6SN7 based Pre-Amplifier

Mr Wu, are you mixing the 6sn7's? Maybe two grey glass and the short bottle Sylvania?

Otto, if you have not done anything yet you may want to wait until Mr. Wu gets done evaluating. I love mine though if that helps.

Tried the stubby Syl chrome top in the regulator and RCA smoke for the input/gain stage 6SN7s. Sounds OK but top end is colored (there's a frequency in the mid high thats recessed and a little above that its pronounced).
I think tube-rolling among these competent NOS tubes (not EH not Sovtek not Shuguang) will only give a different flavor but wont contribute to night and day improvement or fix a "problem".
I'm happy with my smoke-glass regulator and CBS combo for now as the new coupling / output caps settle in.
Bass is still lethargic (not nearly as bad as the SLP98P I owned a year ago). I strongly suspect its the 0NFB design.

Found 2 major design flaws of the Ming Da:
1. Filament power supply in series, 2 supplies, one shared between 2 6SN7 and the other among 3 6922, non regulated.
Be prepared for it to sound good one day and bad on another.
2. Grid resistor from 6SN7 stage to 6922 stage is 1/10 of what it should be. In the unit and the schematic there is a 220Ohm resistor but with that you'd have to have a 6SN7 and the middle 6922 to perfectly jive with each other to avoid a high frequency oscillation which sometimes has the side effect of inducing a pronounced hum as if grounding is not proper.
This is why this preamp is both a tube-roller's dream AND nightmare.
Took me a while to figure it out. Swap in a 2-5k resistor and all is well.