I emailed Ameritron and they suggested I check three possible scenarios: 1) One of the tubes is in backwards. Look at the tubes and make sure all four are lighting. .
Bad suggestion. That would cause low current, or cause no output, or cause backwards grid current with no drive.
2) The 0.6 ohm resistor in the circuit is open.
Bad suggestion. That would cause the plate meter to pin against the stop really hard, even at idle.
3) One of the grid pins has come loose from the ground point
That causes low output, and some really weird things.
You probably have an older amplifier before the bias system was added. Check to see if there is a string of diodes at the bottom of the board on the rear panel, b down at the bottom near the filament choke.
My bet is you have an amplifier that is older and does not have the bias diodes added. Newer tubes have higher idle current than the first tubes had, and will idle around 180 mA without drive when keyed.