Radio Valve Repair Tips
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Written by Herm Willems 1992, translated by Wouter Nieuwlaat 2002.

1: Determining the proper connections.

When a non-transparent valve has come loose from its base, with the connections broken, we face the problem: which wire to which pin? A solution has been found. First, take a ohmmeter and determine the filament wires. Supply the valve with heater current, for example with crocodile clamps. If the value of the filament voltage/current is unknown, start with a very low level, 0,5 V for instance. Set a regulated D.C. supply at 0 Volt. If your supply has current limitation possibility, set it to about 5 mA. On a directly heated valve, connect the minus to one of the filament connections, and the other wires one by one with the supply's +. Now, carefully raise the voltage to a few volts. The wire through which the highest current flows is the first grid. If the valve has multiple grids, the second and following grids each have a lower current than the previous, because they are separated further from the cathode. Thus, the anode pulls the lowest current. Usually, the voltage will have to be raised to 100 V or more to measure the outer electrodes. This way, you can detect and mark every connection of the valve.
Take care to keep the voltage as LOW as possible, especially in the beginning
Because we are temporarily using the grids as anodes, we have to take care not to destroy them by letting them take up too much current.
If you're dealing with a indirectly heated valve, you will have to try just as long until you find a noticable emission. The wire to which you connected the minus is the cathode. For the rest, act as described above. 
This method can also be used to determine the pin configuration of a unknown valve.
And, last but not least; on a valve of which one of the electrodes has a loose contact with the corresponding pin or notch. This way, we can concentrate on the "guilty" connection, saving us the trouble of soldering all pin connection over.

2. Re-attaching the base to a valve

Of course, you can try to aim the wires through the pins, but that way you can never be sure that it is connected well, risking to have to take the base of again later. A better method can be found in stripping a piece of flexible power chord, and using one of the copper wires to solder to the original  connection wire. solder as thinly as possible. Be sure the pins are free. You can heat them with a pencil torch, and blow trough them, freeing the way for the wire. Put the wires through the pins, and gently pull them through one by one. Put some powerglue on the base, and put it together. Bend the wires and solder them to the pin. The surplus wire can be cut off. This way, a good connection is ensured.

3. Fixing the filament 

If you run into a valve like the 5Y3G or AZ1, of which the filament has come loose on one of the point-well spots, (can be diagnosed with a ohmmeter connected over the filament connections, tapping on the balloon from several directions) you can usually fix it up, using one of the really old tricks; charge a 50 uF electrolytical to 250 volts. Connect it to the filament wires. Needless to say that touching the wires can give you a severe electrical shock!!! Firmly tap the valve several times. At some point, you will see a small spark, and the filament is closed again. Unfortunately, on accumulator lamps like the A409, this method is of no use. If valves like these have a broken filament, there is nothing left than waiting for a successor of mr. P. Middelraad from IJmuiden Holland, who could open the tubes, repair it, and close it again!!

4. Making a new top connection.

On a valve like the E442, the top connection is prone to get loose, and misses entirely sometimes. It can be repaired optically (and electrically, of course...) by filing away some of the glass around the wire. file away as little as possible!! solder a small copper wire onto it. If the original cap is still there, you can attach it with conductive glue, if not you can easily make a replacement; put the wire into the slot on top of a M3 bolt, and solder it together. You can use a toothpaste cap to make the new cap. put the cap on top of the bolt, and glue it all together. A better-looking replacement can be made from a "duracare" lens fluid bottle. The top nut can be made by heating a M3 nut with the soldering iron, and pushing a dust cap of a bicycle vent over it. The valve is ready for its second life!!

Part 2; written by Hans Hilberink, translated by Wouter Nieuwlaat

5: Refinishing a valve with conductive paint

A lot of old radio valves still work fine today, despite their age. Unfortunately, most of them don't look that attractive anymore; dust, dirt, grease, moist and a lot of other residues have attached to the bulb. By the dirtyness of a tube, you can usually tell how moven its life has been during the decades. I have come across upon valves that must have spent the first half of their life in a restaurant, and the other half on a dusty attic. These were covered by a thick layer of smelly grease, on top of which an even thicker layer of dust and spiderwebs had gathered.

When you're handling a valve with a clear glass bulb, cleaning is nice and simple; usually resulting in a beautiful specimen that reveals all of its secrets.

Handling a valve, that has originally been painted with conductive paint, is a totally different thing, however. Especially the golden and silver-gray paint types, commonly used in the thirties, have hardly survived the years. If you want to take it out to restore the radio, you are faced with a dilemma; grabbing the valve to remove it destroys the original layer of paint, leaving it will interfere with a thourough restauration of the radio.

In my opinion, the best thing to do is to take it out very carefully, (for instance by prying the lamp base with a cotton-wound screwdriver) and try to keep the paint as intact as possible. A fixed base on a wooden block can be used to protect the valve from rolling around on your desk. That way you can also make good photographs, in case you want to start over with the exterior.

For the correct colour of the new paint, it is crucial to keep a flake of the original paint. If the original layer has dusted away, you can also take a similar valve with its paint still intact.

Unfortunately, it's almost unavoidable to use modern paints for the new finish. Sometimes the original paint composition is still known, but most of the ingredients used in the twenties and thirties are hardly available nowadays, or even forbidden by environmental protection laws. Try to make a schellac-based paint; only the shellac has become sheer unaffordable, let alone the other ingredients!! 

As you have chosen a paint that suits the job, you still have to tackle the "conductivity" aspect of it. Mostly, the conductive paints were used to give some sort of shielding against outer influences, causing interference. With modern laquers, this is not archievable, since they are all more or less insulative. 

The conductive paint on a radio valve is very important for its proper functioning!!

Once I put a Philips "Pionier" V6A and a Philips "Ouverture" 461A on my workbench, and tested the difference between a original HF valve, and one missing its paint. The "naked" one had less amplification, a strong tendency to oscillate, and suffered a lot from some kind of microphonia. I also got the suspicion that the separate stages of the radio were influencing eachother more than usual. Knocking on the cabinet gave wonderful effects.

A good and simple solution for this problem is to prime the tube with some sort of conductive ground layer. A excellent result can be reached with "graphite spray", made by Contact Chemie. They also make Contact 60, 61, WL contact sprays, and a lot of other useful spray fluids. The Graphite spray is contained in a pressure spray can, and can simply be attached. use tape to cover the base and top-contact (need to be insulated from earth!!) spray 2 to 3 layers, each time let it dry in between. If it has dried out sufficiently, you can re-attach the earthing wire around the base. Afterwards you can put on a few layers again, and let it dry completely. If all has gone well, the conductive layer now contacts the earthing point of the radio. If you put the valve into the radio, put one probe on the chassis, and one probe on the valve, the reading should be close to 0 Ohm. Now, the coloured layer can be put on, any method will do. If you have a steady hand, you can also try to make a new type number print on it.

Now, the valve is in good shape, and working the way it should again!

Several radio valves I handled this way have started their second life, and are doing their job in nice old radios again.