MBL 8011AM need your help ......

I bought a used pair of MBL 8011AM 2 months ago but didn't open the boxes to use until last week.

In one amp,kept powered on, after about 30 minutes of no signal input, the amp would shut down spontaneously.
Is this abnormal, or , just from a previous setup of the amp to power off automatically?

In another one, the same condition was noted, but just after only 3-5 minutes of no signal coming in.

Anyone with experience of 8011AM use or now with these under use? ( so you must know )

Thank you in advance!