[Amps] Ten-Tec Centurion Mod. 422 Tubes Look Biased on at, Turn on

[Floyd Sense]

John - a couple of points on the Centurion.  The first has nothing to do 
with your problem, but concerns the metering.  As you may know, there's 
a mini-pot in series with every meter function source voltage, and the 
mini-pots tend to change value over time.  Maybe due to vibration or 
oxidation?  At any rate, in the two Centurions I've worked on, all of 
the meter readings were off and required re-calibration.  Calibration of 
the multi-meter can be difficult if you have one of the Centurions that 
was shipped with a meter with an incorrect full-scale current rating.

The intermittent nature of your problem suggests a tube problem as 
someone else mentioned.  But, the Centurion is prone to failures in the 
bias circuitry and any HV fault is likely to zap one or more components 
in that circuit.  Such faults include a tube fault and discharging the 
HV via the cover interlock switch.  You could tell if the bias circuit 
is damaged by checking to see if there is any plate current with the amp 
in Standby - the meter should show zero current.  With the meter switch 
in Operate, I see a plate current of approximately 125 ma. when 
grounding the relay line.

73, Floyd - K8AC