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RE: Idea for Prolonging Tube Life

Some primary reasons for premature tube failure is over-zealous design (running the tube near its maximum dissipation limits) and cathode stripping.

Many designers, especialy nowadays, run tubes way beyond the CCS rating for dissipation of the respective components, be it plate, screen or grid dissipation. For instance, the absolute maximum plate voltage for a 12AX7 is around 330V, but it is very common to see voltages around 370v or even higher. These higher voltages are used to increase gain yet also increase signal to noise ratios as well, with the compromise of higher distortion. Usually NFB is employed to compensate for these distortions and then creating a problem of increased dissipation on grids as well. Higher plate voltage is also used to improve plate impedence matching with stock transformers currently available (of which most have fixed values). For the best distortion figures and long term reliability a plate voltage of just under 300v is best for this tube, 285v being ideal.

It is very common to see this "hot-rodding" of tubes, especially in Class A high gain designs. NFB schemes can, if not implimented well, can exacerbate the problem. While these designs work well and sound warm (primarily due to higher & pleasant sounding THD), they tend to make tubes fail faster. It's kind of like running around town in your car in first gear all the time because it sounds cool. It may sound cool but your engine isn't gonna last long.

Cathode Stripping comes from applying plate voltage before the emission cloud has a chance to fully form by heating. Applying plate voltage before the tube reaches its quiescent temperature causes the negatively charged material used to offer up electrons on the cathode to directly strip off the surface of the cathode and fly to the high level positive charged plate. Eventually this stripping of material is substancial, leaving not nearly enough free electrons on the cathode to allow for full emission. This is the purpose of a standby switch, something you see less and less of in modern tube gear, and a fatal design flaw in my opinion. Plate voltage should only be applied to a tube after it has reached its quiescent operating temperature. This is responsible for about 80% of tubes failing.

Keeping a tube cool is not always going to extend its life. In fact if cooled too much it may actually lead to the same cathode stripping by lowering the quiescent temperature below that which causes emission cloud formation. Only in adverse conditions where it is recommended to cool a tube by external means is this advisable. Cooling all tubes in all cases for longevity is a classic audiophile myth.

There are some circumstances where this is good, like in tight cabinet designs where many tubes are closely spaced and ventilation is poor, or in cases where individual tubes are dissipating at very high levels, like in the hundred of watts each. You see these conditions more in communications gear and not very often in audio designs. Most modern designs the tubes are in the open. Tubes that operate at those higher levels (813, 833, 572A, 811A, etc) generally have described in their specs as to the proper method for cooling. But in most audio designs you do not see this and most regular convection cooling, on most normal commonly used tubes in the open, is usually all that is necessary, and in fact recommended.

Incidentially, Cathode Stripping due to plate voltage being present is also why it's bad to use a variac to "bring up to voltage" an old vintage piece of gear and NOT remove the tubes first. This creates massive cathode stripping in all the tubes. The plate voltages comes up to reasonably high levels and the heaters do not, or at least high enough to form a cloud from cathode heating. Also running tubes at lower filament voltage causes the filament to draw more current than it is design to handle and the heater may fail as well. Remove the tubes first, then bring the gear up on the variac. The capacitors in the supply will charge regardless of whether the tubes are present.

Any leakge of gas, of any kind, as you propose would radically alter the overall perfomance of the tube and would be evident by loss of the getter material within the tube. This issue you mention does exist (gassy tube) but always renders the tube useless long before the cathode would break down as you theorize.



Edits: 11/25/12 11/25/12 11/25/12 11/25/12 11/25/12

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  • RE: Idea for Prolonging Tube Life - jrlaudio 00:01:46 11/25/12 (0)

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