Miniature valve

The miniature thermionic valve (vacuum/electron tube) is a development of the older style of thermionic valve in which the pins for connecting the valve to circuitry were mounted in a Bakelite base, into which the valve envelope ("bulb") was glued. It was invented in 1938.Miniature valves are rather more space efficient. They do not have a separate base, rather the pins are mounted directly in the glass base of the envelope. This forces the sealing tip, which was previously at the bottom of the envelope unseen inside the Bakelite base, to be moved to the top of the envelope, creating the classic tear drop-like shape, seen in valves such as the ECC82, EF86 and 6CA4.However, making valves smaller and more compact reduced the voltage that they could work at, and also the power of the filament, so the older style continued to be used for high power rectifiers and audio/radio amplifier output stages.Development continued, and led to the sub miniature valve, the "acorn" valve (named due to its shape), and valves in D.I.L. packages.

Sources

# http://www.tubestore.com/ef86types.html
# http://www.r-type.org/static/story.htm particularly the section entitled "Glass Base Construction".