If you are searching for this term today, your results will likely be split between:
Many vintage tube amps were modified in the 70s and 80s. A technician might open a chassis expecting a stock circuit only to find a "surprise" custom-wound transformer that completely changes the gain structure of the unit. 2. Historical Context in Media tube tranny surprise
Conversely, the "surprise" might be a blown transformer caused by a leaked capacitor. Because vintage transformers are often hand-wound and specific to certain circuits, finding an exact replacement can be incredibly difficult and expensive. If you are searching for this term today,