We all know that it is a Bad Idea to run an amplifier with no speakers connected to the output. The reason is that the output transformer secondary winding will try to reflect the impedance of nothing to the primary winding and the output tubes. The output tubes respond by increasing the voltage, to extreme levels.
This voltage can and will arc to ground, or arc inside the transformer, or inside the tube itself – all of which are Bad.
Fender (and other manufacturers) usually employ a grounding switch on the speaker output jack which shorts when there is no plug inserted in the speaker output jack. This provides some protection, at least, in the event you forget to plug your speakers into the amp. Unfortunately, this does not offer any protection if the speaker voice coil fails open.
This Traynor YGM-3 had the original Marsland speaker fail open, which tripped the YGM’s circuit breaker. The user tried several times to reset the circuit breaker to no avail. A look inside the chassis revealed the reason:
The EL84 tube socket pin #7 (plate) has arced to ground rather spectacularly; melting the plate wire and even scorching the nearby cotton-covered wire to the grid.
Here is the amp with the damage repaired:
In this instance the output transformer survived, since the arc of the plate to ground ate most of the excess electrons.
How do we prevent this from happening?
We could put a 270 Ω, 5W resistor across the secondary of the output transformer. This should prevent catastrophic failure if the speaker should fail open. There are those who insist that this will change the tone of the amp. There are also those who insist that this will not change the tone of the amp. So it goes…