8street

Also the problem that this transistor is even more enhancement than SK216 and also more current. Which gives more nonlinear distortion.

Of course I read the datasheet. And I know maximum parameters in T2. You should use only tubes in excellent condition and in fully turned and working amp. But I preffer EL34. Probably KT88 would work as well.

Once I have used KT66. They do not require any mods. Just put and listen.

What is the type of green resistors in batteries?

Please make a few photos your T2. Perhaps the problem is simpler than it seems.

Yes, same as Inu's mod. For confidence, do for all 2sk216. The resistors can be cheap 25-50ppm grade. I have Vishay MKP-1837 0.1uF from partsconnexion. You can use any film/ceramic capacitors. This mod can be used if the previous mod has not helped. Sorry for google translate.

I have 2sk246 BL grade and 10k fixed resistor instead RV2 in source 2sk246. In any case, does not necessarily turn exactly 708V. Less than 730V is already very good sounding.

Maybe it helps: Add 750-1,5k series resistors in gate all 2sk216. For me it was awesome helpful. Add 0.1uF MKP or NP0 capacitors parallel 22k resistors in batteries. Also for fantastic sound I turned batteries to 708V.

No. Touch the isolated copper in PCB between ptf56 and sk246. It will add capacitance for stability current source. Noise would maximize in the other battery in the same channel. In any case, just try it.

Probably the noise from the current source 2sk246 - 10k. Test it: touch a isolated metal screwdriver to 10k pot and also the circuit between 10k and 2sk246 (don't touch the soldering place). Noise should disappear or minimize by touch one battery and maximize in other battery. And do not forget to change resistors R24, R31 to 20 ohm in PSU.

Check the supply voltage -500. For me, the current limit has been reached 90mA in rail -500V. One channel worked noiseless with the consumption of 48mA at -500V.