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7. Which circuit and tube for the second gain stage?
The second stage allows for more latitude in design options, as its noise contribution is less than that of the first stage's. The first stage's topology and tube could be duplicated, but as the gain fell a little short of the desired 30 dB, either a different topology or a tube with a higher mu should be used to make up for lost gain. The latter approach was taken, as it allows for an increase in gain and, potentially, a decrease in current draw. A quick review of the tubes shows the following tubes as good candidates: 12AT7, 5965 and 5751. As the phono pre-amp must work into the load resistance presented by the volume attenuator, the 5751 would be less desirable than the first two tubes, as it would be overwhelmed by the 50-100k load the volume control represents. The 5751's much lower current operation relative to the other tubes forces the use of a high valued cathode follower's load resistor, which when paralleled with volume control load would nullify the constant current draw feature of the circuit. On the other hand, the 5965 or 12AT7 would require a much lower valued cathode resistor on the Cathode Follower portion of the circuit, which would prove less sensitive to the 50-100k input load of the volume control. Because the 12AT7 is more readily available, this tube was chosen over the 5965. A plate voltage of 150 volts and current of 3 ma per leg was chosen.
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