Whole lotta testin' goin' onWith some certainty, from further testing, I've drawn these conclusions:
- Some tubes will hum to some degree, at least in CTH, regardless of heater supply implementation and regardless of their heater draw.
- The primary cause of hum/noise at issue here is backfeed/line-in A/C EMI by switcher to common A/C supply line.
- Disabled on-board SM heater circuit in my CTH. Done by removal of D1H & D3H (LM2595 DC in) & removal of L1H (as well as its unraveling destruction
- Ran external wire for CTH heater pins (#4 & #5). Re-cased CTH to move it away from assorted bench EMI (fluorescent lights, etc)
- With 4 D cell heater supply 6922 has NO hum. 6BZ7 has faint hum.
- Used P2P POC heater circuit to power tube heater (same components as on-board CTH heater)... When CTH A/C + outboard SM heater circuit are fed from same A/C supply get good deal of hum. Feeding CTH & outboard SM heater circuit from separate A/C supplies (adapters) - very, very faint hum... Not easily distinguishable from ever-present low-level hiss/white-noise amp exhibits (even w/battery heater supply, w/R18s shorted & D2000s)
Note: While "Feeding CTH & outboard SM heater circuit from separate A/C supplies (adapters) - very, very faint hum..." I carefully waved the outboard P2P switcher supply very close to the CTH PCB & very close to the tube & this did not elicit noise, so I no longer suspect general EMI radiation from switcher as cause of noise. Ground (plane) contamination is also unlikely the cause (based on this test & wiatrob's raised switcher coil results).
My goal now is to get the hum level at or below the ever-present hiss level, IOW not easily discernible from it, for tubes that have no hum w/battery heater supply - e.g. my Mullard 6922).
I've a few line-in EMI filtering components in my parts graveyard to experiment with (to inject between outboard switcher diode bridge & its CTH-shared A/C supply/adapter). And that's what I'll be doing as time allows.