317 looks good to DMM, but sounds bad

[petemoore]

  Hmmmmm.
  Reads ~13vdc of 15vdc supply voltage, so the LM 317 does seem to be connected properly.
  and the Tremolo tremolo's at that voltage, all the caps are big enough voltage ratings etc.
  The tremolo is tapping the Theremax's supply voltage, at the switch.
  I can't imagine the Theremax is picking up hum because only when the tremolo is on does the hum hum, so it must come from the tremolo/power supply..
  Another option for hum might be that I've bolted [and probably connected] the two cases, theremin is in a 19'' rackmount, tremolo is in a RACO.
  Both circuits share Gnd. point on switch, the LM317 is mounted to the switch, has both input and output bypass capacitors, and produces a useful voltage.
  The "DC" wire from the tremolo to the 317 is the long one, about 8''.
  It sounds like a 'directly induced' hum, like what you'd get using a less regulated WW to power up a gainy circuit, strong hum sound, tremolo too.

[earthtonesaudio]

Don't know if it helps, but in general regulators like the 317 work better the more voltage they have to work with.  Dropping from 15 to 13 volts is just barely turning the regulator "on," especially if you take into account losses through the device.  You get better regulation (and more heat, unfortunately) if you drop a larger voltage across the regulator.  Maybe try running the tremolo at 9V.

These are the opinions I've gathered from my talks with a seasoned technician, not "facts" from datasheets or other more official sources.  So please take at face value.  Good luck!

[petemoore]

  Thanks earth !
  an Eazy try to try...
  Got a chance to try that for a minute and fixed it right up with an 18k across 2k2 = 12.31vdc, not a touch of hum, beautiful, Theremax [w/RCA mod] > Proverb > EA Tremolo > Tube Amp = Sweet 'lil setup.

[earthtonesaudio]

Cool!

[scratch]

earthtonesaudio is bang on, most of the 78xx, 79xx, 3xx regulators like to see at least 2.5~2.8V more on the input than their specified outut voltage, 3V minimum works even better. other regulators referred to as LDO (Low Drop Out) can work with as little as 1.5V differential between the input voltage and the specified output voltage.