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Strange failure mode of cathode resistors




Some years ago I had built this 8W OTL for my brother.
Some weeks ago, while listening some loud music, my brother heard a sharp crack coming from the left channel, he saw a sudden blue glow inside one of the 6080 tubes and then the left channel was dead.
During the internal inspection, apart from a blown 2A fuse (of the negative HT rail) there was no other apparent damage.
I replaced the fuse but the bias pots were still not working.
After further investigation I have found out that the small power resistors on the cathodes of the 6080 tubes had changed their resistance from a few ohms to over 20K.
To me this is a mistery!
How could several power resistors possibly undergo such a huge simultaneous electrical change without any apparent physical damage?
I have replaced the faulty resistors (all) with new 10W ceramic resistors, re-adjusted the bias and so far everything seems to be working fine.

Best Regards
Luca

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Topic - Strange failure mode of cathode resistors - 6AS7_6SN7 01:19:14 05/20/12 (8)
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