The 77 is a classic 6-pin UX-base sharp-cutoff pentode vacuum tube introduced in 1932. Engineered as a high-gain voltage amplifier and detector for early superheterodyne radio receivers, it features an ST-12 shoulder glass envelope and a top grid cap for high control grid isolation. It operates on a 6.3V heater and provides high amplification with low grid-to-plate capacitance. The 77 serves as the 6-pin predecessor to octal sharp-cutoff pentodes such as the 6J7 and 6SJ7. Available NOS and tested used.