ron soyland
To get the high vacuum for the vacuum tube the diffusion pump is used in combination with the mechanical pump. The diffusion pump will only operate at very low pressures around 100 microns or lower. A few specially designed diffusion pumps will operate at pressures up to about 1 torr, but these are hard to find. They do come on ebay quite regularly though.
There are two basic kinds of diffusion pumps. These are the water cooled and the air cooled. We will examine the characteristics of both. Either type is suitable for tube making but not having to fool with water connections is quite convenient!

Diffusion pumps come in all sizes from less than an inch inlet diameter to huge monsters over six feet across used for space simulation chamber evacuation. The size of pump needed for tube making is determined mostly by what the vacuum system will ultimately be used for. If it is strictly for tube making and will not be connected to a larger chamber such as a large bell jar, then a very small pump will do fine. There are several models of pump that are 1 inch inlet diameter that come on ebay occasionally. These are almost all water cooled, but can be modified easily to be air cooled.
If you are going to use a larger chamber on your system, you will need a pump accordingly larger. A rule of thumb is to have the pump inlet diameter approximately 1/4 the diameter of the chamber, or greater. Thus, for a 12 inch diameter bell jar, you would use a 4 inch diffusion pump or larger. The time required to reach a given vacuum will be determined by this ratio.
The best pump size for the tube making system is a 3 inch air cooled type. These are available by many manufacturers and are available used on ebay for a hundred dollars or so. The three inch size is large enough to give excellent pumping speed and yet it is not so large as to be difficult to mount and get valves for.
The photo above is of a small water cooled diffusion pump. This pump is a 2.5 inch size and operates off of 120 volt power. The pump needs a constant flow of water through it for cooling. Small pumps like this cannot be easily converted to air cooled. If you have water and a drain available in your shop, these pumps will do just fine. Water cooled pumps are somewhat smaller physically than air cooled pumps because of the greater efficiency of water cooling. The vacuum attained by the pump is in the low 10 -6 torr range, which is excellent for vacuum tube operation. The small size of the inlet makes it lower cost to find a suitable high vacuum valve for it. The main disadvantage of the pump is the water cooling.
Here we have a 3 inch Varian air cooled diffusion pump. Note that the length of the pump is considerably greater in proportion to the pump diameter. The pump requires a constant airflow over the finned part. This is usually provided by a shrouded muffin fan or even a small squirrel cage blower. The three inch size is still small enough to find a reasonably prices vacuum valve for the inlet. The price of valves, even used, begins to escalate rapidly as the size increases, especially over 4 inches diameter. The tall size of the pump indicates an efficient pump design that provides a greater outlet pressure tolerance than a shorter pump. This pump will operate with an outlet pressure up to about 250 microns, which is easily achieved with most mechanical pumps.
This is a small air cooled diffusion pump from an Alcatel leak detector. Note the blower shroud on the body. The muffin fan is mounted on the right hand side. The inlet of the pump has a blank off plate with an elbow pipe joined to it. The pump itself is about 11 inches tall, noticeable shorter than the Varian pump. The penalty for this shorter size is the reqirement for a lower outlet pressure to keep the pump operating. This pump stalls at outlet pressures above about 75 microns. This pressure is still quite easy for modern two stage mechanical pumps to maintain so it is not a serious disadvantage. The advantage of the small size and air cooling is easily worth the small loss in pumping speed due to the lower stall pressure. It is an ideal pump for tube making. The pump will achieve a vacuum of below 10 -5 torr.

Each of the pumps above is suitable for tube making systems. The water cooled pump makes the smallest system however cooling water is necessary. The taller Varian pump is commonly available on ebay for a hundred dollars or so but is tall, almost 16 inches, which makes it somewhat clumsy to mount unless you make a floor mounted console. The small Alcatel pump is ideal in size and performance but is harder to find.

These pumps operate on Corning DC 704 silicone diffusion pump fluid. Using this fluid, the vacuum achieved by any of the pumps will be completely suitable for tube making. The use of expensive and critical fluids such as Octoil should not be considered.
Octoil is an obsolete fluid that was originally specified for each of the pumps above. The DC 704 was formulated to directly replace the Octoil and not have the critical problems Octoil has. Octoil is flamable and is far above its flash point when operating at normal temperatures in the diffusion pump. If the system accidently or from operator error is opened to air while the Octoil is at operating temperature, it instantly bursts into flame inside the pump. This can even cause an explosion.
This is a stack from a 3 inch diffusion pump that was using octoil when a gasket failure caused the inlet to be opened to air while the diffusion pump was operating. It took about an hour for this mess to happen, before the break in the system was noticed and shut down. The caked on material has the consistency of epoxy glue! It had to be chipped painstakedly off and the stack sanded to get it clean. Amazingly, the inside of the stack where clear octoil was present with no oxygen there was no burned oil. It was completely clean!