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40 dB example. Thus, we realize a savings of one capacitor and the remaining capacitor will be cheaper, as it is smaller in value. Of course, if you wish, you can retain the symmetry of the 40 dB version.
If perfectly matched triodes are used in the first stage, then the coupling capacitor and bias resistor on one leg of preamp will not be needed. But as tubes vary not only from each other but themselves over time, it is best to retain these components, as even a 1 volt difference between plate voltages on the first stage will greatly unbalance the second stage. (Yes this paragraph is identical to the earlier one. It is just that I know many will not read this article in its entirety and will jump ahead to the parts that interest them and would this miss this warning.)
Choice of Tubes
Because of the need for high gain and low noise, the available choice of tubes for this preamp is limited. If you want to follow Yoon's lead and use subminiature tubes, then the 6021 is a good starting point. Although its mu is on the low side, its transconductance is high enough to lower the resistance noise of the tube.
If conventional miniature tubes are more to your liking, then the old preamp standby, the 6DJ8/6922, will work well here; in fact, the values displayed in all the schematics are optimal for this tube. An alternative tube might be the 6AQ8, which has a higher mu (57) than the 6DJ8 and might be still readily available on the surplus market (look for the old Amperex 6AQ8's). Another possibility is to mix tube types, say for the 40 dB version a 6DJ8 for the first stage and a 12AT7 or 6072 for the last stage; for the 60 dB version, a 6DJ8, a 5695, a 5687.
Octal fans will find a more limited choice of tubes. In fact, the ploy might be to build the 60 dB version and use 6SN7's throughout. Odd tubes with internally bridged cathodes such as the 6N7 might work well.
Heresy
Because noise and microphonics are the curse of tubes, a better choice might be to use a matched, low noise J-FET for the first stage. Toshiba made a great matched FET in the 2SK146, but it has been replaced with something new and, we hope, better. Of course, the FET will not be able to withstand the high voltages that a tube can, so its power supply must be limited. Here is where a zener diode could be used not to set the voltage, but just to prevent over voltages.
// JRB
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