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  1. NOSValves's post in fisher kx-90 schematic and manual, help please was marked as the answer   
    The voltage that lights those 3 12AX7's is derived from the cathodes of the 4 output tubes. So the problem is not actually in those 3 12AX7's. For some reason those output tubes are not conducting. Because a tube is lighting does not mean its working what lights in a tube is the heater. The tube can not work without the heater lighting but being lit does not mean the tube is operational.  
    You can get the service manual here
    Oh wait I misspoke a bit. One of the 12AX7's could possibly be the problem if one has a heater winding that is shorted open. To check all 3 12AX7's plug them into the sockets where the two 12AX7's are now lighting and see if all 3 will light. If they light then the problem is elsewhere.
     It could even be that for some reason the amplifier has no high voltage B+ present. But usually when something goes wrong with the high voltage B+ the amp will blow fuses. 
  2. NOSValves's post in Moondog 2A3 is Lagging was marked as the answer   
    When you swapped the preamp tube did the dim glow move with the tube or stay put on the troubled amplifier? If it stays in the troublesome amp I bet Mike's advice I is spot on, if the heater voltage for the preamp tube is low it could extend the warm up time before that tube starts to amplify the signal.
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