Re-filamenting of Large Transmitting Valves – Eric Vast

A short time after Alamein there was a distinct lack of power valves in the Middle East. As ever The Army closed ranks and blamed the shippers. There was a widely held belief that the last time that ten transmitters had been sent from the UK all the valves had been placed on one ship, all the chokes transformers etc placed on a second, and the rest of the bits and pieces on a third one. The ship carrying the valves was then sunk by the enemy and all the valves were lost. I believe that this did indeed happen once, but the great expansion of the use of Radio during the war was the main factor. Enough valves of the type of the 809 810 812 etc were not being made. Also before the war a lot of this class of valve were made in Italy and they were on the other side.

Civilian Broadcasting in the Middle East suffered most and they were only able to get by getting valves refilamented. This process became a flourishing local industry for such places as the Hebrew University and like establishments. Authority ordained that if they could do it, so should we. I had before the war brushed shoulders with valve manufacture in the course of my job a number of times, and had some knowledge of the art. accordingly a number of duds from Polygon Wireless station down the road,  First of all valves of this type have to have the large copper radiation fins removed. These come off pretty easily using a blowlamp, but it was a bit of a surprise to find that solder was not used to stick the fins to the copper anode, but pure tin. Tin is a very soft metal at room temperature, but up near its melting point at some 210 degrees Centigrade it gets very hard. As we could think of no good reason for changing this we stuck to the tin joint in subsequent work.

Next the valve had to be neatly opened. Our final solution was to rotate it on a lathe at low speed, and cut a groove in the glass with a thin cut-off wheel. An iron wire was laid in the groove, held by a pair of tongs, and this wire was then made red-hot from a battery.. This resulted in a very clean cut. The nasty mess inside could then be cleaned up. Oddly enough, we had no trouble getting new filament rods. The local electric light bulb factory provided these. In the process of producing filaments Tungsten in a form of powder is put through a series of hammer dies, and progressively becomes metallic and thinner. Just a question of choosing when.

The filament mounts on the work that we were processing were of an unknown alloy. However it was found that the tungsten would solder to the alloy using nickel. This work required  a workspace to be flushed with “Forming Gas” a brew of Nitrogen with a small amount of Hydrogen, which stops oxidation during the heating. The hydrogen was acquired from the local RAF fliers of Barrage Balloons, The nitrogen from the local Oxygen Works where it is a waste product. It boils off first from the liquid air which they make, The next problem was a Glassworkers Lathe.

This is a lathe with a driven tailstock at the same speed as the head, which can also be very readily moved along the bed. This enables the two pieces of work to be offered up together while rotating. If the edges of the glass are kept molten the speed of rotation can be adjusted so that the molten edge neither collapses to the centre nor flies out. Glass can be kept molten on its edges by a ring of oxyhydrogen flames. After many trials we gave up using the lathe for two reasons. Firstly we had great trouble in getting both stocks to turn exactly at the right speed. A trace of flutter and the joint cracks. Secondly the jointing process is a slow one, hard glass is a solution  of silica in a lower melting point glass, prolonged heating results in the silica coming out of solution. This makes the join cloudy and weak.

We developed a firebrick oven heated by radiation. Glass becomes very easy to work above a certain temperature. The two sections to be joined were assembled on a turntable for ease of working and were welded together with a tiny oxyhydrogen flame, in an atmosphere of nitrogen. None of these joints ever failed. We had considerable trouble   with the fact that every batch and indeed even valves in the same batch of valves seemed to be  made of a different glass and that after we had opened up the old pumping stub our tubing would not stick. In the end we kept forty different types of tubing. Hard glasses can however be “soldered” with certain soft glasses to pump the air completely out of the valve requires a Diffusion Pump and a Backing Pump.

The former needs a low pressure to operate, provided by the latter. When the world was young Diffusion pumps were made of glass with boiling mercury We used sheet copper pumps with boiling oil. We got ours from a gent in Tel Aviv. I understand that the ones used nowadays use heavy Ions. A diffusion pump directs a stream of heavy particles to trap air diffusing from the valve to where it may be collected by the backing pump. A sort of sequence was evolved. First the backing pump was switched on until it ceased to emit bubbles of air. Then the oil was set boiling until again bubbles stopped. The filaments were then lit and slightly overrun and some more gas would be obtained.

With normal filament voltage the grid was heated by passing a current to it from the filament, Next the anode was treated the same way. Lastly the glass was heated by radiation. Alas the repaired valves only lasted a few days.  At this time I was sent to the UK for a few week. I called on some of my mates in the Mazda works at Hammersmith, where I learned the error of my ways. It takes more than heat to get the air off the inside glass. Air and other things trapped on the surface have to be attacked with high voltage the so-called “Spot Knocking”. When I got back to Egypt we made such a one.

We acquired  a 33KV pole transformer and used it wrong way round. A tripler was made up of some condensers won from the Italians  in Eritrea from an high-power radio station. Some TV high voltage Rectifiers fead the biggest of the condensers. The drill was turn off the filaments make them and the grid positive and the anode negative. Starting at about 10KV one worked up to 80KV and kept on until the bangs stopped.

The valve was then sealed and this usually sent the grid current way up. Left overnight this current disappeared the glass taking it up. We also learnt not  to use Carbon-tet for cleaning, a trace produced copper plating of the glass. Valves with this additional processing lasted well. I never heard the total of repairs as I was shipped home shortly afterwards. My successor had about a year before the unit was closed down.

Eric Vast. circa 2000.

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