"Nutcase Kook"

a corroded break rather than shorted turn/s? now taking Tx readings
DVM White 61.8R , Red 75.3R was 61.3/66.4
RLC(L) 20.9mH, 34.0mH
RLC(R) 215 imp, 338 imp
Things certainly not right but what is wrong inside?


** Valve OTs do not get hot enough to melt the enamel insulation on the
wires, as often happens with power trannys. What brings about their demise
is the very high voltages that exist during operation into a loudspeaker,
particularly when driven into clipping.

Voltage peaks can then occur on the anodes of the output tubes that range up
to thousands of volts - resulting in insulation breakdown and arcing
between adjacent layers of the primary winding.

Sometimes an OT will test OK with a multimeter or at low power only to arc
internally as soon as full power operating conditions are applied. This is
due to carbonised material in the area of the previous arcing reigniting in
a new arc.

Marshall amps using EL43s are the prime example of this kind of OT failure.

Some amps, notably Music Man and some Boogie models, have high voltage
diodes fitted from output valve anodes to ground to prevent the sort of high
voltages that damage OTs.


..... Phil