[codefox1]

First of all, the set with the 50L6 does not have a power transformer. The best way to test this tube is substitute a known good one. If there is leakage or a short involving the filament to any other element of the tube you will indeed get a lot of hum. Be careful, these chassis are always shock hazards.

You should use an isolation transformer when working on transformerless AA5 sets, especially if you want to measure things with other instruments that are AC Powered.

Minimal filtering is usually fine for the smaller sets since the output transformers and speakers will not pass much of anything audible for 120 HZ ripple, and are even less capable of reporting 60 HZ from a half wave rectifier. Anyway you can measure the AC element remaining after each stage of the B+ supply with a digital meter or VTVM, or you can have a look see with your scope. If properly wired and componnents are good, there should not be any objectional hum coming from the stock speaker and cabinet under normal operating conditions.