The following text was deducted from information provided by ND2X, UR4LL, SM5BSZ, SM6EHY and W8JI - on the aspects of reviving surplus PA tubes, unused but aged on the shelf.
"As amateurs, we often use old tubes. They may be unused, but may have spent a very long time stored away. A high power tube should work at very high voltages without arcing so it has to have a very good vacuum. When the tube is stored, vacuum gradually deteriorates over time and procedures to restore a good vacuum will be necessary."
Please note that this document is a compilation of individual ideas, experiences, results of experiments, interpretations and suggestions presented by fellow hams. As you will see, the suggested methods all have a personal flavour. In other words, do not consider anything in this document to be irrevocable facts. Go webhunting, there may be new opinions - maybe even some new facts - out there!
The described procedures are valid not only for spares on the shelf. Tubes left unused for years in power amplifiers require the same considerations before activating the PA.
Due to the nature of physical materials, a certain amount of gases are always trapped inside the materials used to construct each tube. A vacuum tube depends on a relatively hard vacuum to function without arcs and other undesired, often disasterously destructive, internal current flow.
While in storage, a certain amount of the gases trapped in its materials is "leached" out into the vacuum of the tube. If one were to plug such a tube into an amplifier and apply all voltages and drive, the small amount of gas within the tube would ionize and provide undesired internal conductive paths; such conduction often reduces an otherwise useful tube to trash.
Fortunately, tube manufacturers considered this situation, and inside each vacuum tube is a metal surface called a "getter" - a surface made from some particularly reactive metal like barium. Being very reactive, it will react with more or less any molecule that hits its surface, and form a non-volatile reaction product. The trick is to get the atoms of gas to move around enough to strike the getter and become absorbed.
PREPARING SURPLUS CERAMIC TUBES FOR USE
The picture shows the jig made by SM5HF, holding GU74B during the process. Forced air cooling is applied.
An external power supply provides 12.6VDC at 4A for the filament and 24VDC for the fan - through two PowerPole connectors.
Ceramic tubes have very good shelf life, unlike large glass tubes. The only source I found to report a difference between ceramic and glass tubes was W8JI. A ceramic tube has - because of the low operating temperature of its external anode - its gettering agent applied to the cathode or filament assembly. This is the only area inside the ceramic tubes that heats enough to activate most common gettering materials.
W8JI suggests on his homepage that for ceramic tubes, steady application of the rated filament voltage during 1 day is all what is needed. The GU74B filament dissipates about 46W (12.6V @ 3.6A) and forced air cooling must be applied during the process.
According to W8JI, the below steps 2-3-4-5 are needed only for glass tubes. However, some surplus tubes may be faulty and do not respond to reviving attempts as expected. Hence, when a reconditioned tube is installed in the PA and subjected to the high voltage (Ua), arcing may still occur and cause damage to instruments and surrounding components.
Therefore, anode systems should have series-resistance to limit peak current in the event of a tube arc or failure. Diode clamps should be installed to protect meters, especially the grid meter since the grid is in the normal path of any internal tube arc.
Before testing the revived tube in its PA socket, we recommend the temporary addition of a 50-100 kOhm series resistor in the anode lead between the filter capacitors and the RF choke. It will prevent any violent current rush caused by arcing. It goes without saying that this resistor must be removed or shorted to prepare the PA for normal operation.
PREPARING SURPLUS GLASS TUBES FOR USE
suggested by UR4LL:
- 1 apply Ufil only, during 12 hours minimum
- 2 ground all grids
- 3 apply 25% of Ua during 4 hours
- 4 apply 50% of Ua during 4 hours
- 5 apply 100% of Ua during 4 hours
to above steps:
To step 1:
As suggested by SM5BSZ and ND2X - bring the filament voltage (Ufil) up slowly, forced air cooling to be applied after the 1st hour:
- 20% Ufil for the 1st hour
- 40% during the 2nd hour
- 60% during the 3rd hour
- 80% during the 4th hour
- 100% for 8 hours
To steps 2 & 3:
ND2X has a variac on the input to his HV supply and would vary the plate voltage (Ua, or anode voltage) as required to obtain the specified percentage for the specified period.
If you don't have a means to vary your HV, you may follow the SM6EHY suggestion: "Connect the anode through a 50 to 100 kOhm resistor. Leave the tube with the plate voltage through this resistor for 2 hours. If the tube has been unused several years, leave it for 24 hours."
UR4LL suggests grounding of all grids before applying Ua in step 3. SM6EHY says nothing about the grids until step 4.
To step 4:
Again, ND2X would vary his Ua using the power supply variac.
SM6EHY suggests: "Now the vacuum is improved, and you can replace the 50 to 100kOhm resistor by 1kOhm and apply the screen grid voltage. Start with a reduced screen grid voltage for half an hour and then apply full voltage and wait for half an hour again. In both cases, the control grid voltage should be adjusted for negligible plate current."
ND2X adds: if you see plate current starting to creep up, you should unground the control grid and apply bias as suggested by SM6EHY.
To step 5:
After step five, UR4LL says: "Tube is ready for use." Step five per SM5EHY: "Now it is time to remove the plate resistor and start to use the tube with some RF."
Most of ND2X's own tubes are from surplus "spares stock" and are several years old. Based on this, he would add more time at each level, rather than trying to shorten the process. At SM5HF the GU74B spare valves are dated September 1978 and October 1979, respectively.
SUGGESTIONS OF W8JI TOM RAUCH
W8JI has designed a number of Ameritron HF amplifiers. He approaches the same matter from a slightly different angle. His views are expressed in full on his home page www.w8ji.com under the headings "Amplifiers/Vacuum Tubes". The below text is an extract from his page. For users of GU74B (4CX800), 4CX250 and similar tubes, the part dealing with ceramic tubes is of particular interest.
Using Old Tubes
Tubes that have been stored for a while often collect gas. This gas either comes from slow leakage through tube seals or outgassing of tube elements. This is the number one problem with old vacuum tubes. The section below describes how to remove gas in old tubes.
Gettering and Arcing
Gettering is very important, since even miniscule amounts of gas will cause a low-resistance arc path from anode to grid or cathode. The normal results of such arcs are blown grid chokes, collapsed anode chokes, damaged meter shunts, and other problems caused by high fault currents. While a few people blame high fault currents on parasitics, it is actually impossible for a parasitic to create such arcs. All the anode and grid can do is deplete the electron cloud from the area of the cathode, and the available current even with a parasitic is limited by the available emission. Uncontrolled arcs are always the result of gas or element alignment in the tube, rather than excessive current from oscillations.
An arc by itself will break down and getter gas inside the tube. This is why an amplifier with a gassy tube may sometimes operate without problem after a sudden tube arc.
Anode systems should have series-resistance to limit peak current in the event of a tube arc or failure. That series resistance should always be in the anode lead between the filter capacitors and RF choke. A diode clamp should be installed to protect meters, especially the grid meter since the grid is in the normal path of any internal tube arc.
Ceramic external anode tubes, because of low anode operating temperatures, have the gettering agent applied to the cathode or filament assembly. This is the only area inside the tube that heats enough to activate most common gettering materials. Ceramic tubes without internal flaws or broken seals can generally be gettered by running the filament at rated voltage for an extended period of time before application of any high voltage. The normal time for gettering is between one hour and one full day. If the tube does not getter within a day it is most likely never going to be restored to a relatively pure operational vacuum. Ceramic tubes have very good shelf life, unlike large glass tubes.
Glass internal anode tubes generally have the gettering material coated on anodes, which must be operated at high temperatures to activate the getter. Glass tubes have a propensity for seal leakage and element out-gassing, both of which lead to a short shelf life for large tubes. It isn't the glass that leaks gas, but rather a Kovar alloy used to bond the glass to the metal protruding through the envelope.
Kovar is also subject to rusting. As odd as it seems, glass transmitting tubes should be stored in a dry location. Glass tubes should be operated at full temperature every few months.
Under some conditions a glass tube can be restored to operation by running low anode voltages, and positive bias on the grid. This will sometimes allow full operating anode temperatures to be reached, and the tube can be "cooked" for several hours. W8JI had about a 50% success rate restoring old 3-500Z's that have sat for years without use. Even though they initially arced severely at full voltage, by cooking them at low voltage and positive grid bias to show anode color, vacuum was restored.