You must always use a load with tube amps because, without it, the primary impedance will approach infinite resulting in an excessive voltage on the plates of the power tubes.
Here are the loads I use during testing: a pair (one per channel) of 8.2 Ohms
50 Watts (you can use 100 watts or more models if you're experimenting high power amps)
resistors mounted on a large radiator.
They are connected to the amp via banana plugs.