I have an odd situation that I've never experience before. While trying to set the bias on my ST-70 (which uses Shannon's driver board) I was able to read and set the bias on 3 of the 4 EL34s. However, on one of them I got no reading whatsoever (in other words, the reading on the meter did not change at all when I connected the leads to the amp). There did not seem to be any ill effects when the amp was in operation: no tubes glowing red, no problem with the sound, the other tube in that channel did not need to rebiased to 'compensate' for the other tube, the odd tube glowed and seemed to function normally). I expected that I must have a loose connection inside the amp to the bias check point. This turned out NOT to be the case. So, on a whim, I substituted another EL34 that I had lying around and, presto...I was able to read and set the bias for that tube as normal.
Is this a somewhat common occurrence? I'm going to guess that perhaps there is a missing connection inside the tube that causes this phenomenon but does not render the tube inoperable...?
Any thoughts? I'm just curious...
Thanks,
Joe