[Mondial]

Most likely not in a AA5, but theoretically the voltage across the output transformer primary with no load can reach infinity.

Say the speaker becomes disconnected and the output tube is driven into cutoff so whatever DC current suddenly stops. The magnetic field in the core collapses and tries to maintain the current flow. The energy stored in the magnetic field still has to be dissipated somewhere. The voltage across the winding can reach thousands of volts in this situation.

Here is an example of what I am describing. The trace shows the positive voltage across a 12 VDC relay coil. About halfway across the trace the relay current is suddenly shut off. Even though initially there is only 12 V across the coil, suddenly there appears a negative spike of several hundred volts. The same thing can happen across a radio output transformer winding

[Image: http://www.yoctopuce.com/pubarchive/2013...iode_2.png]