The record amplifier is based on tube section 6A which provides some signal amplification. Additionally passive 1st order filtering before the grid provide -6dB/octave roll-off of low and high frequencies.
The oscillator circuit is based around tube section 6B, a handful of capacitors, resistors and an inductor. The oscillator circuitry generates the A.C. bias voltage – a high frequency (50KHz to 60KHz) sinusoidal waveform. In layman’s terms, biasing “shakes up” and randomises the atomic-sized magnetic particles on the recording medium (the Binson magnetic drum) to create a uniform “blank slate” for the record head to “print” the audio signal onto. The audio signal is mixed with the bias signal and routed to the record head. The bias voltage aross record head is around 300 volts peak-to-peak (210 volts R.M.S.) and is much, much larger than the audio signal level, which in the order of just a few volts. The impedance of the heads used in the tube Echorec B2 models is around 650Ω.