this app doesn't support download video from youtube.

Video Downloader - Play Tube - Video Tube is a free client for Tube,
allows you to find great videos and free music.

Play Tube & Video Tube can search videos, play free music with creative common licence.
Video Downloader is the fastest downloader on market.
Play videos for free.
Search videos and download free videos.

Important: you can't download videos from Youtube.

+ Watch unlimited free videos
+ Search videos
+ Display general information about a video
+ Download licenced free videos

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Source: https://play.google.com/store/apps/details?id=com.streaming.player.tube.view