6SL7

The 6SL7 is the high-mu (mu ~70) companion to the 6SN7, sharing the same octal envelope but with higher gain and higher plate resistance (~44k ohms). It was widely used as a voltage amplifier and phase inverter in 1940s-50s amplifiers and fills a role similar to the 12AX7 but with the sonic advantages of the larger octal bottle construction. Its higher plate resistance compared to the 6SN7 makes it less suitable for driving loads but excellent as a first-stage gain element. RCA grey-glass and Sylvania versions are well-regarded NOS options, though availability has become increasingly scarce.