OT: why are tubes different shapes?

[marrstians]

i understand the pin-out differences and stuff, but why are some coke bottle shaped, some round top, some have that pointy tip, ect?...  is it a manufacturing specification or just the way someone started making them so they still do? just curious...

[petemoore]

Tubes are experiments. Some have accumulated very strong track records 12ax7   EL34 etc.
 Tubes of the same type number can have different sizes shapes and construcion.
 Different type #'s have different 'innards' and must have room for the structures, therefore different shapes of glass 'hold' the air' out...why the call e'm vaccum tubes.
 THis is my little contribution to explaining tube shapes,,,I'm sure I missed a few things

[R.G.]

For thermal and mechanical reasons, which vary from tube to tube, and for special RF reasons for the really odd RF tubes.

All the simple cylindrical tubes are that way for mechanical reasons involved in making building the tube easier. The pins are imbedded in a round button of glass under special processing to get a reliable glass to metal vacuum seal. If I remember correctly, each pin is actually a series of five different metals end to end to ensure sealing. Once the buttons are made, the pins holding up the elements are welded on, and the heater, cathode, grid(s) and plate are welded to the pins.

The outside body of the tube is a cylinder of the same diameter as the finished tube, which tapers at one end to a slender open tube. The full-diameter open end is placed over the assembled metal elements until it bottoms on the glass button holding the external pins, and the cylinder is heat-welded to the button, leaving the finished tube but with a slender open tube coming out the top. That tube is where the vacuum goes in.

The tube has a vacuum pulled on it through the evacuation tube, and the evacuation tube is melted closed. This is what is responsible for the pinty topknot on all the small cylindrical tubes. On octals, the evacuation tube usually leads down through the bottom button and is covered over by the center pin of the octal base.

So they're primarily cylinders to make that work. Non-cylinders are usually that way because the plates get hot. The plates are cooled only by radiation because the vacuum disallows convection, so heat comes off the plates. The glass envelope is all that can absorb the heat, so the glass is the tube's cooler. Since there is a melting temperature that can't be exceeded, the glass can only absorb and re-radiate or convect a certain amount of heat per unit area, and if the plate has more heat coming off than that, the glass has to be moved further away to increase the glass area hit by the radiated heat. As tubes got better with technology, the glass got better and fewer odd-shaped tubes were made. The really old ones were the odd shaped. The later a tube was designed, the more likely that it was purely cylindrical, smaller - and hotter per surface area!

I have in my box of curiousities an EL34 with an indented cone on one side of the glass cylinder. It literally got hot enough where the heat radiated away from the plate to soften up and the atmospheric pressure pressed the glass in until it touched the plate, vented the vacuum out, and the tube died.

The tech who gave it to me swears that the owner of the amp told him "It really sounded great just before it blew out. Can you fix it so it sounds that way all the time?"