Since my lathe was already set up for glassblowing, I decided to make a third mold for making small envelopes. The mold is very simple and compact, so I included it on the other side of the first mold. This completes a set of three sizes, which can be seen above. This way low-powered tubes can be made more compact, and tubes that produce more heat can have a larger surface area to radiate from, and a larger space inside for components.
I made six of each type of envelope, and in doing so I discovered the cause of the off-centering problem, which so often leads to envelope failure. It appears that blowing hard into the envelope increases the friction between it and the mold; this causes the soft envelope to roll, rather than slip, along the surface of the graphite as it spins. This can be averted by blowing loosely into the blow-hose, allowing some air to escape around the sides of the mouthpiece. The envelope doesn't inflate as quickly, but there is no tendency for it to off-center itself.