SV2-M36 Voltages

[Steve Ward]

Hi Kevin,

Good progress today. I worked through R5 as you suggested and it looks as though the board is fine and simply needed calibrating for the actual device spread.
With the original CTS 1M audio PS pot back in place (measures 990k), I progressively reduced effective R5 from the original 323k:
- 323k → about 51V loss at max PS
- 313k → 42V
- 294k → 21V
- 281k → 8V with the CTS pot
- ~270.4k → 5.37V loss under load with the EL84s fitted
So I’m going to replace R5 permanently with something around 270k, selecting the actual value if necessary.
I also recalibrated the low end using the PS3 trim. It only needed about ¼ turn adjustment. At minimum (barely audible) PS I now get approximately 16.1V plate, 12V screen and only ~0.1mA cathode current per EL84, so it is scaling right down very effectively.
At full PS the output valves are now biased around 39mA cathode current. Final quietest balance was 39.5, 39.5, 38.8 and 39.0mA. Interestingly, setting all four exactly equal introduced a little hum; a slight imbalance nulled it.
Screen resistor drops are around 1.9–2.05V across the 470R resistors, so roughly 4mA screen current per valve.
Current full-power voltages are roughly 331V B+, 322V plates and 314V screens.
So the SV2 now appears to be working properly from full output right down to almost zero.

Thanks again,
Steve