Never run your tube guitar amp without a speaker or dummy load connected to the speaker output jacks! In doing so you can do irreparable damage your output transformer and other components. The output transformer needs to “see” a load which is the speaker (rated in ohms) or a dummy load. Another point is to always match the amplifiers output impedance with a cabinet of the same impedance. This will ensure that you will get the most output power from your amplifier and avoid premature tube wear and possible damage.