I had a very strange experience this weekend with my ST-120. I was checking the bias and the rear left tube was running a bit high at 0.60mv. As I was trying to set the bias down a bit, the tube began glowing WHITE, almost flourescent, I shut things down for a few minutes. When I restarted the amp, the bias was way, way off, left tubes somewhere around 1.2mv. and the right ones down around 0.2. As I tried to bring them into line, the right front tube did the same glowy white thing. I shut things down again and fired off an e-mail to Bob (thanks for the response) who suggested a bad tube (new ones ordered).
I re-read the manual about biasing and decided to switch tubes side to side but only put the left side ones in per the start-up instructions. I turned the pots all the way down and fired it up. Blam went the tube rectifier and took out the fuse. Next I placed the other side tubes in, these were the ones with the light show, installed my SS rectifier and tried one more time. No hysterics, bias around 0.2mv. on all tubes. I slowly tweaked that up a little on each pot until the bias was 0.5-0.51mv. and turned up the preamp. Humm, sounds came out, no weird crackling or flashy stuff in the tubes, Switched from talk radio to CD; sounded fine, tried a few LPs; OK here too. So I ran it all day and checked the bias a few times. A-OK. Someone explain this to me please. I have a set of KT-120s on the way from Jim McShane but they will take a week to get to me. The tubes in use are Sovtek KT-88s. Any help would be appreciated. I don't really trust things that fix themselves.
I re-read the manual about biasing and decided to switch tubes side to side but only put the left side ones in per the start-up instructions. I turned the pots all the way down and fired it up. Blam went the tube rectifier and took out the fuse. Next I placed the other side tubes in, these were the ones with the light show, installed my SS rectifier and tried one more time. No hysterics, bias around 0.2mv. on all tubes. I slowly tweaked that up a little on each pot until the bias was 0.5-0.51mv. and turned up the preamp. Humm, sounds came out, no weird crackling or flashy stuff in the tubes, Switched from talk radio to CD; sounded fine, tried a few LPs; OK here too. So I ran it all day and checked the bias a few times. A-OK. Someone explain this to me please. I have a set of KT-120s on the way from Jim McShane but they will take a week to get to me. The tubes in use are Sovtek KT-88s. Any help would be appreciated. I don't really trust things that fix themselves.
