sorenb

Yes, check your rectified input voltage ...probably too low ...or you might be pulling too much current >3A or so

not sure how you can measure currents like that using 18V ....sounds like something else is wrong ...

detach the HV rails and use the +LV or two 9V batteries

Have you checked the rail as described in the link I pointed to above?

you might find this useful, and be aware that the negative rail is identical to the positive, so the voltage ref 'ground' ref is the negative rail rahter than ground.

what is the voltage at the drain of the Sic?

check the terminal where the leads from the transformer is attached. Might also be that you populated the CPC1117?

only one at a time. Both will keep the amp offset ~ zero, and both are altering the operation point of the particular stage.

in any case the OpAmp is needed. Servo1 acts on the input, and Servo2 on the output stage rather. You need some ~ +15V offset to use the Servo2.

no issue running it a bit low, and leave it at that.

secondaries are too low for getting 400V out ... try 375V rather ...lower the two 390k to 365k, or parallel to get there

no current limiter, and less filtering ...if you don't want current limiter, remove 3904 rather

assume you GRHV voltage out is 390V under load and +404 and -410 without? what is the voltage at the drain of the Sic (middle leg) against ground? and what is the voltage between R9/R7? both with and without load

you need > 6.0V, 3x 1ohm might get you there, else use 4x

Check your CT from the transformer is really going to CT at the board. Sounds like thermal runaway for the 10m90; the dissipate as much heat as the SiC and thus needs to be properly heatsinked. Cannot see from your pics how you have attached the 10m90 to the sink ...use an alu oxi pad, thermal past on both sides of the alu oxi pad, and a screw to tighten ) If not the above check the CSS using 2xbattery9V or similar, and see if you can adjust the current - if not, the DN2540 has probably gone south

...and those sink's are 1C/W then?

With those heatsinks, output Stage current is probably around 5-6mA ... miniature doesn't come free

calculate the resistors based on measured values when everything are hooked up, and tubes are running warm - depending on the transformer the spec'ed voltage might be different under load.

Not needed. Soft START is a good thing to be easy on the tubes. T2 uses 60 seconds delay. Some uses constant current sources to feed the filament, to avoid the high current when the tube is cold - high current is stressing the tube (and usually why incandescent bulbs fail)... some runs the filament a bit below spec e.g. 6V rather than 6.3V, (even seen 5.5V) as far as I have measured 6V is pretty safe, where as lower voltage increase the risk for "cathode stripping"

Yep

Yep - you might mount D1 on the separate board and use it's legs for connection to the main pcb

10mA * 360VAC should account plenty for that ;o)

( 2 channels x (2x Output stage current + 10mA for input stage )*sqrt2*360VAC + 4x 6.3VAC*1.5A ) * some_factor depending on how much overkill ~ 100VA * some_factor I usually end up using a factor somewhere above 2 and close to 3

only reason to use more resistors is spreading the heat dissipation

What is the drop of those special diodes you are using? 2.8V drop seems to be about twice what could be expected. In any case you might consider using higher secondaries to provide the regulation some 'room' ... as far as I have experienced 3V is minimum to get regulation, but if the current is above 1.5A is more likely to be 4V .... as for your experience it seem like you have some loss between the VAC input and collector of the pass transistor