UAV tether systems are provided that reliably deploy and retract cables capable of high-power and high-bandwidth data transmission. The tether system spools cables in a single layer to promote heat dissipation thereby allowing the use of lower-diameter cables. The tether systems further utilize a lightweight converter that achieves a high-voltage power transmission across a cable that is stepped down to a plurality of lower-voltage outputs usable by a UAV and accompanying payloads. The converter is constructed in a manner that promotes heat dissipation through the use of multi-layer PCBs and separate power modules PCBs mounted above main PCBs to create a cavity where forced air can reach heat sinks affixed to the power modules that extend into the cavity.