Using drones for ear training is a very powerful tool. It throws a microscope on the tuning of the notes and can be a great way to get into the moment when you begin your practice.

I like these Cello drones on YouTube: D Drone https://www.youtube.com/watch?v=ygo0ZimLfsQ

G drone: https://www.youtube.com/watch?v=IvXXdGhJA1Q

Here is a great app for iPhone that recreates the Indian tempura drone : https://apps.apple.com/au/app/isruthi-tampura-sruthi-box/id669394303

Here is one for android: https://tanpura-droid.en.softonic.com/android