Can you help me identify this amp so I can get a schematic for it. It is marked 7621 1. Also has a small sticker that says 311. It has 3 tubes- one 12ax7 and two 50c5. I was told it came from a zenith phono

May be chassis 3A04, 3Z04 if mono, 3L04, 3L05 if stereo (two output transformers) https://www.radiomuseum.org/r/zenith_hf11l_ch_3a04.html

This is not isolated from the AC power line and an isolation transformer should be used when operating it outside its normal enclosure.

Attached below, Sams Photofact Set-346 Folder-20
for chassis 3Y04

Be careful. This is a hot chassis, powered by line voltage directly. You should add an isolation transformer to protect any equipment you intend to plug into this amp.