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Binson Echorec Schematic

The Binson ‘Echorec’ echo-delay is a superlative example of innovative 1950s engineering. In their quest to create the perfect delay effects machine Binson got many important aspects of the Echorec’s design right, maintaining a balance between simplicity and adding genuinely useful features and, no expense was spared. For instance, the Echorec utilised an ingenious metal drum, rather than a fragile ribbon of rust—oxide coated plastic tape—as a recording medium. The drum offered superior reliability, stability and sound quality in comparison to ¼″ magnetic tape

Further, the hi-fidelity circuitry within Binson’s flagship Echorec machine—the model ‘T7E’—was based on seven miniature B9A tubes; five 12AX7s (ECC83s), one 12AU7 (ECC82) and an EM81. The 12AX7 and 12AU7 tube types are still manufactured today and, although EM81 tubes are no longer made there are still new old stock supplies to be found. The following block diagram gives a functional overview of the various tube circuit sections within the Echorec. This article examines the inner workings each section and elucidates some of the not so well-understood aspects of the Echorec’s operation.

Input Buffer

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.

Input Mixer

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’ potentiometer, which is a 220KΩ linear taper type manufactured by the Italian hi-fi equipment manufacturer LESA.

Record Amplifier and Bias Oscillator

binson_echorec_cct4b — Record amplifier and bias oscillator

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 magnetisation 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. This upper limit is realised in machines fitted with the newer Photovox heads, however earlier machines, such as the ‘T5E,’ were fitted with heads that had a wider gap and therefore the frequency response tops out at around 5KHz.

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 250 volts peak-to-peak (175 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 thin, lacking that full, rich tone Binson Echorecs are renowned for. And, if the bias level is too high then the delay repeats will sound mushy, lacking clarity. So bias adjustment is therefore always a compromise between distortion and high frequency content.

Biasing is an important, and often overlooked, aspect of Echorec setup. The bias voltage level should be adjusted so that it’s in the region of 250V peak-to-peak. An oscilloscope should be used to monitor the bias level at the record head—do not use an R.M.S. digital voltmeter. Further, the heads must also be aligned and set up properly to obtain the best audio performance from the machine. Note: the impedance of the record head is approximately 400Ω for the tube model ‘T7E’ Echorec.

Playback Amplifier

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. Note: in the tube model ‘T7E’ Echorec, the impedance of the four playback heads are approximately 600Ω.

Switching Circuitry

binson_echorec_cct5 — Switching circuitry

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
1	’1′	’0′	’0′	’0′
2	’0′	’1′	’0′	’0′
3	’0′	’0′	’1′	’0′
4	’0′	’0′	’0′	’1′
5	’1′	’1′	’0′	’0′
6	’0′	’1′	’1′	’0′
7	’0′	’0′	’1′	’1′
8	’1′	’0′	’1′	’0′
9	’0′	’1′	’0′	’1′
10	’1′	’1′	’1′	’0′
11	’0′	’1′	’1′	’1′
12	’1′	’1′	’1′	’1′

Note: ’1′ = head on; ’0′ = head off

Tone and Volume Controls

binson_echorec_cct6 — Tone and volume controls

Basic passive tone and volume controls operating on the delayed signal. The ‘VOLUME ECHO’ potentiometer is LESA 470KΩ has a logarithmic taper and the ‘BASS/TREBLE’ pot is a 220KΩ is logarithmic taper type.

Output Mixer and Buffer

binson_echorec_cct7 — 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

Level Indicator

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.

Power Supply

The mains transformer output windings generate Low Tension (L.T.) and High Tension (H.T.) A.C. voltages. The L.T. winding supplies 6.3VAC (2A minimum for the 7 tubes in the Model: T7E) to power the tube heaters and filament lamps that illuminate the lettering on the ‘Plexiglas’ front panel.

The H.T. winding supplies 250VAC at around 100mA (for all those tubes). The A.C. (Alternating Current) from the transformer is converted into D.C. (Direct Current) by means of a Siemens type B250C75 selenium bridge rectifier and a π inductor-capacitor L-C filter circuit. Some additional, rudimentary smoothing is also provided by two cascaded resistor and capacitor R-C filters—these reduce power supply ripple even further before it reaches the more noise-sensitive parts of the circuitry, such as the audio input gain stage. The capacitors are multi-section types where there are two 350V 47uF capacitors housed in one can. The inductor (D.C. choke) is in the order of 2 to 5H and 130Ω resistance (Hammond part# 156L).

Note: Hammond part# 369AX appears as if it will work as a replacement mains transformer. All Binson Echorec mains transformers feature primary windings with multiple taps to allow the machine to operate internationally. The taps are colour-coded as follows:

A.C. Mains Voltage	Wire Insulation Colour
280	Blue
220	Black
160	Brown
145	Green
125	Yellow
110	Red
0V (fuse)	White

And that just about covers everything… well, almost. There’s just one more thing; Binson also manufactured a 4-knob Echorec, known as the ‘Export’. And, before the 6-knob model ‘T7E’ there came the ‘T5E’. And what about the ‘T6E’ they made for Guild in the U.S.A.? And the PE 603, the ‘B1’ and Binson ‘Baby’? In fact, there are numerous Echorec variants, all with slightly different circuitry. But keep calm and don’t get bamboozled—all Echorecs, 4-knob or 6-knob, tube or transistor, old or new, essentially operate on the principles outlined in this article. This really isn’t rocket science you know, or is it…?

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Binson Echorec Schematic