Tom - is the Mullard CV181/ECC32 compatible?
I've been using a Red base 5692 and noted Currawong's experience that it can be a bit harsh so keen to experiment. I did order a Tung Sol VT231 round base black glass on eBay but only one channel was working (arrgghh!). It was used (not NOS) but won't buy from eBay again.
The other option I was considering is the Sylvania VT231. NOS tubes are available from tubedepot.com -
https://tubedepot.com/products/sylvania-vt231-6sn7gt-1945 - so no need to resort to eBay for this particular tube.
Just looking to take a very slight edge off - I'm using HEK v2's which sound amazing with this amp especially when I switched in a pair of RCA VT-107-A/6V6GT/G grey glass.
But I don't want to go too far the other way and dull the sound.
I would advise caution if attempting to substitute a CV181/ECC32 tube in place of the Studio-Six's 6SN7. The pin-out is the same, but the CV181 is a very different tube electrically than the 6SN7.
Personally, I have not tried using the CV181/ECC32 in the ST-6, and have not made bias/operating point calculations, and so can only offer a few thoughts:
* The CV181/ECC32 consumes .35 amps more filament power than a 6SN7, which is not a problem unless one is also using the optional phono stage attached, then the additional current draw could be excessive and cause filament voltages to sag.
*The CV181/ECC32 has a higher amplification factor than the 6SN7, and a lower plate resistance. These two factors will possibly increase high-frequency roll-off somewhat.
* I have not sat down and calculated the operating points for the CV181/ECC32, and so can't presently say if either insufficient or excessive current would be drawn if such a tube were substituted for the 6SN7, and/or whether distortion would be increased.
If one desires to use the CV181/ECC32 in the Studio-Six, be advised you are doing so at your own risk. Personally, I'd advise against it, and advise staying with tubes which are actually in the 6SN7 family electrically.
One would have to ask ALO directly, I can't speak for them, but I would caution that any malfunction or damage resulting from use of tubes outside of the recommended/specified types would likely not be covered by warranty.
I hope that helps