Well, I connected it up, and something ain't right. I've disconnected the inputs from the SP12 board at the volume control to eliminate any issues there.
After connecting to my power amp and powering up after a few seconds get a lot of hum through the speakers, increasing in volume quite rapidly. The first time I wasn't quick enough and my power amp went into protection mode. No harm done anyway.
Not really sure where to start after that, like I said I disconnected the inputs to eliminate anything from that side. It's only the SP12 connected up.
Tubes being used are:
Checked all the voltages and everything seems to be OK
Transformer output 13.7VAC
Heater regulated output H+/H- 12.34VDC
Transformer output 252vac
B+ regulated output 256VDC
Voltage across C4 233VDC, both channels within 1V
Pin 3 of 12x4 tube 13.3VAC - I am guessing I might need to drop this down a bit with a higher value resistor? Standard in the kit is 1 ohm and I reckon I need to go up to around 3.3ohms or so to get 12.6v here. Will have a look and see what I can find in my
My 12v transformer seems quite noisy, I disconnected the output from it and it didn't make any difference. It's one of these: