SM 5 BSZ - A procedure to recondition power tubes
(Sept 16 2006)

Background



The information on this page is based on my own very limited experience and on information from a few other sources. This page was initiated by SM6EHY, who kindly pointed out to me that this highly relevant information to the experimenting amateur was missing.

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 the procedures below will help to restore a good vacuum.

Part of the procedures may be unnecessary, or misinterpreted by me. I have tried to explain what I believe the different procedures are intended to do and why, but if you have better information please send an E-mail so I can make corrections.


The cathode

The cathode is the "heart" of the tube. There are several types of cathodes, and they have different properties. Read in the manufacturers data book how to treat the cathode of the tube you are going to use.

A pure tungsten cathode will give maximum tube life if the heater voltage is made as low as possible for the desired output power, while a thoriated tungsten cathode may be damaged if it is underheated.

For QBL5/3500, the PHILIPS handbook (1968) says: Maximum life is obtained when the heater voltage is held within 1% of the nominal 6.3 volts (measure directly on the tube pins). Temporary deviations from the nominal value must not exceed 5%.

When the heater is switched on for the first time, it may be important to allow it to reach its final temperature slowly. I have been told that a slow heating, raising the voltage from zero to nominal gradually, over a time of several hours, will increase tube life considerably in normal professional operation. I guess this has something to do with a rechrystalisation that takes place when the heater wire is heated for the first time, but I do not really know.

The vacuum pump

Maintaining a good vacuum is a fundamental problem in vacuum tube technology. Some atoms of various kinds, such as nitrogen, carbon, oxygen and hydrogen may be dissolved in the metals or other materials from which the tube is built. These atoms gradually leak out and form molecules that make a very thin gas inside the tube. When the pressure of this gas becomes high enough, the gas may become ionised and form a short circuit from the anode to the nearest electrode that will be the outermost grid. If the power supply contains a big capacitor, and there is no current limiting resistor, the tube may become destroyed by a single discharge. Even with reasonable protection, a discharge is likely to release more gasses and make further arcing more likely.

In order to remove the gas from inside the tube, the tube is equipped with a vacuum pump. A getter is a surface made from some particularly reactive metal like barium. In a small glass tube, the getter is the shiny surface on the inside of the glass wall. It is very reactive, and will react with more or less any molecule that hits the surface, and form a non-volatile reaction product. If there is a leak in the tube, this surface becomes white.

In a big power tube the pumping may be arranged differently. I do not know how, but as I understand it, the pumping does not work unless the tube is heated. Maybe the gas is bound by chemical reactions that occur at high temperatures only, or the pumping is obtained by acceleration into the metal of ions produced by ionisation of the gas molecules.

In any case, the tube should be brought into operation slowly, and the bigger the tube is, the more important it is.

Reconditioning of a tube, SM6EYH style

1. Apply the heater voltage, but no other voltages. You will probably have to start the fan to avoid overheating. Leave the tube with heater only for 1 hour.

2. Connect the anode through a 50 to 100 kiloohm 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.

3. Now the vacuum is improved, and you can replace the 50 to 100 k ohm resistor by 1 k ohm 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.

4. Now it is time to remove the plate resistor and start to use the tube with some RF.

Reconditioning of a tube, ON4ADN style

1. Apply full heater voltage for 24 hours with no other voltages applied.

2. Apply 33% of the normal plate voltage and adjust the control grid and/or screen grid voltages for the plate current to become about 5% of the nominal current at full power output. Run the tube like this for at least one hour. Be careful to place a fuse rated no higher than twice this current in the anode supply line. Make sure you use a fuse capable of breaking the current at the high voltage. It may be a good idea to use a fuse on the primary side of the mains transformer instead. During this step it is a good idea to disconnect large filtering capacitors from the high voltage supply since arcing will discharge capacitors and dispose all their energy into the tube. If you leave capacitors in the power supply, add a safety resistor for the anode current. This resistor should be in the range 10 to 100 ohms and it should be capable of withstanding a short pulse of 66% of the full plate voltage. Find a big wire-wound resistor with a reasonable separation between the turns.

3. Increase the plate voltage to 66% of the full voltage and adjust grid voltage(s). for the same current as in step 2 while protecting for arcing as in step2. Run for at least one hour.

4. Increase the plate voltage to the full value and set the current as in step 2 with the same protection.

5. remove the small fuse and run the amplifier with 50% of full output power for several hours.

Other alternatives

If you have a variable mains transformer, or other means to run the tube at reduced voltages, you may simply start to use the tube at drastically reduced voltages. Reduce both anode and screen voltages in proportion and start very low, something like 25% of full voltages (and 0.25 squared = 6% power output.

Run the tube like this for several hours (local FM QSO's) and then increase the voltage in steps of 5% of nominal voltage, each time allowing the tube to run with full RF output for several hours at each new voltage level.

When you have reached 80% of nominal voltage it may be wise to allow even more operation time between each voltage step.

The maximum plate voltage for a tube in amateur service is well above the maximum voltage stated for professional use in class AB(SSB) or class C(FM/CW). Look for the maximum plate voltage in class C, anode and screen grid modulation. The plate voltage at the modulation peaks in this mode of operation is twice the given values. Not even an amateur should apply quite that much voltage, but somewhere 50 to 75% of the extra voltage can be used.

As an example, 4CX250B is rated 2000 volts, but with anode and screen modulation it is rated 1500 volts (allowing peaks up to 3000 volts) In amateur service this tube works fine at 2500 volts, but going much above that level may easily cause arcing and destroy the tube.

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