Hello. I have been lurking here for a few months and finally registered. I have an old ST-70 that was my daily driver in the early 2000's. It went out shortly after the economy crashed in 2009, so to be practical, I mothballed it because I didn't want to spend any money to repair it. I recently got out the multimeter and went through the circuits. There were several issues mainly related to the driver board being the original half-century old board. I installed a new board that changes to a 6GH8A tube circuit. I had already replaced the old can capacitor with a larger capacity and the selenium rectifier with a diode. I had previously rewired for triode mode, and I undid that to go back to ultralinear. I'm running some 15 year old Tesla EL34s, and an ancient Sylvania 5AR4 (mislabeled as 5V4GA). I added the diodes on the 5AR4 socket using the yellow sheet diagram. I'm using the preamp section of a NAD C740 to drive it through ELAC Debut B6.2s, and the old Dynaco is working better than ever. I've weaned it off of the variac, and it's my daily driver again. Dead silent. No hum, vibration, or distortion. Cheers! 
