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. |
Sorry to be late to this thread, but yes, when my ARC Ref 150 SE blew at the input fuse and board, ARC refused to take my amp in telling me that they were too backed up. I am the original owner of this amp and had registered it with ARC so I not a happy camper. I got put on a waiting list and was told I would be notified when I could send my amp in and that even THEN, it would 6-8 months once sent in for it to be repaired and shipped back. Btw, to this day ARC has never contacted me to tell me I can send in this amp and this was over a year ago. I sent some photos of the blown board to my local tech, Dan Abell of Abell Audio here in Columbus OH. He had worked on my previous ARC amps and I should have just gone to him in the first place. The burned circuit board had me thinking only ARC could properly repair it. Dan repairs mostly guitar amps, most of which are tubed. Columbus OH has a fair amount of musicians and Dan has seen everything. For him, repair of my Ref 150 SE was easy. I am guessing he saved me well over $2,000 in repair cost-my cost of repairs was just over $1,000. |
I found my local guy by calling a reputable music store and asking who the best guy was around here to repair tube amps. They had one favorite and that guy had never worked on an ARC before, but immediately diagnosed the problem as "multiple issues- not just bias" He was right- a weird resistor failure, a bad voltage controller tube which he had NOS stock at a low price, 2 6550 Sovekteks (I supplied them). He burned it all in while biasing as he went. $350 and a very short time later I'm listening to a wonderful quiet amp. Moral of the story: Great audio repair guys may be guitar amp guys unknown to us hifi gear heads. |
