Tube rolling today!
As it works out, this circuit, because of the use of CCS's - Constant Current Sources - allows for the use of different tubes.
So far, 6922/7308/6DJ8 has been the basis of design. I wanted to evaluate the 6CG7 however, because it is the 9-pin equivalent to the 6SN7. I.e. it IS the same tube, but in a small bottle - check the curves! I also tried the 6H30. All three of these have the same pinout and all require 6V filaments. The PS 9.0 is capable of providing the higher current needed for the 6H30, so no problem there.
Because CCS's are used, the different tubes settle in at different operating points. I checked and all operate in a comfortable range on their curve.
The 6CG7 (same as 6SN7), not surprisingly, has a bit thicker and richer tone than the 6922.
The 6H30 is operating at a bit lower plate voltage than ideal, so is not as fast sounding as it typically sounds, but it sounds good.
I will have to go back to the 6922 again and compare with the same music.
The cool thing however, is that no matter what tube I settle on as Basis of Design and that will ship with the mod, all three of these tubes can certainly be used.
I know that many people love the 6SN7 tube and this has been the Basis of Design for our Oppo 95, 105 and Sony XA-5400ES mods. However, I used the 5687 tube in a radically different circuit in the Marantz SA8005 and I know that many customers who also previously had both Sony XA-5400ES and Oppo 95 or 105 mods were VERY pleased with the greater level of resolution, detail, dynamics and overall resolution.
I want to achieve the same with the Oppo mods. The SABRE DAC itself is VERY revealing, to the point that I want to be sure that the final product has the RIGHT sound.
More as I settle on this and fit it also into the compact Sonica DAC enclosure....