Binson Echorec Schematic
by Phil Taylor
The ‘Echorec’ echo-delay is a superb example of 1950s of engineering! In their quest for perfection Binson got many important aspects of the design right, maintaining a balance between simplicity and adding genuinely useful features to their iconic echo machine. The engineers developed their own unique magnetic drum storage medium that offered superior reliability (‘permanent guarantee’) and stability in comparison to the 1/4″ magnetic tape utilised in the echo effect machines built by other manufacturers. No expense was spared in the design of the mixing and amplifier circuitry – there are seven miniature B9A tubes within the model T7E Echorec! All the tubes are still readily available 12AX7 and 12AU7 triodes. The signal level indicator – an EM81 ‘magic eye’ tube – is no longer manufactured, however there’s still a plentiful supply of new old stock EM81s available.
This webpage contains a description of each functional part of the T7E circuitry, a flowchart clearly showing the signal flow, schematics for the Echorec IA and II.
The input buffer is an inverting gain stage based on section 4A of a 12AX7 tube. To optimise this to work with electric guitar pickups the the 47KΩ resistor before the input coupling capacitor can be removed. Virtually all guitar amp input stages are direct coupled i.e. there is no capacitor, effectively ‘hot’ biasing the first input stage. This is okay as a guitar pickup only generates an AC component with no DC offsets. Note: there is no cathode bias resistor. Typically triode amplifier configurations have a 1K8 resistor to maintain the grid voltage below the cathode.
Mixer stage with common anode resistor. This is where the ‘dry’ signal and delayed signal are mixed to create multiple repeats. Trimmer, T2 is used to set the range of the ‘LENGTH OF SWELL’ knob.
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.
The playback amplifiers are based on tube sections 1A, 1B, 2A and 2B, boosting the small signal from the playback heads. The 220pF capacitor filters out the HF bias signal from the audio signal. Trimmer, T5 is a tone control and T4 output control for each of the four playback amplifiers.
The ‘SELECTOR’ and ‘SWITCH’ knobs form the switching circuitry. The SELECTOR knob allows selection between ECHO, REPEAT and SWELL settings. ECHO is just one distinct repeat; REPEAT is multiple repetitions, depending on the ‘LENGTH OF SWELL’; SWELL introduces a little signal from all four playback heads into the final mix, to produce reverb type effects. The REPEAT setting is the ‘one’ to use to create early 1970s Pink Floyd delay effects.
The 12-position ‘SWITCH’ knob enables selection of different heads and combinations of heads for single and multi-tap delay. The Echorec schematics only show a simplified interpretation of the action of this switch and there is no explanation in the Binson user manual of which head combinations can be selected using the switch. In theory there are 24 (16) possible head combinations (including the possibility of selecting no heads). The table below shows the actual combinations available with the 12-position selector switch:
|Position||Head 1||Head 2||Head 3||Head 4|
Note: ’1′ = head on; ’0′ = head off
Tone and Volume Controls
Basic passive tone and volume controls operating on the delayed signal.
Output Mixer and Buffer
The output mixer buffer is a cathode follower configuration. Suggested modifications to extend the H.F. response a little are to increase the 47K resistor on the output to 100K and increase the 270nF output coupling capacitor to a polyester 680nF
The level indicator is based on an EM81 ‘magic-eye’ tube which displays the level of signal repeats. The ‘fan’ can be adjusted using trimmer, T3.
Low tension 6.3V supplies the tube heaters. High tension power supply with various levels of smoothing for different parts of the audio circuitry.