I'm 150% sure that my ST-70 and other components are not at fault here. However, the ST-70 does not have a grounded power cord. It is 2 prong. Perhaps I should try upgrading the ST-70 to IEC connector and power cord. I've seen it done by others but I don't really want to mess with it and I also don't think it would solve the problems at hand. Somebody correct me if I am wrong in that thinking.
Before I messed with the SP12 today, I had disconnected some things to try to find where the hum was coming from. I disconnected the PH12 board inputs and the B+ and ground and removed it from the risers. No change had resulted from that. Today I reconnected the PH12, although I am waiting for a pair of resistors that are currently missing because I damaged up 2 of the original ones trying to remove them (had them in the wrong spot!).
I had the HV connected from the transformer to the Rectifier board in parallel for 250 V without center tap and thus was using the pair of resistors and pair of diodes on the rectifier board. Today I switched it to wired in Series for 500V with center tap and removed the resistors and diodes from the rectifier board as per Roy. STILL HUMS! However, it seems a little different in that when I play music through it the volume is less than it was before. It is not possible to make the music loud enough to drown out the hum
I replaced the metal risers below the PH12 with plastic ones that I had in my parts box. NO CHANGE that I can tell. The board should be grounded through the ground wire that connects to the Aikido board but I may find later that I need to ground it to the chassis as well. I'm not sure.
I played around with the grounding jumper on the back corner of the Aikido. I forget the number right now but it's the one that connects ground to one corner where the screw holds the board to the riser post. This made the Hum Quieter but still present.
I previously had disconnected the Mute switch. I had also wired it incorrectly the first time. It did not seem to affect the hum before I disconnected it in either switch position. Today I wired it back in correctly and the hum COMPLETELY disappears when I have the switch on Mute, even with the power on. On or off, it doesn't matter. Mute switch on Mute - NO HUM. Mute Switch on Operate - HUM with power on and power off.
So, it seems that there is definitely something allowing AC to get into the signal path on the Aikido board somewhere, but the dang thing plays music so I have no idea where to begin. So I won't. Not today anyway. I have reviewed all the connections on the Aikido but I am afraid I'm going to have to pull it and check it all again.
I'll probably go back to 250V HV without the center tap and put the resistors and diodes back into the rectifier board. I think it worked better that way.
I was not my usual methodical and scientific self by only changing one thing at a time like I should have but since nothing has really changed, I guess I'm no worse off. If I had managed to fix it, then I'd really not know which solution had worked. Alas, none of them have worked thus far.
What will I try next? I'll have to ponder that one. I did NOT see any improvement by placing my hand between the transformer and the PCBs. There was no change. I hope I don't bust any more resistors!! I made my second order with Tube Depot (AWESOME STORE) and I always feel bad for having a 50cent order so I end up shopping the site. Today I added a sweet T-Shirt:
Last time it was upgraded tubes for the PH12. I hope there isn't a next time! Who knows what I'd blow extra money on
Until next time...Best Wishes from Oregon.