The design of the circuit shown in Fig. 1 is very similar to the well-known Mullard 'Five-Ten' monophonic amplifier. Only one channel and the common power supply are shown in Fig. 1 except for the loudspeaker phase reversal switch SC, the other channel is identical. The complete amplifier uses two EF86 and four ECL86; and a suitable rectifier is the GZ34.
Input Selector Switch
The input stages of both channels are connected to the 3-way input selector switch SA, which provides facilities for stereophonic reproduction from a crystal or ceramic pick-up head, and dual-channel monophonic reproduction from a monophonic pick-up head or fm tuner unit.
Input Stage
The input stage uses an EF86. The voltage gain of the stage is approximately 120 times. The stage is capacitively coupled to the phase splitter.
Phase Splitter
The phase splitter uses the triode sections of two ECL86 in a 'long-tailed' pair. Fixed bias to the grids is provided by a potential divider across the ht supply. The voltage gain is approximately 24 times per half.
Output Stage
The push-pull output stage uses the pentode sections of the two ECL86 opterating under class AB conditions with distributed loading (20% taps). With an anode-to-anode load of 9kΩ and an anode-to-cathode voltage of 300V, the distortion without feedback is less than 2% for outputs up to 10W.
The output transformer should be of the construction normally used in high-quality amplifiers. In particular, it should be free from any pronounced resonance up to about 50kHz The turns-ratio should be such that the anode-to-anode load is 9kΩ after allowing for winding resistances. The transformer used in the prototype has the following specifications:
Total primary inductance 100H
Primary resistance per half 265Ω
Leakage inductance (whole primary, with secondary short-circuited 21mH, Secondary resistance at 15Ω terminals 0.9Ω
Negative Feedback
Overall negative feedback of approximately 20dB is applied from the secondary winding of the output transformer to the cathode circuit of the input stage. Coupling time constants which are considerably greater than the transformer time constant ensure low-frequency stability. A 'phase advance' network in the feedback path gives adequate high-frequency stability.
The amplifier design is such that more than 17dB of negative feedback is effective over a frequency range of 30Hz to 30Hz.
Tone-control Circuits
Passive tone-control circuits giving bass boost and cut and treble boost and cut are provided. The input impedance of the tone control circuits is about 500kΩ.
Balance Control
The balance control consists of a dual-ganged potentiometer, one track obeying a logarithmic law, and the other an anti-logarithmic law.
Power Supply
The power supply requirements for both channels of the amplifier are:
Heaters 6.3V 3.2A for 4 x ECL86 plus 2 x EF86
HT 320V No signal 138mA, 315V 10W sine wave l60mA per channel
PERFORMANCE
Sensitivity
The sensitivity of the basic amplifier for an output power of 10W is 2.3mV without feedback and 23mV with feedback. The sensitivity of the complete amplifier, including tone controls, is 210mV.
