c1962 Bird Golden Eagle 4/25, courtesy of Music Ground, Leeds, England
The next development from the Golden Eagle 2/15. The 4/25 had an increased power output of 25 watts produced by twin EL34 valves. In order to handle this large increase in power, a 15" speaker was fitted - usually a Goodmans unit. Early amps had an all ply cabinet, but the later ones were probably the first in the UK to use chipboard.
This was a twin channel amp, i.e. 2 x 2 inputs with individual volume and tone controls for each channel. Tremolo was fitted to one channel, and the spring reverb acted on both channels. The Bird spring reverb used crystal (record player type) cartridge drivers at each end of the springs, and these are notorious for breaking down over time. In addition, a "sort-of" repetitive echo was fitted - a feature no doubt to enable the amp to compete with the new Watkins Joker, which had a complete tape echo unit built in. The Bird's echo was not tape driven however, but used a valve circuit in conjunction with the spring reverb to give a vaguely similar effect.
The Golden Eagle 4/25 scanned from the 1964 Bell Musical Instruments catalogue.