It was our IOT transmitter. For the benefit of other readers, an IOT (Inductive Output Tube) transmitter uses vacuum tubes. It is a bit misleading to refer to it as an older type compared with solid state (transistors), as both are actively sold and in-use. IOT's are used for high power applications. They are more efficient and can have better linearity than solid state, and watt-for-watt are considerably less expensive. Their main drawback is the additional complexity of their liquid cooling systems and the need for high voltage (35 kilovolt) power supplies. A solid-state transmitter has, in place of one or two tubes, a good number of modules (16 per 7 KW cabinet), so the loss of a single module does not take the transmitter off air or reduce its output power by much, compared with 50% in the case of the loss of an IOT in a dual tube transmitter, or 100%(!) in a single tube transmitter.