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Improved Octal Tube Base Clamp

In order to clamp the tube in a V-block, the clamp must position the tube’s centerline so the envelope will clear the V groove, thusly:

OD3 Octal - V-block clamp

OD3 Octal – V-block clamp

The clamp now extends into the V-block and surrounds the entire Bakelite tube base:

Octal base compression clamp - Slic3r preview

Octal base compression clamp – Slic3r preview

The little divot captures the clamp screw and the slot lets the whole affair compress just enough to firmly squeeze the entire tube base.

The tube data table now includes columns for the envelope OD and the base OD, although only the 0D3 (and similar) Octal tubes in my collection have a bulging envelope and a smaller base. You can build clamps for cylindrical glass tubes if you like; I don’t vouch for the accuracy of the table contents.

For whatever it’s worth, the 6SN7GTB tube I started with has a 32 mm Bakelite base and the 0D3 tube has a 29 mm base. That should probably justify two separate entries in the table, but I’m making this up as I go along.

The OpenSCAD source code as a GitHub Gist:

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  1. #1 by madbodger on 2016-10-04 - 13:02

    Light bulb (and vacuum tube) envelopes have a shape, naming, and size convention that goes WAY back. The first tubes had a spherical envelope (known as a “globe” or G shape), with its diameter measured in eighths of an inch (G12 would be an inch an a half in diameter). Later tubes used a “sign” or S shaped envelope (although they’re known as “globe” tubes in the audiophile community, confusingly). Later still, they went to an ST envelope, which is a combination of an S envelope and a T (“tubular”) one. This allowed the top mica to be stabilized by the tubular section. Your 0D3 is this shape. It’s also known as “shouldered tubular” by some folks. All of these are generally larger in diameter than their bases. Later tubes went to a plain tubular envelope, which is easier and cheaper to make, supports both micas, and doesn’t (usually) extend beyond the base. Fluorescent tubes are similar: an F40T12 is a tubular envelope an inch and a half in diameter.

    • #2 by Ed on 2016-10-04 - 13:31

      Ya learn something new every day around here… [grin]

      I knew about fluorescent tube sizing, with old-school 40 W T12 tubes versus today’s downsized 32 W T8 tubes. You’ve tied some random bits I (thought I) knew together!

      The ST envelope wins, hands down, for good looks, but the regulators tubes (around here, anyway) have dark mica sheets and a getter flash on top that severely attenuate the top LED; there’s barely a hint of color up there.

    • #3 by Red County Pete on 2016-10-04 - 22:41

      Until we moved up here, I had a 4 pin Type 80 dual rectifier. The envelope was unusual; conical from the base to about the top insulator, then a hemisphere to top it off. I suppose it dated back to the 1920s. I recall it having a lot of getter flash, but don’t recall just where.

      FWIW, Costco is now selling T8-style LED shop light tubes. I passed on them, and got a couple of shop-light fixtures for $48 each. I think the tubes are currently on sale for $13 a pair. Might try the tubes later on; I’ve had another one of the circa 2004 fixtures die due to a flaky ballast. The brand name for the Costco stuff is FEIT. No signs of any bare tubes at Home Depot just yet.

      • #4 by Red County Pete on 2016-10-05 - 13:23

        Correction, LED shop lights cost $30 each, not $48.

      • #5 by madbodger on 2016-10-05 - 13:51

        Some of the early tubes (Kellogg was one such brand) had the odd conical envelopes. The gettering was done differently then too, so sometimes the getter flash covered the whole tube (01 tubes often look like that). Those LED tubes are great when you have a dead ballast, since you have to rewire the fixture to deliver line voltage to the pins for most of them anyway. FEIT has been around (making low-end bulbs and fixtures) for quite a while. My main squeeze works at Home Depot, I’ve seen some of the bare LED tubes there on a clearance rack out by the lumber department (don’t remember the price, but I was tempted).

        • #6 by Vedran on 2016-10-06 - 09:32

          “main squeeze”… implies the existence of auxiliary one(s) as well :)
          Hopefully she’s not into Ed’s blog :P:P:P

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