The 12AX7 high impedance amplifier triode was designed specifically for audio use and has a high amplification factor of 100. This is an American design that was copied by many other companies.
The 'EH' suffix appears to just identify the brand. Double mica sheets are present as would be found on military and aviation valves subject to vibration.
The twin triode sections follow the original pattern of no central screen.
A three quarter view of the anodes.
One triode seen face on.
The thin glass tube envelope is 22 mm in diameter, and excluding the B9A base pins is 48 mm tall.