Measuring/tracing hum on preamp using digital oscilloscope

Hi,
I tried to use a mini digital oscilloscope (Zeeweii DSO3D12) to visualize hum, but it's kinda act weird. Not sure if I hook up correctly.

I've just recently built a tube preamp with recommendation from this thread (https://www.diyaudio.com/community/threads/6j1-china-preamp-thoughts.309876/). It sounds fine but it has some hum, louder on the left channel. I connect probe ground to signal ground and probe to left output. Despite loud hum from the speaker, the scope show no signal. However, if I float the probe ground, it show 50hz signal at few Vpp. Using a DMM, it also show 50Hz.

What should I adjust my setting? Please suggest.
AP
 
I followed the thread and video by TheStuffMade (
) and Tha(t)oneGuy (https://www.diyaudio.com/community/threads/6j1-china-preamp-thoughts.309876/post-7539158) so credit goes to them.

Here's the schematic by Tha(t)oneGuy post. I have all the mods except power supply bleeding resistors.
Debugged schematic.png


My test setup is using this pre-amp with a power amp to the speakers. I used 10X probes. I tried floating power amp input and it was quiet. It only hum when pre-amp is connected, regardless of pre-amp input is connected to source (battery-powered tablet) or not.

My questions are;
1. If I follow the signal path, I should be able to "trace" the hum? I meant from "Audio in" to "Audio out". I connect probe ground to input ground.
2. I don't see hum. What should I look for? I expected some frequency at 50Hz with some amplitude, but I see nothing at "Audio output" yet I can hear it from the speaker.
3. I haven't tried FFT yet. I'll try.

Thanks for any recommendation.