Question 6A429

In a radiotelegraph transmitter using grid-bias keying, the key typically switches a negative voltage (blocking bias) to the control grid. When the key is open, this bias should be sufficient to drive the vacuum tube to cutoff, preventing emission. * **A) Insufficient grid blocking bias is applied to the grid when the key is open:** If the negative bias voltage applied to the grid is not strong enough to cut off the tube's current flow, the tube will continue to conduct, resulting in continuous emission. * **B) Shorted bias resistor:** A bias resistor helps establish the correct negative grid voltage. If this resistor shorts, it reduces or eliminates the intended negative bias, causing the grid to become less negative. This would allow the tube to conduct when it should be in cutoff. * **C) Shorted key-click filter:** A key-click filter (often an RC network) is typically connected across the key contacts or in series with the keying line. If a component within this filter shorts, it can effectively short-circuit the keying line, bypassing the key. This prevents the blocking bias from being properly applied when the physical key contacts are open, keeping the transmitter on. Since all three scenarios prevent the tube from being properly cut off when the key is open, "All of the above" is the correct answer.