I’m coming up on 2 months of enjoying the music with the Mogwai SE. Below is a summary of my tube selections and recommended approach for the Mogwai SE with high sensitivity headphones and speakers (which may potentially be applied to other amps).
For my chain and desired sonics, my tube preferences lean toward more detailed, incisive, airy as opposed to warm and punchy.
UPDATE: See subsequent post below.
My current preferred tube combination for Qutest 2V > Mogwai SE 32 ohm out > VC with Auteur/Be2 pads (all NOS except JJ):
12AU7 Amperex Bugle Boy Holland (or Telefunken/Mazda)
5U4G Tung Sol blackplate ST shape
KT88 JJ Slovak Republic made blue glass
For less sag:
Same input tube
5AR4 Sylvania fat boy
GE 6L6GC
The above essentially reflects either a high power rectifier coupled with low power power tubes or low power rectifier coupled with high power power tubes. In my listening, going high-high resulted in too much energy and going low-low was too relaxed. In all instances, a low gain input tube was preferred. Eventually I might see if low-low matches with a higher DAC output voltage, and vice versa.
For those wanting a warmer/punchier tilt, they may like RCA (especially 6550 and 5691 redbase), Mullard, Gold Lion, Matsushita. Bendix (with adapter) and Tung Sol were also highly regarded in my research.
For new production, Tung Sol and JJ have been good, especially TS 6550.
If someone can please report back on Tung Sol 6550 grey plates no holes, early 1960s, they are supposed to be the next tier up for power tubes.
For high sensitivity headphones, I would start with a low gain input tube. Next, figure out whether you like more or less sag, or whether your chain responds better to a higher or lower power rectifier tube. The power tubes then fill in to best complement the selected input and rectifier tubes. Depending on any audible noise/hum and sonic preference, adjust accordingly for more power/gain. Once nailing down the tube type, you can proceed to selecting the tube manufacturer (not necessarily the tube brand). Other approaches may work; this worked and was manageable for me.
12A_7 tubes require the use of a tube adapter with the Mogwai SE. I got better results with these tubes over NOS 6SL7/6SN7.
Switching to high sensitivity speakers, using the highest power and highest gain tubes was preferred.
Below is a cheat sheet for the gain ranges for 9-pin 12A_7 input tube variants (not comprehensive, someone check my math):
100: 12AX7
70: 5751
60: 12AT7, 6201, ECC81
47: 7062, 6211, 5965, 18ECC, E180CC
20: 12AU7
My sweet spot for performance and value for non-directly heated tubes is generally $60 to $100 per tube. Before I commit to spending more on a tube, I’d need to confirm that an elevation in tubes would result in the highest ROI compared to other enhancements to my chain.
@PaisleyUnderground I’m posting here as the content is specifically related to the Mogwai SE and other A&S amps. We can link in the other thread.