I guess the aim is to prevent feedback howling by delaying the amplified waveform long enough so it's detached from the live waveform?
I'll describe my experience with a reel-to-reel tape recorder. It has 3 heads. The playback head is separate from the recording head, about one inch distance. This allows me to compare the taped audio right after it's recorded. In other words, an echo. The longest delay available is when running at 1+7/8 inches per second. The delay is half a second. The repeat can be dubbed onto my audio (a feature known as 'sound-on-sound').
So does this setup make feedback impossible? I find I can get feedback even with 1/2 second delay, when the mic is placed near the speaker-box. The echoes build over a few passes across the tape heads, if the volume is loud enough. The howling gets equally annoying as that from a live PA system.