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Guide Pin and Octal Base

TubeGeek
2017-01-29 20:01:24

We added an article about broken guide pin and what you can do about it.

Let's talk a little bit about octal base. On octal tubes, the base serves important functions.

Octal tubes like EL34 actually have wire leads coming out from the glass. The leads are soldered inside the octal base pins. These pins are what makes contact with the socket receptacles.

This pin soldering is tricky to do well. Quality of this soldering work tells a lot about the quality of the manufacturing.

The octal base has to meet a lot of demands. Octal base insulates high voltages between the pins. Octal base is cemented to the bottom of the glass tube. The octal base also needs to conduct heat away. Octal base must be flexible enough to accommodate thermal expansion of glass tubes. And octal base has to be cost effective.

For these reasons, tube manufacturers use plastic in octal bases. Even the metal bases use plastic, as seen on this Svetlana KT88.

Kt88 Svetlana

Being made out of plastic makes an octal base more prone to blemishing and, of course, guide pin breaking off.

Octal base itself does not affect the sound of the tube at all. But you want to exercise some caution when installing/removing the tube from the socket. Guide pin on the octal base happens to be the most fragile part of the tube.