6BU6

In one 1950 RMA release, release 820, Sylvania put out 7 market failures; a series of duo-diode triodes: 6BK6, 12BK6, 26BK6; 6BT6, 12BT6; 6BU6, 12BU6. These were largely identical in concept. All have the same 7BT basing. Besides the filaments, the difference is in the triode gains. The ‘BK6 triodes have a gain of 100; the ‘BT6 triodes have a gain of 70; and the ‘BU6 triodes have a gain of 16. I have had 12BT6 and 12BU6 tubes in my collection for a while, but have recently run across the 6BK6 and 12BK6. The ‘BK6 and ‘BT6 lost out to the 6AT6 and 12AT6, and the ‘BU6 lost out to the 6BF6 and 12BF6. (Source: Jim Cross)