Mallette Posted September 5, 2013 Share Posted September 5, 2013 Got home yesterday and headed up to the music room. System had been left on and I started to play something. No sound. Looked at the ST70 and the tubes were dark. Shut off power, and checked the fuse. Blown. Being unusaually optimistic and assuming the blow had come from our crappy and inconsistent mains power, I fetched a fresh fuse and put it in. Turned it on and the tubes came up. Watched it for 30 seconds or so and headed for the turntable. Heard a smidge of hum, then a louder one, then nothing. By that time, I was back to the amp and the tubes were still lit, but no sound. Shut it down. All looks fine with no signs of magic smoke exit on the exposed circuit board, but I really didn't feel like opening the bottom yet. If I don't see anything obvious I suppose it will be off to someone competent to figure out. Through my Soundcraftsman PCR-800 into the breach, but I WANT MY TUBES BACK... Dave Quote Link to comment Share on other sites More sharing options...
Don Richard Posted September 5, 2013 Share Posted September 5, 2013 I'd check the rectifier tube first, then measure the power supply output voltage for DC and AC, there should be no AC on the B+. If there is AC on the B+ there is probably a bad filter capacitor. If power is OK the amplifier has a problem. If you are lucky it will be a bad tube. If not the amp will probably require bench servicing. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted September 5, 2013 Share Posted September 5, 2013 I'd check the rectifier tube first. Yup I bet 90% chance the 5AR4 took a dump on you! But it also could be a power supply filter failing that cause the rectifier to go... some investigation is in order before placing a new rectifier tube in. Feel free to give a call with multi-meter ready and the bottom cover off. The first filter section that is directly on the output of the rectifier tube is suspect. Quote Link to comment Share on other sites More sharing options...
Mallette Posted September 5, 2013 Author Share Posted September 5, 2013 Going to try to reply to all. Thanks DR. My very limited knowledge (let's say, none of consequence) caused me to assume that if it was the rectifier then there'd be no power to the tubes. I'll check it. I actually have a tube tester out tin the garage somewhere. As to the others, here again, in my ignorance, I assumed that each set of tubes represented a channel so if one of them was bad the other channel would be present. So, my inaccurate assumpt based on insufficient evidence was that it was a power issue between the rectifier and the audio circuits. Mark, it's a Van Alstine rebuild (about 13 years ago) and I don't think it is modded beyond updating some of the older circuits. BiaSet and such is not required. Might take you up on that, Craig. I'll shoot you a PM to see what your convenience is when I see some available time. You've had it on your bench once before for something 8 or so years back. Dave Quote Link to comment Share on other sites More sharing options...
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