Old amp blows fuses with no tubes in: fault of rusty PT?

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1/30/2006 1:16 AM

Carlo Pipitone

Old amp blows fuses with no tubes in: fault of rusty PT? I bought this 1482 Silvertone amp off ebay. The seller admitted that it blew fuses.   Now I have done a cap job and replaced a few drifted resistors. All the negative lugs of the e-lytic caps are connected to ground, so there should not be a miswiring problem at this end.     PROBLEM:   about two seconds after startup the 1A slo-blo fuse blows. This happens with NO tube in the sockets.   New fuse, same behaviour: about two seconds of life.   The power transformer is rusty outside, both on the black casing and on the laminations (here less than on the case). The screw heads are totally rusty.     How do I check if rust is the culprit?   What else should I check?     TIA,   Carlo

 

1/30/2006 2:42 AM

Peter

I would check the .047/.05 600v cap to ground   after the fuse then check the 6.3 pilot/ indicator light socket .   Then to remove all doubt disconnect all the secondary windings from the transformer and see if the fuse blows .   Usually rust is mainly surface rust ....unless the amp has been "dunked".   I would think a major internal short in the transformer would blow the fuse in less than 2 seconds..my 2 cents worth ...see what everyone else thinks

 

1/30/2006 3:00 AM
Carlo Pipitone	Thank you Peter.     I have replaced all caps in the amp (except the small ceramic discs and those in the tremolo section) with new Mallorys. So the .047 cap after the fuse should be ok.     I'll check all the rest and post my findings.     Carlo

 

1/30/2006 4:11 PM

Carlo Pipitone

Fuse blows with secondaries disconnected! I have disconnected all the secondaries from the PT, and the fuse still blows in about two seconds.     I have taken VAC readings from the free ends of all the secondaries (power switch ON):   - heater wires (including the ground wire): 41 VAC;   - B+ wires (including the ground wire): 51 VAC.     Also, I have to wiggle a lot with the meter probes if I want to have continuity between the B+ wires.         Carlo

 

1/30/2006 6:06 AM
Carlo Pipitone	Re: Old amp blows fuses with no tubes in: fault of rusty PT? BTW:   no smoke or evident smell when the fuse blows. The fuse itself just gets warm.     Carlo

 

1/30/2006 2:49 PM
R.G.	Re: Old amp blows fuses with no tubes in: fault of rusty .. tube amp debugging page...?

 

1/30/2006 4:18 PM
Carlo Pipitone	For some reason I often forget to check the TADP, although I saved it on my pc a long time ago...     Anyway, since the fuse blows with the secondaries disconnected and with no tubes, I suppose I have to check the wiring between the mains and the PT, right?   This puzzles me, because I can't see anything wrong in that district...     Carlo

 

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