The vertical position of the JET plasma is normally unstable and feedback stabilisation is therefore needed. A new power amplifier (East ,Radial Field Amplifier - FRFA) based on GTO inverters has been procured to cope with configurations characterized by plasma having a high degree of vertical instability, beyond the stabilising capability of the phase controlled thyristor converter used so far. The new power amplifier is composed of four identical sub units which can be connected in two different configurations to achieve output voltages up to 10kV (at 2.5kA) and output currents up to 5kA (at 5kV). The amplifier is characterised by a peak output power of 25MW and by switching frequencies of the individual GTOs of 1kHz. The series connection of more inverters allows more voltage levels (up to nine) to be made available on the load. The paper presents the results of the extensive series of tests on a dummy load performed on the amplifier both at factory, where a complete sub unit was assembled, and at the JET site. The tests included the achievement of the full performances and the assessment of the correct operation of all the required control modes.