Electron tube 6L31 Tesla is a low-frequency beam power tube with indirectly heated oxide-coated cathode and small 7-pin miniature base. It is direct replacement for 6AQ5 and EL90 tubes. After mechanical adjustment it may be used also instead of the russian 6P1P (6П1П). 6L31 was also common in many of the Czechoslovakian receivers made by Tesla (for example: Tesla 420A, 506A Arie, 510A, 514A, 521A Populár, 526A Kantána, 612A, 615A Dunaj, 616A Opera, 618A Kriváň, 621A, 622A and others). New old stock tubes, tested on BM215A. Years of production may differ. Produced in the 60's.