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

 

128x128Ag insider logo xs@2xyesiam_a_pirate

Wish I could find a tech like that in Australia 

Maybe I should ask the guitar shops…

Yes, I hade a McIntosh MC30 red plate tubes, from bad resistors. Trashed the tubes. Definitely a serious condition which needs repair. 

Support your local electronics man.
Surely would if there were one. Fifty years ago there were three crackerjack electronics repair nerds in a town of 6,000. Today, 12,00 people, zero repair guys.

Wish I could find a tech like that in Australia 

Maybe I should ask the guitar shops…

@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.