To transmit Morse code via radio, we on-off-key a radiofrequency (RF) transmitter to form the elements—dots, dashes, and spaces—of the code. Multiple on-off keying methods can be used, depending on the RF power level and active devices (vacuum tubes or transistors) used. In this page, I discuss the commonly used technique called cathode keying, in which the direct-current (dc) flow into the cathode connection of one or more vacuum tubes is interrupted to achieve on-off keying, and how cathode keying becomes much more potentially hazardous B-minus keying—interruption of the negative lead of the high-voltage power supply to one or more stages—with a subtle reconnection of just one component per keyed stage.

The Circuit Difference Between Cathode Keying and B-minus Keying

We see it especially in vacuum-tube-based oscillator circuits that are on-off cathode-keyed to form the dots and dashes of Morse code: Sometimes the oscillator grid-leak resistor is returned to ground and only the cathode dc return is keyed, and sometimes the grid leak is connected to the cathode and both the grid-leak dc return and the cathode dc return are keyed simultaneously. The difference is profound and potentially dangerous: Keeping a stage's grid leak grounded and keying only its cathode dc return results in an open-circuit voltage equivalent to only the stage's cutoff bias—plus the peak value of the stage's grid drive voltage, if it is driven continously across key-up and key-down transitions. Connecting a stage's grid leak dc return and cathode dc return together and opening and closing their ground connection results in an open-circuit voltage equal to the stage's full anode supply with an available current equal to the stage's full cathode current. This is why we differentiate between these keying arrangements by referring to them with different names: Keying only a stage's dc cathode return is cathode keying, whereas keying a stage's grid dc return and cathode dc return by opening and closing one wire common to both circuits is B-minus keying because a tube's anode supply is traditionally referred to with the alphabetic identifier B (with A used for filament/heater supply and C for grid bias supply).

The Greater Danger of Electrical Shock with B-minus Keying

B-minus keying is potentially much more hazardous—pun ongoingly noted—to a human operator manipulating a Morse code key than cathode keying because it involves opening and closing the keyed stage's full anode supply. As an example of the difference, an 807 or 1625 beam tube operating at a plate (anode) voltage of 750 and a fixed screen-grid voltage of 300 cuts off at –45 volts or so. (In actual operation, its peak grid voltage may be much higher, as for Class C, but if the stage is not driven when keyed, the open-circuit voltage at the key will rise to no higher than cutoff.) Cathode-keying such a stage therefore involves making and breaking a 45-volt circuit at the stage's full cathode current. B-minus-keying the same stage involves making and breaking a 750-volt circuit, also at the stage's full cathode current. Although a potential of 45 V can produce a nasty electrical shock under the right conditions, a voltage of that order is much less likely to be lethal than a potential on the order of 750 V.

The Greater Danger of Heater-Cathode Breakdown with B-minus Keying

Why Use B-minus Keying at All If It's So Dangerous?

Ensuring the Safety of People and Tubes with Cathode and B-minus Keying

[to be continued...]