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Forum » Valves / tubes - Semiconductors » Valves / Tubes; practical use and procurement » TUBE REACTIVATION / REJUVENATION
Bruce Morgenstern
CDN  Articles: 11
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30.Dec.13 01:43

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When a tube has lost its electro-emitting capacity and is partly or wholly paralyzed so that it passes a very small plate current it can often be restored to an active condition by a process of reactivation or, “Boosting.” It should be remembered, however, that only tubes having a thorium-impregnated filament can be reactivated, and fortunately, most of the tubes in use are of this type, except the WD11 or C11 and WD12 or CX12 types, which have oxide-coated filaments.

The electronic emission of these tubes, that is, their plate current, depends upon the presence of a layer of thorium atoms on the outer surface of the filament. The filament is not thorium-coated, however, after the manner of the oxide-coated filaments, but is rather, permeated throughout its whole substance with this rare element, thorium. 

In case of thoriated filaments, the coating of thorium on the surface gradually gives out and/or evaporates. This would correspondingly reduce the emission current and render the tube very short lived were it not for the fact that the thorium is continuously replenished from the interior of the filament. As long as the filament voltage, in normal use, is not raised over ten percent above the rated value this evaporation and replenishing continues at an equilibrium rate, so that the constant layer of thorium is maintained on the surface.  

When subjected to an over-voltage on the filament, however, the evaporation becomes excessive so that the thorium surface layer is partially or completely diminished, and the tube accordingly more or less paralyzed. Operating these tubes at subnormal voltages is also liable to paralyze them slowly as the filament temperature is then so low that the process of boiling out the thorium from the interior of the filament becomes abnormally retarded. Hence, it is important that the thoriated filament tubes be run at their rated filament voltages. It may be noted here that the maximum life of the “dry Cell” tubes is attained when they are operated with a voltage of 3.3 volts across the filament.

While the great majority of thoriated tubes after a long and useful life gradually die a natural death, others are rarely executed by excessive voltages. In either case, if the filament is not actually burnt out, the chances are very good that the tube may be restored to life and vigor by the simple process of reactivation.

By reactivation the thorium inside of the filament is brought to the surface, thus permitting the tube to function normally again. Since the symptoms exhibited by exhausted tubes and weak B-batteries are practically the same, the owner / service man is apt to ascribe the condition of his set to the latter, and to change the batteries, when it is really the tubes that are at fault.


If it is found necessary to reactivate tubes every month or at frequent intervals, this is proof that the tubes are being overloaded, and in such cases the following precautions should be observed: Do not burn the filament voltages in excess of the rated filament terminal voltage. Keep the filament rheostat set as low as possible. Do not us high plate voltages unless proper grid bias is provided. Without a grid bias, it is best not to exceed a potential of 67 ½ volts on the plates of the radio-frequency and audio-frequency tubes, while the detector-plate voltage should not exceed 45 volts. 


If one is not sure that the tubes need reactivation, a simple test of the tube emission or plate-current flow, will show its condition. When plate current of the UX and CX tubes, given in the following table below, drops to the values indicated under “Minimum Emission,” reactivation is necessary. 

To do this, the circuit shown below is used. The grid and plate are tied directly together and then joined to the plus terminal of the B battery through a milli-ammeter. The negative B battery terminal is joined to the negative end of the filament and a key switch, normally open, is included in the plate circuit. The voltage across the filament should be adjusted to the values specified in the table, which give also the proper values of B battery to use with different tubes. Now depress the key just long enough to obtain a reading of the emission current in MA. (Disregard the change in the voltage reading caused by the emission current.) If the emission current obtained under these conditions is zero or any value less than the minimum specified in the table, the tube can, undoubtedly, be improved by reactivation.    


Tubes which have internal short circuits between elements cannot be reactivated and it is advisable to check for such conditions with a C-battery and a pair of phones, as shown below, before attempting to reactivate them. Today, a good ohm meter would suffice. If a click is heard when touching the grid and plate terminals of the tube, or when touching the grid and filament terminals, there is a short circuit, and the tube should be discarded. This test should be made when the tube is not connected in any circuit. Tubes in which the vacuum is impaired cannot be reactivated. This is usually indicated by a filament-current reading above the rated value. If considerable air is present in the tube, the filament will not light up at all unless the filament voltages are raised well above normal, in which case the filament will usually burn out.



