reason for overheated shielding winding ?

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nashkato

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forgot to turn off my pp el34 monos over night .
next morning the mains fuse of my room had tripped .
i had a short between primary and shielding coil of the mains transformer of one of the mono blocks .
obviously the transformer got very hot .
on that one i changed the selenium rectifiers to a resistor to drop the HT .
thats the only difference between those two .
what might have caused this ?
bad tube ?
 

CJ

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Jun 3, 2004
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California
Bad tube.

EL34 always good for thermal runaway/unstable bias.
Install fan or lower screen volts, or buy nos metal base mullards that are good for 3000 volts plate and 500 watt plate disp.

Check the opt for dcr.
 

abbey road d enfer

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forgot to turn off my pp el34 monos over night .
next morning the mains fuse of my room had tripped .
i had a short between primary and shielding coil of the mains transformer of one of the mono blocks .
obviously the transformer got very hot .
on that one i changed the selenium rectifiers to a resistor to drop the HT .
thats the only difference between those two .
what might have caused this ?
bad tube ?
Have you checked resistance between shield and each side of primary? Also measure primary end-to-end.
If you have a short between a part of the primary to ground, it results in excessive current in the primary.
 

nashkato

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shield was connected to earth via the transformer via chassis .
gnd was not connected to earth/chassis .
the shielding wire connected to the mainstransformer broke as soon as i touched it .
primary DCR of mainstransformer reads correct .
the short was gone as soon as the shieldingwire connection broke .
 

abbey road d enfer

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shield was connected to earth via the transformer via chassis .
gnd was not connected to earth/chassis .
the shielding wire connected to the mainstransformer broke as soon as i touched it .
primary DCR of mainstransformer reads correct .
the short was gone as soon as the shieldingwire connection broke .
It is very likely the shield is still connected to the primary, and there is an insulation fault there.
It may or may not have consequences...
 
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