I've been listening blissfully to my ST-70 for the past year or so with no issues. I turn it on in the morning around 6 when working from home and off around 3 or so. It is being ran from the pre-amp output of my Pioneer SX-980 and powering my HPM-150's.
When i purchased the Amp (was sold to me as tested and working) I cleaned it up, replaced the main cap and the rectifier (it was making a buzzing noise) and maintain the bias at approx. 1.54. My output tubes are Winged C EL-34's.
Recently the amp. had an issue where the front left output tube started to run hot. There was a popping through the tubes so I looked up and noticed the issue. I powered it down and let it cool. I swapped the front and back left tubes and powered it back up. The left channel sounded weak but no red plating or running hot so I powered it down and swapped the output tubes to the right channel and vice versa. Let it cool and powered it back up. Same issue. Turned it off and swapped the 7199's. This seemed to fix the problem. It worked for a few days and then the weak channel issue was back. When i went to check the bias and put the multi-meter ground wire to the Amp housing the weak channel issue went away. (Perhaps a grounding issue?). Later on the amp wanted to run hot again and blew a fuse. So I powered it off and purchased a new Rectifier Mullard GZ-34.
I installed the Mullard GZ-34 Rectifier, checked the bias and set to 1.54 approx. and ran the amp for about 20 mins. the weak left channel was there at first and then went away as I touched the multi-meter lead to the housing. After a while the front left output tube went hot again and the amp blew a fuse Note: these left tubes are the ones that were originally on the right. So now i'm not sure what to do. Any help appreciated!