Built a board. 4x INA128 instrumentation amp input plus 2X single-ended inputs, all powered on 24V.
INA128 can run up to 36V, LM358 up to 32V. SRPP offset voltages are around 45V or higher, so maybe with the divide-by-two inputs it would be better run on 28V or even 30V.
- Design: http://edf.bu.edu/svn/edf/EricHome/TubeAmp/BiasMon/bias/
- Schematic: http://edf.bu.edu/svn/edf/EricHome/TubeAmp/BiasMon/bias/tags/RevA/bias.pdf
11/8/20
Stuffed board, trying to test. Completely screwed up the programming connector. It's a 2mm pitch numbered wrong. Managed to wedge in a 2x3 header, can maybe fix the pinout with a knife.
MISO (18) -- 1 2 -- VCC SCK (19) -- 3 4 -- (17) MOSI nRST (1) -- 5 6 -- GND
Now I can at least download blinky light firmware :)
Box:
- Hammond 1590BB (metal) https://www.hammfg.com/part/1590BB
- Hammond 1591B. https://www.hammfg.com/electronics/small-case/plastic/1591t
- Would require board < 2.89" wide board
INAs would measure the four bias voltages. Additional channels would measure the DC offset from GND of the cathode circuits on each side.