Manley Labs - Bias Instructions for Manley Amplifiers
About half way down the page ..."I can't get a bias reading on any of my tubes". We know you were really rocking out with that Def Leppard CD when that happened... So, is your amplifier turned on? Yes of course it is. Lights on but nobody's home. No readout at any of the test points usually indicates the B+ fuse blew. "
Sounds like this could be your issue?