Here is my latest HP amp: 6AS7 with Sophia Electric Mesh Plate rec.tube - 274B. This build was inspired by the Woo Audio HP amp which can be purchased with the same rec. tube.
Take a look at the write-up: Sanguine
Dropped the amp on the CRO and here are some specs. I think the most impressive is the Ultra low noise figures:
With 33ohm load:
-3db points -10Hz to 240KHz
max power with 2.03Vpp input = 1Vpp @ 1KHz
noise with input S/C = 0.45mV to 0.88mV RMS (that's really low) knob at 10:30 position my comfortable listening level
Xtalk - 1Khz at 2.03Vpp input in one channel = 1.6mV in the other channel (excellent)
With 180ohm load
-3db points -10Hz to 164KHz
max power with 2.03Vpp input = 6Vpp @ 1KHz
noise with input S/C = 0.45mV to 0.77mV RMS (that's really, really low)
Xtalk - 1Khz at 2.03Vpp input in one channel = 1.6mV in the other channel (same, excellent)
Channel matching was perfect 100%. Noise didn't change with attenuator fully open.