Don't want to sidetrack - but a question for Dave. I understand the concept, but thought the output transformer provided the current limiting - the primary/secondary ratio stays the same, so increases by a factor of 4 when the output is loaded with 16, rather than 4? With the HT supply fixed - somewhere between 200-400V or so, increasing the primary impedance lowers the current? Have I got this mixed up? I'm re-thinking my understanding of valves now. It's also stranger because with jack plugs still being quite common on speaker cabs, I'm not sure how CE marking would be possible with HT possibly on the easy to touch jack? I too have had a decent belt from a turn shorted transformer.
Ok, think not of "transformer" think "inductor". Basic electrical theory, the voltage across and inductor is proportional to the rate of change of current through it. Ever done a 'buzz' test on a big ole mains traff and got a belt? Everytime you see a relay coil in a sstate circuit you will see a 'clamping' diode across it and possibly a CR 'snubber' network.
So, worse case, no speaker at all. Secondary load is infinity therefore primary load is infinity X n (ratio) squared! In theory, in practice there is interwinding capacitance but the anode (plate) voltage can still get very high indeed. A higher than optimum load will still give excess voltage and two other factors conspire to help the destruction.
1) The impedance of a speaker rises with frequency. It is measured at 400Hz but at say 4kHz it could be 2 or 3 times that value.
2) Distortion produces lots of harmonics, HF (remember the "rate of change" bllx?) So, driving a valve amp into distortion especially with a higher than proper load is fraught.
The fact is almost all the failures in guitar amps are due to valve failure due to overdriving. There have been several designs over the years where not enough testing/thought has gone into what happens when the tits are driven off the thing!
I am not familiar with Fender amp history but I seem to remember that the early one were less rugged than other makes? Nothing against Leo! He was making CLEAN amplifiers and they have always had that reputation for pure, bloody loud sound!
Note, that it is not the VALVE that is the villain here? It is the TRANSFORMER! If you forget to plug a speaker into a valve amp, the traff will "sing" and you MIGHT see a flash in a valve but the seconds or so will not be long enough to damage much more than a fuse. Transistor amps with OPTraffs are rare but try that stunt and POOOF! Buggered in microseconds.
Bottom line. I personally would "misload" one of 'our' amps 1 to 2 and even hit it hard. I would NOT do that to a valuable old Fender. I would misload mine 1 to 3 but only to make gentle "is it on" test noises.
Dave.
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