Voltaic Construct, Mana Vault, and Karn, Silver Golem

Prerequisites

• Voltaic Construct on the battlefield
• Mana Vault on the battlefield
• Karn, Silver Golem on the battlefield
• Mana Vault does not have summoning sickness
• available

Steps

1. Activate Karn by paying , causing Mana Vault to become an artifact creature until end of turn
2. Activate Mana Vault by tapping it, adding
3. Activate Voltaic Construct by paying , untapping Mana Vault
4. Repeat from step 2

Results

• Infinite colorless mana