Tube amps do get HOT. That power transformer is made to run at 125 - 140 degrees Fahrenheit. And yes - at that temperature you won't be able to keep your finger on the power transformer for more than a second or two.
1. It's warmer in the house in the summer and it's harder for the amp to get rid of the heat.
2. The power transformer has hot tubes around it causing a small thermal gradient. (the relative difference between two temperatures). On an ST-70 type amp the power transformer has warm output transformers on the sides and hot tubes to the front. Most of the heat will rise but some will radiate from the rear of the power transformer. (unless you pushed that amp against the back wall)
3. Tube amp's usually don't have heat sinks like solid state amps. The heat sinks absorb the heat from the solid state amp and air moving between the fins on the heat sinks takes the heat away.
Below is a photo of an ST-120 amp that is also on another post on the forum here. The ST-120 was being stressed last year under conditions that would cause it to get pretty warm. The amp was driving Tung-Sol KT120 tubes which use more current than any tube you can put into this amp. The Weber WZ68 gives a higher B+ voltage. The amp was played fairly loud in another room for about 3 or 4 hours while I was in my workshop. The infrared thermometer gives the Fahrenheit temperatures on various parts of the amp. I did neglect to take a temperature off the Weber WZ68 solid state rectifier. The Weber also gets pretty warm .. Probably as warm as the power transformer ...