AMATEUR VACUUM TUBE MAKING
The vacuum necessary for the vacuum tube to operate is extreme. It takes carefully designed equipment to achieve that level of vacuum. The production of proper levels of vacuum will be one of the most challenging parts of setting up the equipment to make vacuum tubes. Since the vacuum system will be used constantly for tube making, it is advised to make the system as nice as you can afford. A clumsy to operate system can cause tube failure, and in some cases certain kinds of tubes will not be able to be successfully made. First, lets examine just how much vacuum is necessary for making a vacuum tube.
PRODUCING THE VACUUM FOR TUBE MAKING
The chart above shows the range of vacuum that will be encountered during tube making. The left side of the scale is the lowest vacuum that will be used and the right side shows atmospheric pressure.
Observe the range the vacuum tubes operate. For optimum results, the final vacuum achieved by your system should be between 2 x 10-6 and 2 x 10-5 torr. This is not particularly difficult to do if certain design parameters are followed. Vacuums below 10-6 torr are not necessary and spending extra money to achieve lower levels of vacuum is not productive.
Note that the range of operation of vacuum tubes cannot be reached by a mechanical pump alone. Even the best mechanical pumps cannot pump below about 2 microns in any reasonable time period.
Thus, a second type of pump is necessary to pump the pressure down into the area where the vacuum tube operates. The only practical pump for doing this is the diffusion pump. Other pump types are available such as turbomolecular pumps and cryogenic pumps however they are much more expensive and in some cases dangerous to operate.
By using the diffusion pump, the pumping requirements of the mechanical pump are substantially relaxed. Most diffusion pumps will operate with an outlet pressure of 50 microns, and some really nice pumps will operate to 500 microns. Most two stage mechanical pumps will easily pump to 50 microns to operate the diffusion pump.
The characteristics of pumps are covered in other pages of this site. (see index.)
The drawing above shows the minimum system that you can get by with in tube making. Note that there are no valves and no gauges. While this would work, it would be extremely clumsy to use and you would not have any idea on what pressure you were working at.
Here we have a more practical system. We have a complete set of valves and a set of gauges.
Valve 1 is used to bleed up the system when it is shut down. This prevents the vacuum in the lines from drawing oil up out of the pump into the lines over time. This valve is closed when the system is operating.
Valve 2 controls the outlet of the diffusion pump. When the diffusion pump is on, this valve is open to keep the outlet pressure of the diffusion pump below 100 microns.
Valve 3 is the roughdown valve for the outlet port. This valve is opened to quickly pull the tube down from atmospheric pressure to below 100 microns to where the diffusion pump can then continue pumping. This allows changing tubes while the diffusion pump is hot.
Valve 4 closes off the outlet of the diffusion pump during roughdown so the diffusion pump can be left operating and ready to continue pumping.
Valve 5 is a needle valve. It is used in nixie tube and gas tube making, and in fusor vacuum systems. It allows careful adjustment of the pressure in the tube or fusor chamber. Valve 4 is closed and valve 5 is used to allow slow pumping of the tube or chamber.
The baratron on the diffusion pump outlet line monitors the pressure to make sure the pressure is within the operating range of the diffusion pump. This is desired because if the pressure is too high, the diffusion pump can overheat and burn out.
The baratron on the diffusion pump inlet is used to monitor the roughdown of the tube to make sure the pressure is below 100 microns before changing over to the diffusion pump. Pressures above 100 microns can stall the diffusion pump and cause it to overheat.
The ion gauge is used to measure the high vacuum while the diffusion pump is pumping the tube. The position of the ion gauge is on the inlet of the diffusion pump. This is so that the gauge can be left under vacuum continuously while the system is shut down. The gauge takes hours to stabilize after being exposed to atmospheric pressure so it is desired to keep it under vacuum all the time.
While this system is useable, it is prone to operator error. It is easy to forget to close a valve before opening another thus causing a system malfunction. For example, valve 3 and valve 4 will never be opened at the same time. Likewise, valve 1 will never be opened while the system is operating. Valve 2 and valve 3 will never be opened at the same time. Valve 4 will never be opened while valve 2 is closed.
The operator must be very careful to always control the valves in the correct sequense to prevent system malfunction or even system damage.
The ideal system uses electrical solonoid valves for each valve. The control circuitry is then arranged such that incorrect valve positions are not possible. This is more expensive to do of course.
It is possible to use solonoid valves for some of the valves to prevent serious errors, such as opening valve 1 while the system is operating. Or, leaving valve 2 closed while valve 4 is open, which can cause the diffusion pump to burn out.
High vacuum valves are somewhat different in design from ordinary liquid valves. The major difference is that the valve actuator stem is usually sealed with a hermetic bellows seal. This is a true hermetic seal that has no leakage. Valves with packing seals can be used for high vacuum work if they are properly prepared. There will always be some leakage through the packing but it can be so low that it is not a problem.
Refrigeration valves can also be used successfully for high vacuum work. Many of these valves use a diaphragm seal on the actuator which is truely hermetic. The main disadvantage is usually these valves do not open particularly wide. This can lead to the pumping being choked off and slow if used in the inlet of the diffusion pump. They would be suitable for the micron pressure range parts of the system just fine. (valve 1, 2 and 3 in the drawing)
Valve 1 is a high vacuum bellows valve that is suitable for the inlet valve of the diffusion pump. There is no leaks with this kind of valve.
Valve 2 is a small high vacuum bellows valve that is suitable for the roughing valve of the system.
Valve 3 and 4 are smaller bellows valves that are suitable for low flow areas in the system.
Valve 5 is a packing type needle valve. It is suitable for gas manifolds. The packing is teflon so the leakage is minimal.
Valve 6 is a typical refrigeration valve. It is suitable for use in less critical areas, but has more leakage than other types.
Valve 7 is a ball valve. The valve uses a polished ball with a hole throuth the center to seal the valve. These can be used in less critical areas, but are subject to leakage while they are being activated.
THE INSIDES OF THE TWO BELLOWS VALVES ABOVE.
CONNECTING TO THE VACUUM SYSTEM
New users sometimes make drastic mistakes when making connections for vacuum use. The use of rubber or plastic tubing for example. Rubber and plastic tubing is not suitable for use at pressures below about 50 microns. The outgassing of the tubing will slow the pumping speed down to where it is useless. This effect becomes overwhelming as the pressure is dropped even lower.
Thus, all equipment connections should be made of metal or glass. Limited use of plastics like delrin, teflon, plexiglas, and other hard plastics can be done for small parts that will only temporarily be in the system, but all permanent system components should be made of metal or glass. All common construction metals are suitable for pressures down to 10 -6 torr. Pressures lower than this are not necessary for tube making so they will not be covered here due to the much more stringent requirements that add very high cost to the system.
Brass and aluminum are the most economical metals to use. Stainles steel and plain soft carbon steel are useful but are substantially more difficult to machine.
The industry standard for vacuum connections are the simple but efficient K clamp connectors. These use a rubber O ring seal and are effective to pressures down to 10 -6 torr. These flanges are easy to connect and disconnect in tight places since wrenches are not necessary. They are available in sizes from about 3/4 inch across to 3 inches across. The cost of these flanges can be quite high for an amateur system but surplus is available on ebay regularly.