The European Spallation Source (ESS) will be the world's most powerful spallation neutron source when it comes in operation at the end of this decade. The ESS linac will accelerate 50mA of protons to 2 GeV in 2.86 ms long pulses at a repetition rate of 14 Hz on a tungsten target for neutron spallation. The linac contains 26 superconducting spoke cavities, which are being developed by IPN Orsay, France, and which accelerate the protons from 90 to 220 MeV. The FREIA laboratory at Uppsala University (Sweden) has developed the required RF power sources, procured the RF distribution and will test the cavities at nominal power. As no suitable amplifier was available at the ESS specifications (352MHz, 400kWp / 20kWavg, pulse width = 3.5 ms, pulse repetition frequency = 14 Hz), various technologies were compared and tetrode technology was selected for the first technology demonstrator RF power station at FREIA. We discuss the design of the technology demonstrator and present the first test results.