Cell Phones & Tube Amps

I have, for many years, avoided plugging my cell into tube amps. I occasionally used my laptop as a source, but never the cell.

But recently I moved the charger near the tube amp, so I figured why not try it? It was convenient, easy, and gave me access to a wealth of music, even if I wasn't using a fancy DAC as part of the chain. The volume was limited, but some YouTubes and digital feeds actually sounded half-decent.

But after 10 or 20 minutes, the cell would jam up with a locked display, either a bright white screen or horizontal stripes, and the only way to fix it was to take it apart and remove and re-insert the battery. After it happened a few times, I tried to figure out what was causing this.

It seemed like the charging had something to do with it, so I ended up reversing the plug on the charger, since it didn't have a polarized plug with a fat prong that could only be plugged in one way. It seems to have fixed the problem. 

Are there any reasons that a cell shouldn't be plugged in to a tube amp? Can I damage the phone or the amp?

Perhaps it's better to use a USB dac or something like a Dragonfly. I don't really want to go Bluetooth.