On further listening to the SII-45B with Kaiju iron, 45 filament and stock B+ power supply, I'm a little less enthusiastic.
Likes: the revised amp is really well balanced from low to high, to my ears. Compared to the SII-45 with either stock iron or the Lundahl 70H/60mA and regular core headphone OPT, the Kaiju was so much better in the treble. With the Kaiju iron installed, the SII-45B amp comes very close to the Kaiju in the treble. And what it still lacks may just be a consequence of the Kaiju's bigger soundstage and instrument separation. At the same time, the Kaiju to my ears is more weighted towards the highs. Bass has tons of slam, but something about the presentation of the music always leads my ears/mind to focus on the treble. I'm more of a mids/low-end guy. With Kaiju iron, the SII-45B strikes a balance that I find really pleasing. So that's a big plus for me.
Dislikes: (1) it's a bit slow and, to use a expression of Doc B's, it sounds a bit "fat". It's as if there's a halo around all the notes. Maybe too much 2nd harmonic? (I could try to measure it, but my past attempts to measure THD turned out wonky. My scope seems to be really sensitive to the emi/rfi environment. And my place is filled with electronic stuff. LED lights seem to really throw the scope out of whack.)
(2) Narrow soundstage and lack of separation. Using headphones with the Kaiju, I can almost forget that I'm listening through headphones. Instruments and notes are placed in a 3D space around and outside my head. With the SII-45B, the soundstage is pretty much limited to the span between ear cups, and notes/instruments are often superimposed, making detailed listening more difficult. This doesn't appear to change with different secondary loadings. I've tried putting 17R in parallel with the 200R phones to get an effectivw 16R load to match my 16R OPT secondary wiring, as well simply plugging the 200R phones into the speaker outs. No appreciable change re soundstage and separation. Although the higher reflected load does seem to make the music a bit crisper, which I like.
Now, don't get me wrong, had I never heard the Kaiju, I'd be thrilled with the SII-45B with Kaiju iron. It's a significant step up IMO from the stock iron. But it's still no Kaiju. And notwithstanding that I find it a bit more treble-weighted than I'd prefer, I still much prefer to listen through the Kaiju.
I will eventually rebuild the SII-45B with different Lundahl iron and try to hit one of Jac's recommended operating points that reportedly produces -30dB second harmonic and 4+ wpc output. And I'll use 5670s as drivers and for shunt regulation (any maybe even try to figure out how to use an EML 30A as driver, with some suitable single triode filling in the for 5670 for shunt reg). Maybe that will give me the sound that I'm chasing: a lower centered Kaiju.
If that doesn't get me where I want to go, perhaps the 45 and 45B tube just doesn't fit the bill. And unfortunately I'll never know whether my earlier mishap of mistaking an 8R2 for an 8K2 when converting one of the SIIs to 45 damaged the tubes. I *think* not b/c the EML 45B's sound more detailed and better (to my ears) than my other pairs of regular 45s - including two pairs of NOS RCAs. But who knows? I don't think I want to shell out for another pair of EML 45Bs just to test the question.
And maybe I'll just have to accept that the "perfect amp" doesn't exist and that there's always trade-offs. ... Nah
cheers, Derek
