which looks dull red. Temperatures much above 1100K generally cause a short life, while those below 960K are very susceptible to poisoning of the emission, and require careful attention to maintain a very high vacuum. The thermionic valve is normally operated with its anode current considerably less than the maximum emission produced by its cathode. In the case of one having a pure tungsten filament no damage is done to the filament if all the electrons emitted are drawn away immediately to the anode. This is not so, however, with oxide coated cathodes and these, for long life and satisfactory service, require a total emission very much greater than that drawn under operating conditions. In such a case a cloud of electrons accumulates a short distance from the surface of the cathode and supplies the electrons that goes to the anode. This space charge… forms a protection to the cathode coating against bombardment and high electrostatic fields. ..... If the electron emission from the cathode is insufficient to build up this space charge, the cathode coating is called upon to supply high peak currents that may do permanent injury to the coating and in extreme cases may even cause sputtering or arcing"
   "An oxide-coated cathode, operated under proper conditions, is self-rejuvenating and may have an extremely long working life. The life is,
therefore, largely governed by the excess emission over the peak current required in normal operation...."
   ".....Under normal conditions a valve should be operated with its filament or heater at the recommended voltage ; in the case of an oxide coated valve it is possible to have fluctuations of the order of 10% up or down without seriously affecting the life or characteristics of the valve ... If the filament or cathode is operated continuously with a higher voltage than that recommended, some of the coating material is evaporated and permanently lost, thus reducing the life of the valve. Moreover, some of this vapour tends to deposit on the grid and gives

rise to what is known as grid emission when the grid itself emits electrons and draws current commonly known as negative grid current ....."
  "During the working life of the valve, its emission usually increases over the early period, reaches a maximum at an age which varies from valve to valve and one manufacturer to another, and then begins to fall. The user does not generally suffer any detriment until the emission is insufficient to provide peak currents without distortion."

   Phew ! Wish my darn OCR was working ! Anyway, to back up the claim of +-10% operation, it is interesting to note that valve radio manufacturers (UK!) generally provided tapings on mains transformers for 200-220v, 220-240v and 240v to 260v, i.e. +-5% settings !
   The bit about "self rejuvenation" of the oxide coating makes sense ; part of the manufacturing process involves activating the coating by heating it up. Indeed, a common trick in ye olde days for dulled TV tubes was to use a gadget to stress the cathode in such a way as to attempt reform the surface (but was no more than a kill or short term cure).
   Personally, I'd stick to the manufacturers recommended heater voltages. Far better to seek maximum lifetime from ensuring the tube is operated within its proper ratings (underline that bit for the benefit of guitarists!) and, as you highlighted, some resistance in series for power-up surge protection (lets face it, if you're OTL "Blast furnace" is already chucking out 400W are you really going to notice 5W or 10W extra loss in a couple of resistors?)
   Oh, and on the subject of heaters, it surprises me how often people ignore the heater-to-cathode rating. I've seen at least one OTL with one cathode at -170v sharing the heater winding with a driver stage valve cathode of the order of +350v. Eek ! Anyway, keep up the good work.

Jon