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.
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