This is my first post. I have found the more I learn about the ST 70 the more questions I have. My ST 70 is mostly built from old stock I purchased from Stereo Cost Cutters in the mid 80's. They didn't have any power transformers so I bought one from a highly regarded name in tube electronics (he is still in business). This replacement transformer doesn't have a 5 volt tap for a 5AR4 which he claimed was better because this tube was getting rare and expensive.
So for the next few years I tried a couple of different driver boards and even splurged on a SDS Labs cap board but the amp never sounded quite right so I put it in storage. A couple of weeks ago I pulled it out of storage and installed a Diytube driver board (the one that uses two 12au7's and one 12ax7.) The manual can be found at tubezone. (I have the link but I am too new to post links.) The documentation that came with this board has a voltage chart and that's when I discovered the real problem. The power transformer only puts out 630vct instead of the 720vct for a
PA060. So now I have many questions. On the driver board J19 should be 300v and J20 should be 350v. Both were 50v lower so I changed the 6.8K power supply resistor to 3K and that brought the voltage back up to where it should be. There is only 390v on pin 3 of the EL34's and I'm using the 15.6 ohm resistors. My EL34's are original Dynaco and I would like to keep the bias current on the safe side. How do I figure this out? What value would a safe current be? 35ma or should it be lower? The two pots on the driver board (R29 and R30) are supposed to be adjusted for zero volts but the best I can get is minus .9 volts. How important is this setting? Can I change values on the driver board to get it closer to zero? I would be grateful for any advice. Maybe I need to buy a new power transformer? I wouldn't mind lower wattage if it can be done properly.
Last edited by gwvenus56 on Mon Oct 05, 2020 6:15 am; edited 2 times in total (Reason for editing : left out questions)