Saturday, January 28, 2017

Vaccum tube audio amplifiers – 20W vacuum tube audio amplifier circuit and working - Electronic project

The circuit is designed to give the highest standard of sound reproduction when used in association with a suitable pre-amplifier, a high-grade pick-up head and a good-quality loudspeaker system. Two output pentodes, type EL34, rated at 25W anode dissipation, form the output stage of the circuit. These are connected in a push-pull arrangement with distributed loading, and give a reserve of output power of 20W with a level of harmonic distortion less than 0.05 %. The intermediate stage consists of a cathode-coupled, phase-splitting amplifier using the double triode, type ECC83. This stage is preceded by a high-gain voltage amplifier incorporating the low-noise pentode, type EF86. Direct coupling is used between the voltage amplifier and phase splitter to minimize low-frequency phase shifts.
The main feedback loop includes the whole circuit, the feedback voltage being derived from the secondary winding of the output transformer and being injected in the cathode circuit of the EF86. The amount of feedback applied around the circuit is 30dB, but in spite of this high level, the stability of the circuit is good and the sensitivity is 220mV for the rated output power. The level of hum and noise is 89dB below the rated 20W. The rectifier used in the power-supply stage is the full-wave rectifier, type GZ34. This provides sufficient current for the amplifier (about 145mA) and also for the pre-amplifier and f.m. radio tuner unit (about 35mA) being used with it.
Input Stage
The EF86 input stage of the circuit of Fig provides high-gain voltage amplification, the stage gain being approximately 120 times. High-stability, cracked-carbon resistors are used in the anode, screen-grid and cathode circuits, and they give an appreciable improvement in the measured level of background noise compared with ordinary carbon resistors.
The stage is coupled directly to the input of the phase splitter. The purpose of this is to minimise low-frequency phase shift in the amplifier and to improve the low-frequency stability when negative feedback is applied. A CR network (C1, R3) connected across the anode load produces an advance in phase and thus improves the high-frequency stability of the amplifier.
Intermediate Stage
The second stage of the circuit uses a  double triode, type ECC83,and fulfils the combined function of phase splitter and driver amplifier. The phase splitter is a cathode-coupled circuit which enables a high degree of balance to be obtained in the push-pull drive signal applied to the output stage.
With the high line voltage available, the required drive voltage for an output power of 20W is obtained with a low level (0.4 %) of distortion. The anode load resistors R11 and R12 should be matched to within 5 %, R12 having the higher value for optimum operation. Optimum balance is obtained when the effective anode loads differ by 3 %. The grid resistors R14 and R15 of the output stage should also be close-tolerance components because they also form part of the anode load of the driver stage. High-frequency balance will be determined largely by the wiring layout because equality of shunt capacitance is required. Low-frequency balance is controlled by the time constant C6R10 in the grid circuits of the triode sections,. and the value chosen in Fig  will give adequate balance down to very low frequencies.
A disadvantage of the cathode-coupled form of phase splitter is that the effective voltage gain is about half that attainable with one section of the valve used as a normal voltage amplifier. However, as the amplification factor of the ECC83 is high (100), the effective gain of the cathode-coupled circuit is still about 25 times.
Output Stage
The main feature of interest in the output stage is the use of two EL34 with partial screen-grid (or distributed) loading, the screen grids being fed from tappings on the primary winding of the output transformer. As stated in Chapter 3, the best practical operating conditions are achieved with this type of output stage when about 20 % of the primary winding is common to the anode and screen-grid circuit.
The anode-to-anode loading of the output stage is 7kOhm and, with a line voltage of 440V at the centre-tap of the primary winding of the output transformer, the combined anode and screen-grid dissipation of the output valves is 28W per valve. With the particular screen-grid to anode load ratio used, it has been found that improved linearity is obtained at power levels above 15W when resistors of the order of 1kOhm are inserted in the screen-grid supply circuits. The slight reduction in peak power-handling capacity which results is not significant in practice.
Separate cathode-biasing resistors are used in the output stage to limit the out-of-balance direct current in the primary winding of the output transformer. The use of other balancing arrangements has not been thought necessary although it is likely that some improvement in performance, particularly at low frequencies, would result from the use of d.c. balancing. It is necessary in this type of output stage for the cathodes to be bypassed to earth even if a shared cathode resistor is used. Consequently, a low-frequency time constant in the cathode circuit cannot be eliminated when automatic biasing is used.
Negative Feedback
Negative feedback is taken from the secondary winding of the output transformer to the cathode circuit of the input stage. In spite of the high level of feedback used (30dB), the circuit is completely stable under open-circuit conditions. At least 10dB more feedback (obtainable by reducing the value of R13) would be required to cause high-frequency instability. The most probable form of instability would be oscillation with capacitive loads, but this is most unlikely to occur even with very long loudspeaker leads.
Power Supply
The power supply is conventional and uses a  indirectly-heated, full-wave rectifier, type GZ34, in conjunction with a capacities input filter. The values of the limiting resistors R26 and R27 will depend on the winding resistances of the mains transformer used. Their purpose, when required, is normally one of voltage control only. Where a transformer with a very low winding resistance is used, a secondary voltage rated at 400-0-400V may be found adequate.