Nashville SE 20 - the modified SuperTriode Concept The Input (VAS) Stage (fig. 1) The voltage amplifying stage (VAS) is simply an actively loaded 6SN7 twin triode. The cathode resistors of 220Ohm in each tube section set the plate currents to 9mA (red line in fig. 2). In this area the average tube characteristics change less compared to the lower current area and thus is an acceptable compromise between amplification factor µ (green line) and plate resistance R p  (blue line) while plate dissipation (around 1.2W each) is still low (fig. 2). The dynamic R k  of the upper tube is 3.9kOhm + 220Ohm. Its AC voltage drop is fed into the upper tube through C2 and increases the effective load impedance substantially. The effective impedance Z of the upper triode is given by the equation                                   r p  + (µ + 1) * R k  = Z given a µ of 21.8 (green line) and Rp of 7500Ohm (blue line) the effective impedance is     7500 + (20.8 +1) * (220 + 3900) = 97.3K ≈ 100k               Fig. 1: 6SN7 driver stage circuit. 9mA against a conventional 100k plate resistor would require 900V and a total power supply voltage above 1000V! Yet both triodes operate at a comfortable 290V. Taking into account the 100K grid resistor of the following output stage the dynamic load impedance is 50kOhm and the amplification factor is 18 (25db). That should be sufficient to drive the output stage from a high level source such as a CD player or phono stage. An ordinary plate resistor would be limited to 163V * 9mA ≈ 18k with a corresponding amplification factor of 12 (21.6db). Figure 3 shows the point of operation and 6SN7 average plate characteristics. The price on the other hand is a relatively large input capacitance due to the Miller capacitance. Remind that the cathode of the upper triode has a potential of +160V (fig. 1). Thus the potential of the heater voltage has to be elevated to +70V. The 1K resistors are grid stoppers to prevent parasitic oscillation.                  Fig. 2: 6SN7 average characteristics at 9mA (red line) idle plate current Fig. 3: 6SN7 average plate characteristics at 130V plate voltage (red line), 9mA idle current (green line), 50k load impedance (dark blue line). With an ordinary plate resistor (18k) and 50k in parallel the load impedance slope is substantially higher (light blue) resulting in less amplification and higher distortion.
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