Did I just blow my OT?

[rfgordon]

OK, so nobody's home but me, so I crank up my CrocoDelite--really get those 6V6s banging. Then the music stops, there's a dark hum, the rectifier tube redplates and the fuse blows. So I replace the recto tube. Same thing. I pull the power tubes, and the noise is still there, even without power tubes.

So, did I blow the OT?

[dartanion]

Maybe a bad rectifier tube. If it's redplating, theres probably a short there.

Sovtek (Russian) recto?

[rfgordon]

Like I said, I replaced the recto tube.

So that ain't it.

The dark 120hz noise is present even without power tubes.

[ampdoc1]

It doesn't sound like bad OT symptoms, but unplug the amp, let the caps discharge, and make a resistance check. You should have = resistance from each of the plate wires to the center tap. On the output side, unhook the ground wire, and see if you still have a connection to ground from the speaker jack. You should get very low resistance from the ground wire to each of the taps, but infinite resistance to the amp chassis.

ampdoc

[Johnhenry]

with the rectifier tube pulled, what kind of AC voltage are you getting on pins, 4 & 6, also pins 8 & 2, that should tell you if you blew the power tranny !
Boobird

[rfgordon]

It's official: OT is dead.

I disconnected all the OT connections and soldered in a known good OT (a donut I had Trafomatic make me a couple years ago). Now the amp works perfectly.

So, do y'all think my beautiful old Mullard 5AR4 is history after this mess?

[Johnhenry]

your lucky if that's all it got, the redplating could not have been to good for your power tranny, sounds like with the OT blown you were pullin a bunch of amps, good to know you found your problem.
Johnhenry

[rfgordon]

And here, as they say, is the REST of the story:

I took the end bells off, and lo and behold, the culprit was obvious!

At the factory one of the primary wires was repaired/reconnected/extended (who knows why?) and a piece of shrink tube was put over the solder joint. All this was INSIDE the end bell. Ever hear of hivolt shrink? Me neither. So there it was, staring right at me--a tiny hole in the shrink with blackened copper wire behind it. One leg of the primary was shorting thru the shrink to the end bell.

So that explains why the ohmeter didn't indicate a short--only the hi volts of operating would make it happen.

Weird, huh?

[skyboltone]

And here, as they say, is the REST of the story:

I took the end bells off, and lo and behold, the culprit was obvious!

At the factory one of the primary wires was repaired/reconnected/extended (who knows why?) and a piece of shrink tube was put over the solder joint. All this was INSIDE the end bell. Ever hear of hivolt shrink? Me neither. So there it was, staring right at me--a tiny hole in the shrink with blackened copper wire behind it. One leg of the primary was shorting thru the shrink to the end bell.

So that explains why the ohmeter didn't indicate a short--only the hi volts of operating would make it happen.

Weird, huh?

Actually I've worked plenty with 15KV and 25KV shrink tube. Just not in little sizes. I think it was a sharp point on the solder joint and pressure from the end bell. I'd fix it and go on with life. Just make sure the joint is smooth and you've got plenty of layers. Cambric or even old fashioned friction tape makes a good last layer after the insulating layer.