TABLE OF CONTENTS Unless the amplifier is quite low power, it will require a dedicated driver stage. We need a stage with good linearity, low output resistance, and good output voltage swing.
The differential triode pair is the ideal choice. The output stage probably requires about 25 VRMS to each grid, and has an input capacitance of 40 pF, or more. An output resistance of 10 k ? coupled to an input capacitance of 40 pF gives a high frequency cut-off of 400 kHz, which is perfectly acceptable. Since r a ? R out, the high valves, which tend to have a high r a, are probably unsuitable.
In a properly designed power amplifier, the output stage should be the limiting factor, so we ought to design the driver stage to have at least 6 dB of overload margin. This requirement probably rules out our favourite valve, the E88CC. Very few commonly available valves satisfy our requirements.
Dual triodes suitable as drivers:
| Type | r a | Comments |
|---|---|---|
| *SN7/*N7 | ?10 k ? | Lowest distortion |
| ECC82 | ?10 k ? | 13 dB worse distortion than *SN7/*N7 |
| E182CC | ?5 k ? | Good on paper, but can sound strident |
| 6BL7 | ?3 k ? | Cripplingly high C ag |
| 6BX7 | ?2 k ? | Capable of driving 845. Robust |
The *SN7/*N7 family is by far the most linear of the previous selection, and if the *SN7GTA or *SN7GTB versions are used, V a(max .)=450 V. The 6BX7 and 6BL7...
