Mars Stereo 70 Tube Amplifier
- 35 watt per channel stereo class AB
- Self bias (cathode bias) EL34 output tubes
- OEM transformers: designed and manufactured by Mars
- Ultra Linear output stage configuration for low distortion and accurate sound
- 6SN7GT preamplifer and driver tubes
- 3 stereo RCA inputs, selectable from rotary selector
- speaker binding posts accepts speaker loads from 4 to 8 ohm
- point to point wiring with 2 watt carbon film resistors
The 6SN7 input stage of the circuit above provides medium-gain voltage amplification, the stage gain being approximately 17 times. Carbon film resistors are used in the anode, screen-grid and cathode circuits.
The stage is coupled directly to the input of the phase splitter. The purpose of this is to minimise low-frequenoy phase shift in the amplifier and to improve the low-frequency stability when negative feedback is applied. A small capacitor in the feedback loop (47pF) connected across the feedback resistor produces an advance in phase and thus improves the high-frequency stability of the amplifier.
The second stage of the circuit uses a medium-mu double triode, type 6SN7, and fulfills 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 35 W is obtained with a low level of distortion.
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 taps on the primary winding of the output transformer. 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 6k Ohms and, with a line voltage of 450 V at the centre-tap of the primary winding of the output transformer, the combined anode and screen-grid dissipation of the output valves is 30W per valve.
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 DC 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 is taken from the secondary winding of the output transformer to the cathode circuit of the input stage. The circuit is completely stable under open-circuit conditions.
The power supply is conventional and uses a solid state diode bridge in conjunction with a capacitive input filter.
There is a good article by John Atwood about the 6SN7 here. To quote: “the 6SN7GT is arguably the best general-purpose audio triode. Its large octal format is more rugged and less prone to intermittent pin connections than miniature type.”