WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Look up the tubes datasheet and look at the maximum ratings for the heater, if its over or below that you might want to take action.
Lazy way to fix it is to use a resistor in series with the heater for some voltage drop.
I have seen discussions of some hi-fiers installing in-line UPS to rid their audio systems from the plague of noise and interference that somehow sneaks through. That system would keep heater voltage under control! Or alternatively use a motorised dimmerstat/variac with heater voltage feedback control
The 10% figure is always what I ran by as well. However isn't it mostly the current that we're worried about with the heaters? P,diss more specifically. I mean that's kind of the M.O. of the tube anyway usually, correct?
Saw a chart linked a short while ago on this forum about heater life vs heater voltage. Higher voltage shortens tube life. But a 10 % drop extends it quite remarkably. I guess too low a heater voltage is also bad for the tube . Keeping it at the correct voltage or within 10% lower at normal operating conditions is probably best. If it's a dc heater voltage you can easily keep it at a constant safe voltage.