Sovtek 12AX7 / ECC83 LPS LPS = Long Plate Spiral Heater
This is an entirely new 12AX7 design from Sovtek and a great step up in sound quality. They have very large ribbed plates and great sound reproduction. They are very smooth and well balanced in terms of bass, mids and treble response. The large plates make them more prone to microphonics in combo amps, so they can be a problem if you like to run things wide open. It is still the best thing Sovtek has produced in a 12AX7, with very good gain and low noise.
They have filaments that are almost completely encased in the plate structure. They often don't "light up" when working properly. This is not a problem, it is normal for the LPS.
Sovtek's 12AX7LPS is the best choice for HiFi Audio with excellent sonics. It has a long ribbed plate derived from Siemens design, with a filament that works with both AC and DC supply.