There are two methods which can be used to reactivate tubes which have been overloaded and sometimes even tubes that have dropped in emission at the end of their normal life. The exact process for best results depends on the extent of the overload to which the tube has been subjected. Tubes that have been subjected to only a slight overload may be reactivated by a simple process, which consists in burning the filament with the plate supply disconnected. This “boils” out the thorium from the inside of the filament and provides a new layer on the surface. The electronic emission from the surface of the filament is negligible while this is being done. The length of time required to reactivate a tube by the process is usually from ½ to 1 ½ hours, depending entirely on the condition of the tube. Occasional tests to ascertain the plate emission should be made, and in this way one can readily determine when the tube has been brought back to normal. Any radio owner can reactivate his tubes in this way without the need of special equipment.

Tubes that have been badly overloaded may not improve noticeably when subjected to this treatment, and it will then be necessary to use another method of reactivation, employing the use of a re-activator. The tube must first be “flashed” from 10 to 20 seconds. This is merely burning the filament at a voltage indicated under the heading of “flashing voltage” in the table above. Then the filaments must be burned at the voltage given under the heading “aging voltage,” for about 30 minutes. Test the tube for emission, and if it is not restored, burn the filament at the aging voltage for about 2 hours. If the emission is not normal after this has been done, the tube is permanently impaired and cannot be reactivated. No plate voltage is ever applied during reactivation. It is advisable to be certain of the exact voltages applied to the filament by means of a suitable voltmeter. A small percentage of tubes that are reactivated will burn out when the flashing voltage is applied.


Reactivation of tubes by the “while-you-wait” method is not recommended, for in such cases the work is usually done very quickly, sometimes within 10 minutes, and higher voltages than those given the table above are used.  This procedure materially shortens the life of the tubes, and tubes reactivated in this way soon fall back to their minimum emission value. Besides, the use of higher voltages greatly increases the possibility of tubes being burned out.


When this  original data was written, in 1927, there were several types of tube re-activators on the market. One of these re-activators can be viewed on the Radio Museum Site  ID=213117, Sterling, Tube Tester & Reactivator R-510.

Using Storage Batteries:

It is possible to use two 6-volt storage batteries connected in series, so that a flashing voltage can be obtained. A 6-ohm rheostat is used to control the voltage and a 0-50-reading voltmeter is connected, as shown below.

U-Build Tube Re-activator:

A tube re-activator consists of a step-down transformer, a socket, and a voltmeter. The Transformer is tapped so that a flashing and an aging voltage can be obtained. All the equipment necessary is a toy train transformer, tapped for various voltages; a 6-om radio rheostat, a 0 to 50 reading voltmeter and a radio tube socket. These instruments are neatly fastened to a bakelite panel, attached to a small wooden box, and are then wired up according to the diagram shown below.  


It was found that tubes could, on the average, be flashed and recovered six or eight times before showing any decrease in the saturation current, which would indicate a deterioration of the filament. This deterioration, however, was not rapid, a dozen flashings serving to reduce the saturation current by only 8 or 10 percent. This does not mean, however, that the total life of a thoriated filament properly used may be increased tenfold by reactivation. Reactivation might be expected, perhaps, to triple the useful life of the tube.

Bruce Moergenstern


Radio Trouble-Shooting, By Enno R. Haan, E.E. Written in 1927 and published 1928.

The General Radio Experimenter, April 1927, Article by Horatio W. Lamson, Engineering Department.

E.T. Cunningham, Incorporated.













This article was edited 30.Dec.13 02:27 by Bruce Morgenstern .

David Hennen
USA  Articles: 1
Pict.: 25
01.Nov.20 18:28
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Could a "reverse sputtering" of thorium atoms from other sections of the tube to the filament by applying a high negative voltage to either the grid and/or plate work to restore a filament with more thorium atoms? I heard someone say that applying -300 volts to the plate with a normal 5 volts to the filament restored good performance in a weak 01A tube.


This article was edited 09.Nov.20 22:04 by David Hennen .