"Gareth Magennis"


Well, are you going to tell us then?


** The OT failed first due to internal arcing in the primary winding. This
is induced by overdriving the output stage until "spiking" occurs - ie
spike voltages on each rising edge of the squared off wave. Spiking only
occurs with loudspeaker loads ( or simulations ) and is produced by the
energy stored in the inductance of the voice coils. The voltage spikes are
of the order of +/-4kV. The problem is mainly with EL34 valves and
Marshall type output stages.

Arcing damage will produce shorted turns and sometimes open windings - both
happened here.

Once half the primary went open, the two EL34 screen resistors on that side
began to severely overheat, the screens would have been glowing white.

The two EL34s on the other side began operating under great stress as the
tranny now had shorted turns and a much lower impedance than normal.
Dissipation goes well beyond the max 25 watts, the plates glow red and one
EL34 quickly became gassy and out of control of the grid.

Then the 1 amp fuse blew, another maybe larger fuse was tried and that
resulted in the 22 ohm resistor failing.


.... Phil