The cronjobs (crontabs) for all system users in Debian and Ubuntu Linux are stored in the directory /var/spool/cron/crontabs. To make a backup with tar, use this command:
tar pcfz /root/user_crontabs.tar.gz /var/spool/cron/crontabs
The backup file user_crontabs.tar.gz is be stored in the /root/ folder.