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    the safety precautions around high voltages.

6V6 line preamp

Load still clean of frequency: nothing 50 or 100Hz seen in the tester. AC ripple 1.8mV

1st cap PSU 100Hz & ac ripple more or less the same because it's only a 0R68 resistor between the caps
2nd cap PSU 100Hz & AC ripple jumping 486.9mV-533mV
 
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If you will feel your 50Ω parallel Ro Tx needs something lighter, without 60mA reg CCS, maybe try this simple Mosfet capacitance multiplier of mine. But use D4 Zener in the chain BZX85C62 for lower Vout because your rectified voltage isn't high enough. All other Zener type codes put as shown.
 

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See about rearranging your ground wires to run send return side by side with the hot lines they represent. To tighten loop area. Maybe also compare star ground to buss bar solutions. In general try to reduce hum in the build's wiring. 1.5-2mV AC noise picked by such a wide bandwidth and many digits count DMM on the rails, and a clean scope picture, do not explain hum as coming from some PSU.
 
Yes, keep it as an alternative to maybe unload your Tx enough since its tested and working. Constant current can be tough when asking not far from Tx + rectifier spec capability. This one is not even a series reg, let alone shunt reg, its a series voltage referenced filter but its not power demanding on the other hand. Not as dynamic but at least not against good tone. Works on smaller sink too.
 
The two 22.000uF main filter caps at PSU filaments heaters are not grounded at chassis, could be the hum issue?

6V6 & his PSU only grounded the first cap to chassis.

The input RCA with incoming signal ground also grounded to chassis same point PSU 6V6.

Electrostatic R-Core also grounded to chassis.

All 3 single point.
 
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The input RCA with incoming signal ground also grounded to chassis same point PSU 6V6.
I recommend coax signal wire, make one ground node at Ck (-) pin for all main preamp circuit, then with one cable refer it to chassis. Not to pick ripple from the raw PSU ground point directly. Or refer it to the regulator output gnd line only that goes to the raw PSU ground point through its input wiring. So there's some physical distance to the Raw DC capacitor(s) ground point again.
 
I recommend coax signal wire, make one ground node at Ck (-) for all main preamp circuit, then with one cable refer it to chassis. Not to pick ripple from the raw PSU ground point directly. Or refer it to the regulator output gnd line only that goes to the raw PSU ground point through its input wiring. So there's physical Raw DC capacitor(s) ground point distance again.
Ck (-):+ cathode caps both channels are soldered to the bus ground, not connected to chassis. The bus ground is connected to the SSHV2 output ground. SSHV2 floating and 1st cap after rectifier grounded to chassis, that's all. I don't understant why to do two chassis ground: one for 6V6 PSU and the other for the buss ground?

I'm sorry Salas don't understand you.