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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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oddiofyl's avatar
oddiofyl
3,974 posts
 

Awesome ending.   There are still some good local techs , it’s finding them that’s a challenge 

oddiofyl's avatar
oddiofyl
3,974 posts
 

In the Boston area D&S Tube Audio does repairs and mods on HiFi  and guitar amps.   He’s in North Attleboro MA

johnread57's avatar
johnread57
237 posts
 

Wish I could find a tech like that in Australia 

Maybe I should ask the guitar shops…

dinov's avatar
dinov
846 posts
 

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

hickamore
682 posts
 

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.

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