Factory Warranty Conditions

WHITE SPARK PROBLEM - Notes for manufacturers

These notes are not for end-users.  Please have your amplifier manufacturer read this, in case of a spark problem.

What causes the spark?

The problem is caused by positive grid voltage at the moment of the spark, causing very excessive plate current.   This will evaporate some filament material, which will ionize, and can make the tube can spark.    At the moment of the spark the  tube will short circuit.

In electronics literature this effect is called "trigger".    It seems the tube will discover from it, but each spark will shorten the lifetime of the tube.  So if it happened too often, the tube may still seem ok, but will stop working sooner than expected. 

The effect gets larger when the tubes have better emission.  So you may observe this with large power tubes, but it can happen to smaller tubes as well, when the switch-on effects in the amplifier are very bad. 

Also really excessive overdriving the tube can cause the trigger effect.   (some people try that, just  to hear how crazy this sounds).   The result can be a spark. 
 


Read this: 
Notes from 1925, by OSRAM, about sparking tubes. 
 

You need to check this:


 
  • What peak value do you measure for the grid-to-filament voltage at switch-on or switch-off ? 
  • What peak value do you measure for plate current at switch-on or switch-off ? 
  • DC-coupled circuits  need to be checked very carefully for this, they can behave unexpectedly.
  • Meausere the surge current through the coupling capacitor, to the end-triode. What is the surge current while you switch on and off the amplifier? You will be surprized how high this value can be. This current will flow through ground via the grid-to-ground  resistor, and cause a positive voltage there of 50Volt and higher.  (Grids are supposed to have a negative voltage...) 


    • What can you do? 


     
  • Disconnect the coupling capacitor to the end-triode and repeat the above measurements. Compare the difference.  See if the spark  is gone now.  If yes, you have at least  localized  the problem.
  • A stand-by circuit sometimes solves the  problem. 
  • Hard overdriving power triodes (like in guitar amplifiers)  is not allowed. Will the amp allow for excessive overdrive with the volume at maximum? If yes, you need to correct this. 
  • Reduce the grid-to-ground resistor  to  the datasheet value. The dataheet value is the maximum, so feel free to go below.  A low enough value will protect against the coupling capacitor surge current.
  • Change the couping capacitor the the smallest value you need for good frequency response. Don't oversize it. 
  • A grid series resistor can partially help.  This resistor will give a frequency roll off as a side effect.  Choose the resistor low enough to get no unwanted roll off in the audio range.   Choose it high enough to limit the grid surge current (into the grid, via the coupling capacitor). 


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