I'm not too familiar with that amp but... the specs on the Ameritron site
calls for 2700 volts with the AL80B under load and 3100 volts no load. 2400
is too low...... the HV shouldn't, I believe, change with a soft tube as
it's a measure of the transformer voltage. In my experience the main cause
of low plate voltage is low line voltage at the outlet.

The meter may be off some .... and 120 volts isn't exactly well regulated
voltage. Double check the tap set-up on the power transformer.... make sure
that is set for your line voltage. Measure your line voltage and set the
taps to the closest voltage...The info on the AL80B indicates it's a multi
voltage type transformer with 14 taps from 90 to 230 volts...but I'm not
sure about the AL80A.

DO NOT fool around inside an amplifier if you are not knowledgable about
working around high voltage and discharging capacitors. The HV in an
amplifier is LETHAL. (usual disclaimer grin)

Mike, W9WIS