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True Blue Glass

There’s only one sure thing in the wonderful world of vacuum tubes and that is, no matter how much you may think you know about these fascinating thermionic devices, there’s always something new to discover. Take this Mullard ‘EC91’ pictured above—what’s with the transparent blue paint on the glass envelope? Well, you may already have come across tubes with blue glass envelopes, such as the JJ ‘EL34’. JJ have painted the glass blue to make the tube look different, to differentiate their tubes from those produced by their competitors. This is no recent phenomenon. This marketing ploy has been utilised in the tube industry right from its beginnings back in the early 1900s, for instance, by the tube manufacturing company called “Arcturus” (almost certainly named after the brightest star in the northern celestial hemisphere) whom produced many beautifully styled blue tube types in their time.

Further, Brimar also used blue paint to identify their “blue-tip” premium tubes which were intended for use in medical and scientific applications. However, this Mullard ‘EC91’ has been painted blue for an entirely different, and more interesting, reason—in this case the paint isn’t cosmetic, it’s functional. Inside the ‘EC91’ the anode plate wraps around only part of the cathode/grid structure allowing a significant number of electrons pass through the grid wires and gaps in the anode. The electrons penetrate partway into the inner surface of the envelope causing a gradual release of gases trapped within the glass, which in time impairs the efficiency of the tube ultimately shortening its life. To minimise this effect and improve long-term reliability the centre part of the glass envelope was coated by a vacuum deposition process with a special material which prevented the electrons from penetrating the glass.

So there you have it—there are more things dreamt up in our philosophy or, more accurately, science to keep Effectrode busy creating new effects pedal designs for another hundred years, probably longer. But I don’t think I’ll reach the ripe old age of 152 so, alas, these yet-to-be-designs will remain undiscovered country… for me at least. Incidentally, the ‘EC91’ is a single triode tube, whereas the ‘ECC83’ (12AX7) is a dual triode tube. Like the ‘ECC83’ the ‘EC91’ has a gain factor of 100, but can supply a much greater current of 10mA—so the ‘EC91’ combines the high gain factor of the ‘ECC83’ and the current capability of the ‘ECC81’ and would be an excellent choice of tube for use in a hi-fi or guitar amp preamp stage or, a tube effects pedal for that matter…

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