Pull all the tubes, especially the rectifier tube. Verify your bias voltages are within normal, expected bounds. This diagram might help...
Point H appears at the control grid of the output tubes, pin 5. You should be able to get between -42 volts DC and -67 volts DC depending on the position of the main bias pot (the one on the chassis). Adjust the trim pot on the Poseidon board so that both power tubes will see the same bias voltage. Then set the main bias pot so you get the maximum negative bias voltage (about -67 volts DC). Now put in all your tubes, and begin the biasing process.