It may be your PT has a well-balanced heater winding. Or the CMNR of a PP amp is cancelling it out already. The only way to be sure is to scope the output and/or the heater circuit. The way I’ve done it on a SE guitar amp is by attaching a scope probe to each heater connection, with the ground connected at the CT. Then set one of the channels to subtract from the other. The displayed signal can then be minimized by adjusting the humdinger until it is as small as possible.
Whether that is any better than using your ears is debatable. For my BH build, I haven’t adjusted the humdinger from its factory set position (that I assume is middle).
