6X4 SER = LM Ericsson Sweden "D" getter NOS 1950's slightly tilted glass in white box

Aktiebolaget Svenska Elektronror = Limited Company Swedish Electron Tubes

54/40

54/40 

40=65%

50+=100% 

Recommended for

Audio Note DAC 1.2
LAMM LP2 phono

KONDO KSL M7 preamp
Kondo M77 preamp
Eastern Electric Minimax Phono

Pultec EQ

Wright WPP200C
Audio Note M7