire up. Does it still blow fuses? If it does, then the transformer is not at fault. If it no longer blows fuses that way, we STILL don't know it is the transformer. SO we go further. That transformer has two plate leads and two screen taps. If any one of those is either grounded or arcing to ground, fuses will blow. First, with center tap still disconnected, measure resistance from the center tap wire to ground. There should NOT be continuity there. We want to check for shorts from that winding to chassis, to the frame of the transformer, and to the secondary winding. It should be shorted to none of those. Of course an arc only forms with power on, and it is not something a meter can detect. But if you detect no shorts but the disconnected center tap stopped fuse blows, then reconnect the center tap and disconnect the plate and screen wires. Fire up. if it still blows fuses that way, the transformer is bad. If it no longer blows fuses, then one of your sockets was arcing.

[twilightofthedogs]

Thanks Enzo, I will give that a try