Tung-Sol.com:: FAQs What is an Octal Tube?

FAQs

Question:
What is an Octal Tube?
Answer:
An octal tube is a vacuum tube that has an 8-pin base with an 11/16" diameter keyed locating prong called a spigot in the center. This tube base was introduced by RCA in 1935 for their new line of metal tubes, which they called an "Octal base". The octal base soon caught on for conventional glass tubes as well. The eight available pins allowed more complex tubes to be constructed, including dual triodes. The glass envelope of an octal tube was cemented into a Bakelite or plastic base with a hollow post in the center, surrounded by eight metal pins. The wire leads from the tube were soldered into the pins and the pinched-off nub through which the air was evacuated from the envelope fit into the post. The post had a protrusion along one side that matched the indexing slot in the socket so the tube could only be inserted in the correct orientation.

Until the introduction of miniature (Noval base) tubes, such as 12AX7 and EL84, the octal base was the most common and popular tube base. Some common octal tubes are 6V6GT, 6L6GC, EL34, 6550, KT88, 6SL7, and 5AR4. When looking at the bottom of the tube with the indexing protrusion facing you, pin 1 is the first pin to the left of the indexing protrusion.