Other than Bob, are there any real old-time amplifier builders out there? Not kit builders putting together a predesigned unit according to a specific circuit and layout...but those who bought a couple of transformers, some resistors, capacitors, tube sockets and a breadboard, and wired up an amplifier from scratch? You would remember watching an oscilloscope while trimming an amplifier's feedback loop for optimum, extended frequency response with stability, and would recognize what can happen when there isn't a load across the output transformer's secondary.
The amplifier oscillates due to phase shift at the extreme high frequencies, above the audible range. That resistor is chosen to present a load to the secondary that reduces or eliminates that oscillation so that it doesn't have the potential of burning out something.