We have all heard that it is Bad to operate an amplifier without a speaker cabinet plugged into the output jack.
For many years, Fender (and everybody else), have used a switching jack for the speaker output that grounds the output if there is no plug inserted into the jack. This gives some protection from the output transformer running without a load and frying itself, or something else.
What you may not realize is that this scheme offers no protection if the speaker connected to the amp fails open.
In the case of this Traynor YGM-3, that is what happened. The voice coil of the original Marsland speaker failed open.
This is a 1976 Traynor YGM-3 that a customer brought in because the circuit breaker kept tripping.
The inboard EL84 socket shows severe arcing on the plate pin to the chassis.
An open voice coil on the speaker means that when the amp was turned on, the output transformer secondary saw an extremely high impedance (almost infinite) and reflected that to the primary winding. The EL84s responded by trying to put very high voltage and current into that load. The voltage is high enough to bridge significant distance; in this case, the plate of the EL84 arced to ground and attempted to reach the temperature of the Sun.
Often the arc will form in the primary winding of the transformer (especially cheap transformers), thereby ending the OT’s life in a rapid and often spectacular manner.
Traynor does not employ a grounding output jack on this amp design; not that it matters, because a switched output jack would not have prevented this.
That is the problem that almost all vintage amplifiers have – they cannot detect an open speaker voice coil, and run the risk of destroying the output transformer (and other things).
If you own a YGM-3, you could put a 270 Ω, 5W resistor across the secondary of the OT – this should prevent catastrophic failure if the 8Ω speaker should fail open. The resistor is so high in DC resistance that most of the audio signal will still make it to the speaker.
There are plenty of people who will insist that this changes the tone of the amp, and plenty of people who say that it doesn’t. It is very easy to try it and draw your own conclusion.
