In article , "N_Cook"
wrote:

Fred McKenzie wrote in message
...
In article , "N_Cook"
wrote:
The power amps always survive . In the process of exploring suspect caps etc
and turning over the pcb, the spade-connector for this 0V connection came
off the pcb spade. Looking under x30 on the spade and what I can see of the
line spade-socket, I can see no sputtering or smoke trails or anything
suggestive of a problem there.
I'm aware that a valve output matching transformer working hard and there is
a break in the speaker wire/connection /voice coil then enough voltage can
be induced in the primary side to knock out the output valves and weld turns
on the primary. But I do not know if similar in a mains transformer would
give enough current , from the energy stored in the inductance , to throw
back enough current , at over the mains voltage , to knock out the mains
fuse.

N_Cook-

For tubes, gain = Gm * R(load). A transformer coupled amplifier output
stage has a very high voltage gain if the speaker is disconnected. With
no load, tubes alternate between being saturated and cut off. When
saturation current flowing in the output transformer primary is
interrupted, inductance reacts to generate a voltage that attempts to
keep the current flowing. Voltage = L * di/dt, where di/dt (change in
current with respect to time) is almost infinite as current goes to
zero. This is comparable to the points opening in an older auto
ignition system, and voltage increases until insulation breaks down or a
spark occurs.

The reason this effect would not happen to your power transformer, is
that the two sets of filter capacitors are in series across the full
output voltage of the bridge rectifier. Current interrupted when the
center tap is disconnected, will be supplied to or from the capacitors.

Fred