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Appliction Note: AN-05 


Last Update: 28-Okt-2015->->->

Sometimes, a tube amplifier can cause a power triode or rectifier to spark.  This is bright  spark,  blue or white,  and it causes a very loud  noise from the loudspeakers. If this happens, you may think it's a short circuit in the tubes, specially if it  seems to be connected to a particular tube only, one tube of a pair. However, this is always an amplifier problem.

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. 

To prevent any damage do not use the amplifier any more, and contact your amplifier manufacturer to have it checked up, or better say repaired. 


For amplifier designers, the below links may be helpful.

1) Warning by OSRAM, about design issues,
which causes sparking tubes

2) Spark Warning by RCA, about design issues,
which causes sparking tubes

3) Another Spark Warning by RCA.

4) Table of lowest allowed transformer
windings resistance
5) This App Note, Part 2
Important Notes for amplifier manufacturers

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