A great transistorised limiter from early in the BBC’s popular Am6 series.
Later varients like the white faced Am6/7 and the Am6/17 made under licence by Neve and Calrec are better known and, as far as music production goes, more aggressive or clinical respectively.
The Am6/3 has the option to internally modify to a softer ‘compression’ slope, but sounds fine as it is.
Based on a smaller and more readily available valve, the ECC81 (or 6060 with lower microphony) the Type B became a familiar sight in many BBC studios across the land, including of course the Radiophonic Workshop. It is surprising that so few appear to have survived!
King of the roost was the AMC/5 microphone amplifier with fixed gain stages of 40 to 70dB. This had been developed from the earlier general purpose GPA/4 amplifier which was also used for microphone applications. Further down the gain chain was the C/9 line amplifier with 10dB in hand.
Consoles were passive, constant impedance, utilising Painton stud faders and in some instances with quadrant faders, such as at the Radiophonic Workshop as seen here in the capable hands of Delia Derbyshire.
The CMU/10 also provided for a P.P.M. meter which was powered off the OBA/9 rig.
Not currently in use, but looking forward to getting these lit up like a Christmas tree.
The OBA/8 system was introduced in 1938. Intended for portable use with war in Europe looming, it was more lightweight then predecessors and utilised a pentode valve for the first time in British amplifier design.
The whole system, designed for self operation, consisted of two OBA/8 amplifiers (one on standby), a passive MX/18 mixer, LSU/1 loudspeaker and comms.
Many units, particularly the MX/18 mixer, continued to find a use long after peace had been declared, in provincial studios and of course the Radiophonic Workshop.
Here is a picture of Daphne Oram with three such passive mixers, which utilised wire wound constant impedance attenuators (and an OBA/8 in the foreground).
The OBA/9 system evolved in the 1950s to replace OBA/8, with a smaller footprint and expanded facilities to meet the demands of outside broadcast engineers as the needs of broadcast expanded.
The whole system was designed to be ‘portable’ of a fashion; consisting of an MX/29 passive mixer, OBA/9 valved amplifiers with high gain and power supply, along with monitoring amplifier and distribution facilities. Ancillary items included a loudspeaker LSu/11 and communication units CMU/9 and CMU/10