accelerates the evaporation of free barium from the cathode surface causing emission slump early in life. In addition, free barium may be deposited on other tube elements and parts, resulting in emission from these elements and the possible development of leakage paths between elements. At low cathode temperatures, free barium may not be provided at a rate sufficient to maintain good emission and damage to the cathode results." (Our italics.) Click here to read what this paper had say about sleeping disease.

   I used to use standby switches that halved the heater voltage and quartered the idle current. No longer. I switched to killing the heater voltage while preserving the full B+. It sounds dangerous, but it works well. For example, I built a tube preamp that housed a phono and line stage. But phono stage was used only occasionally. The thought of needlessly wearing out four expensive tubes bothered me much. So I added a switch to turn on the phono stage's heaters only when that stage was needed. The B+ remained on constantly (the high voltage  was mitigated by the fact that the topology used was the constant current draw type with a protecting diode between the cathode follower's grid and cathode, which limited the cathode-to-plate voltage to 150 volts for all tubes).
    As for the 8 tube output stage, you are right about the hassle evolved in regulating this heater supply, as it draws 20 amps. I have a bench voltage regulator that can put out 20 amps and it weighs almost as much as your amplifiers. But since hum is not your concern and extending life expectancy is, I would look into current inrush limiting with thermistors or maybe a two stage heater turn on. This could be accomplished by, at startup, connecting the heaters to one outside transformer winding lead and the transformer's center-tap (if it has one). This way the heaters would see only 3.15 volts until you switched them over to the full 6.3 volts.

Subject: Standby
   Hello Mr. Tube cad, I have a request following on from "heater concerns". Running pre amp and or driver tubes on standby heater voltage i.e. 4.5 instead of 6.3 is easy enough if you already have a regulator in place - simply relay or switch an extra resistor in the "adj" leg divider network.
   What about a methodology for a bunch of output tubes? I have an OTL with 8 - 6AS7G and regulated DC on that lot would not be easy or cost effective. Is an AC method viable. What's your opinion on the benefits. The thing is virtually dead quiet now but what about life improvement expectancy?
   I like the idea on my rare 12SX7 inputs, but the outputs are more driven by the cost of replacing them sooner vs. the cost of implementing some sort of huge DC control circuit.


 Standby mode can damage a tube, if the tube's cathode is hot but draws no current.   "Sleeping disease" is the name given to the cathode interface problem that occurs when a resistive layer develops in between the cathode emitting surface and the metal cathode sleeve. Radically under heating a cathode can also cause   problems. Here is a quote from a 1957 Sylvania white paper covering the optimal use of subminiature tubes:

Operation of tubes at their normal cathode temperature, as determined by rated heater voltage and current, is essential if optimum reliability is to be realized. Operation at other than recommended conditions results in abnormal cathode temperature and may be reflected in poor performance and/or shortened life.

Effects of Abnormal Cathode Temperatures
    Abnormally high cathode temperature



pg. 17

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