I checked entries 2077 [Central Guard House, Letters and orders received, 1862-65] and 2078 [Central Guard House, Orders Received Apr. 1862-May 1865] in Record Group 393, part IV, but did not locate any information about Ambrose Baker, 91st Pennsylvania Infantry.
However, entry 2086, registers of prisoners at the Central Guard House (volume 313, page 294) revealed the following information:
Cell 20
Prisoner: Ambrose Baker, 91st PA
Imprisoned on May 13, 1862, by order of General Wadsworth
"Held for Orders"
I regret that I can not copy this page for you, but the volume is too fragile. Above is a complete transcription.