Very good. Yes try, you may like 2SK170. uPA68 is NEC NOS from same country by the way. But more alike 2SK117.
There is no better IDDS pick if enough. Because the BF256 CCS at their tail forces about 2.5mA in each. Transconductance (Yfs) is the same at 2.5mA for higher or lower IDSS spec if above 4mA threshold anyway.
2SK117 (or uPA68) has lower Yfs and lower pF than 2SK170. Shows the flat THD sweep of post #1 with either 117 or uPA. With 170's more Yfs internal gain of DCG3 (OL) gets higher, THD falls even lower because of more feedback due to that open loop gain, but its sweep tends upwards towards HF because more pF. Sounds different. More forceful. uPA68 & 117 almost same sound more gentle. With K170 more push of body and detail.
2SK117 (or uPA68) has lower Yfs and lower pF than 2SK170. Shows the flat THD sweep of post #1 with either 117 or uPA. With 170's more Yfs internal gain of DCG3 (OL) gets higher, THD falls even lower because of more feedback due to that open loop gain, but its sweep tends upwards towards HF because more pF. Sounds different. More forceful. uPA68 & 117 almost same sound more gentle. With K170 more push of body and detail.
GR or BL same thing when IDSS is enough for this purpose. K117 almost same quality as uPA. But uPA is monolithic well matched pairs by design. He will hear and decide for himself about K170 vs uPA. Difficult to fully explain in words. Its not a practical option for many today because K170 quad is rare, big caution for being faked, expensive, but when available its good to hear in DCG3. Its a more powerful part for transconductance, with more parasitic capacitance also, makes measurable electrical differences.
I don't expect whatever changes of Jfet gonna change the sound dramatically
It's more system synergy thing, one might suit better and in this case only a real can tell
I'm gonna try it but first I'd let some hours of burn in and attenuator test first.
With only few hours of play, still it plays beautifully with Muses 72320 in attenuator duties and BIB in power supply. My previous conclusion was that power supply makes much more difference than all the rest, I'd like to see if I'll confirm this with the new one.
It's more system synergy thing, one might suit better and in this case only a real can tell
I'm gonna try it but first I'd let some hours of burn in and attenuator test first.
With only few hours of play, still it plays beautifully with Muses 72320 in attenuator duties and BIB in power supply. My previous conclusion was that power supply makes much more difference than all the rest, I'd like to see if I'll confirm this with the new one.