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Mystery Of Red Plating

My tube amp (ARC VT130) was blowing up her screening resistor on the left channel. The two Sovtek 6550 tubes would glow red- pulsating with red heat. If it weren’t so disastrously bad for them it would be fun to watch. I assumed it was a bias problem. I also

replaced resistors - more than necessary but as the story will tell not the right one. As it turns out it was bias- and a lot more! I had a resistor with a pin hole in it that was arcing to a trace on the printed circuit board which was pushing 100volts into a triode tube ( a 12BH7A) which is a voltage controller for the 6550s.

https://imgur.com/gallery/f9E1Yqd

 

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carlsbad2's avatar
carlsbad2
1,821 posts
 

Circuit boards are not a good thing to have in a tube amp.  Point to point wiring is the way to go  

Best of luck.

atmasphere's avatar
atmasphere
11,938 posts
 

The most common cause is the tube itself fails.

Sometimes the socket fails with the tube. If high voltages (+500V) are involved, sometimes the socket can develop internal arcing. This is why ceramic sockets are used in higher power situations, as they are immune to this problem. Of course a socket that is arcing must be replaced.

Good luck with the repairs!

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