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impedance of the amplifier. For example, if 1 volt is the peak voltage and 100k is the input resistor value, then 0.01 mA is the peak current flow due to the input signal. Now, if we establish an idle current through the cable at least as great as 0.01 mA, then the current flowing through the cable will never under go a change in direction.
Good enough, but how do we establish an idle current? We add the resistor that was probably missing from last month's schematic.
How do we arrive at this resistor's value? We begin by taking the available biasing voltage and divide it by the desired idle current. Given 9 volts from the battery and 0.00001 amps of current the result is 900k.
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Do not remove the 200k resistor on the preamp side, as it pulls down the voltage at the output of the preamp down to a safe value; without it, the full plate voltage would be present at the output of the preamp.
The math is much like that for the black boxes: we start with the peak input voltage into the power amplifier, the input impedance of the power amplifier and then divide the first by the second. In this example, lets say that the peak input voltage is 2 volts and the amplifier input impedance is 47k. The result equals 0.042 mA, which we round up to 0.05 mA. Next, we divide 10 volts by 0.00005 amps, which gives us 200k. For safety's sake, we add a 200k resistor to the preamp side as well. The rest of the design procedure is outlined in last month's article.
Editor
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To play it safe, we will bump this value down to 600k. Then this value is halved and we have our new resistor values. The 600k load on the battery will certainly work to drain the battery more quickly than the original 10 meg resistor illustrated. Nonetheless, even the cheap 9 volt batteries have a capacity of at least 200 milliamp-hours, which means the battery should last at least 2 years at this rate of discharge.
So much for the black boxes. Since tube circuits are the center of focus, let's go over the two tube based implementations of this technique.
From the preamp side, adding a trickle current to the interconnect requires adding an additional resistor to last month's schematic.
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