[35Z5]

Pin 6(plate) of 6SQ7 should measure at least 220K ohms which is it's plate load resistor. Through the power supply is its only  ground reference. If it measures lower there is a leaky cap or maybe tube socket.

The likely scenario is resistance will measure high, meaning the 220K has increased in value. If resistor value was low, plate voltage should be high. If all resistance is good, likely the tube has issues. If all is good and tube doesn't fix it, quit worrying, a 6SQ7 will operate with only 11v on plate..

For tube heaters, measure voltage across heater pins, if it's 6v(or 25v on higher voltage tubes) you're golden.