The 6SL7 was and is a fine high mu twin triode.
So... here's the thing. These little Asian-sourced boxes are "effects boxes" for audio reproduction -- akin in concept to those used for music
production but serving a different audience.
I would suggest that, if one is interested in procuring and playing with a "hifi effects box" (yes, I recognize the paradox in that phrase, but consider, e.g., the graphic equalizer craze of the 1970s and 80s!), there was, and perhaps still is, a purpose-designed preamp that allows one to investigate this more intelligently and deliberately.
A funky little
Hawaiian Oklahoma company, PAIA (the late John Simonton), for years had a wide range of interesting kits and products including a theremin*, analog synthesizers, and various preamps and other boxes for musicians. They also had one product that straddled the line between musician and, shall we say,
audiophile called the
Tubehead preamp. The kit was sold in two versions, one for studio/production work, and one for home use. The 'secret' to this design was adjustable operation of the 12AX7 (another very good high mu twin triode small signal tube, in a 9 pin miniature package) to allow one to
hear the effect of alterating a triode's operating point.
I write this in past tense. I don't know if PAIA is still a going concern, but the design was published and it should be "straightforward" to DIY a "Tubehead" if one is sufficiently interested.
https://paia.com/manuals/docs/9305-tubehead-manual.pdf (read the design analysis on the last couple of pages)
Out of deference to the OP, I think I'll start a separate thread on "cheap tube preamp opportunities" and see if there's any interest.
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* Of course, I have one! My son and I built it as a project when he was quite young. Everyone should experience a theremin at some point, ideally one they built themselves! It's a very simple gizmo (two oscillators) with magical qualities.
jelabsarch.blogspot.com
