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    Ralph Spracklin
    CVRS Member

    Spray Shield Tubes
    Most radio restorers, are aware that Rogers had their own brand of vacuum tubes, some of which they sprayed with a black metallic coating, which done away with the need for snap-on metal canisters to fit over the tubes. This of course to eliminate radio frequency interference in certain areas of the radio circuit. For ever so long I thought that this was something peculiar to Rogers. But I was surprised to learn that they were not the only radio manufacturer to use this trick. Telefunken also had their own line of Spray-Shield tubes. Mullard, Philips and maybe others, also manufactured spray coated tubes. Some of those were spray-plated to look like gold, and others were spray-plated to look like silver, and I have actually seen red tubes. Some say, Rogers sandblasted their tubes prior to coating to achieve proper adhesion, however I have seen a couple of Rogers tubes which the coatings had lifted???? I do know that Telefunken had a pealing or lifting problem over time, as did others. Some manufacturers also sprayed the base of certain tubes. And, apparently some Manufacturers applied a negative potentials to the thermal spray to keep any possible cathode rays, from interfering with the internal workings of the tubes as well as to prevent emission of electrons from or thru the walls of said tubes. This information published by the Institute of Radio Engineers in 1933. It is interesting to note that some of those tube manufacturers mentioned above, used coatings that was sprayed on with the use of an Oxyacetylene Blow Torch. I am not certain what that process would look like. but in my minds eye, I should like to see that operation.

    If you are thinking of coating some of your own tubes a company called M G Chemicals have a product MGC-841, that should fit the bill. I’m not certain what colors it comes in. Amazon sells it when available, and it comes in spray cans, but it is pricey and requires some safety precautions when using this product. I have not used it, but it is a process; that I, myself would take outside.

    Speaking of tubes and coatings, should you want to blacken up the coating on your own Rogers, Spray Shield Tubes, the black laminations on your power or output transformers, or to blacken-up the back side of your speaker cones, just squeeze a little black liquid shoe polish into a small container and apply it with an artist brush. The tube or speaker will look like new. As for me myself, I do not apply it to the front of the speaker, one does not see that anyway. The product I use, by Brillo and available at Walmart, will blacken the cone, and should there be any white markings or numbers on the cone, you will still be able to read them. Sometimes it can over-power the tube numbers on the tube itself. So go slowly and do a trial test as you go!

    Regards
    Ralph

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