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
@johnread57 Guitar amps can red plate too and some of them use KT88s with some rather high voltages. Any tech that can fix them should be able to fix any other tube amp using similar parts. In short, yes, as the guitar shops. |
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! |
