After finally getting all your arrangements for time travel together, you've landed in the 1880s, just as the battle between Tesla and Edison is starting to begin. For reason, $reason$, and reason you need Edison to win this thing. Unfortunately, Edison's system has some major disadvantages from the beginning that will ultimately lead to its downfall. For one, the lower voltages used by DC transmission results in higher cable prices, where as AC can utilize transformers to step up electricity to much higher voltages for transmission, and many preferred the cheaper alternative.
After a bit of convincing you get Edison to accept the solution of high voltage transmission, provided you can build him a transformer. However, being from the future, you know that DC transformers are absolutely possible. Transistors allow the production of boost converters for stepping up DC voltage and buck converters for stepping it down. Unfortunately, in the 1880s the transistor was a long way away, and the simplest vacuum tube, the diode, wasn't created until 1904. As such, you need to find a way of building Edison a DC-DC transformer in the 1880s.
- All solutions must be achievable with 1880s technology and knowledge. In this circumstance transistors wouldn't be feasible, as the theory for them was not yet established and the technology to produce them was not yet produced. Having Edison "invent" the vacuum tube may be viable, but you must suggest a way to produce them in the 1880s.
- Killing Tesla or sabotaging his operation is not allowed.
- Copper prices were rising during this period, so cheaper is better.