Lost bias in my SE output stage. But no apparent damage.

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Was firing up my KT-90 SET amp (it uses fixed bias for the output stage) and apparently the KT90 300K grid resistor, to which the bias was applied was soldered poorly. I wondered why no sound on the left channel, then tried to measure the idle current (drop across a 1 ohm monitor resistor in the cathode) and it came back -0.0
Then I shut the amp down and eventually found the problem.
After fixing the bad connection both output tubes seem to bias very closely and no complaints about sound, so it doesn't look like any harm was done.
A question however. I assumed if you lost bias in the output stage the tubes would very quickly get cherry red on the plates and probably fail. Here it seemed that the tube went into complete cutoff instead because the control grid was floating and built up negative charge?
In any case, KT-90 seems to be one rugged valve.

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Topic - Lost bias in my SE output stage. But no apparent damage. - A.A. 21:32:43 04/13/07 (1)

Re: Lost bias in my SE output stage. But no apparent damage. - Michael Samra 00:56:37 04/14/07 (0)

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