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 bias oscillator circuit is based around a 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 (Binson’s patented magnetic drum) to create a uniform “blank slate” for the record head to “print” the audio signal onto. More technically, the bias shifts the recorded audio signal into the “linear region” of the recording medium’s magnetization curve; it’s purpose is to cancel out non-linearities, yielding distortion-free recording. The frequency of the bias oscillator in a tape recording machine is typically just over five times the highest audible frequency, which, in the case of the Echorec, sets the upper frequency limit at around 10KHz to 12KHz.
The audio signal is superimposed over the bias signal and the combined signal routed to the record head. The bias voltage across record head is in the order of 300 volts peak-to-peak (210 volts R.M.S.) and is much, much larger than the audio signal level, which is only a few volts. The amplitude of the bias signal is altered by adjusting the 100pF trimmer. To obtain optimum distortion and upper frequency response of the recorded signal the voltage level of the bias must be calibrated correctly—if the bias level is too low then the delay repeats will sound distorted and if the bias level is too high then the delay repeats will sound mushy, lacking clarity. The bias adjustment is therefore always a compromise between distortion and high frequency content. The heads must also be aligned and set up properly to obtain the best audio performance from the machine. Note: in the tube model ‘T7E’ Echorec, the impedance of the record head is approximately 400Ω.