Question 6A162

For a vacuum tube operating as a voltage amplifier, the primary goal is to achieve significant amplification of the input voltage signal. **Transconductance (gm)** is a measure of the tube's effectiveness in converting a change in grid voltage into a change in plate current. Specifically, `gm = ΔIp / ΔVg` (change in plate current divided by change in grid voltage). The voltage gain (`Av`) of a simple common-cathode amplifier stage is approximately `Av = gm * RL`, where `RL` is the plate load resistance. Therefore, a **high value of transconductance (B)** is most desirable because it directly translates to a higher voltage gain for a given load resistance. A tube with high `gm` can produce a larger change in output current, and thus a larger voltage swing across the load resistor, for a small change in input grid voltage. Conversely, a **low value of transconductance (A)** would result in very little voltage gain, making the tube ineffective as an amplifier. **High output tube capacitance (C)** is generally undesirable. It can limit the high-frequency response of the amplifier by shunting the signal, forming a low-pass filter with the load resistance, and potentially leading to instability or oscillation, rather than improving voltage amplification.