valve head question

[nikon F]

is there a particular component that dies/fries goes  bang when impedance is mismatched with an all valve head,as in 8 ohms into a 4 ohm cab ? or could it be anything and its down to pot luck ,,,,,,,,,,,,,,,,,,,,,,,,,,,im asking for a friend of course 

[itu]

This is probably easiest to understand by looking inside one amp.

Preamp is the part where you can tweak the sound (gain, eq etc.). It also prepares the signal to the power amp.

The power amp raises the voltage level so, that the speaker cones start to move.

Because the valve/tube amp output impedance (Z) is very different from transistor amp, the impedance has to be matched with the speaker. That is done with a power amp transformer. This particular component is not the same as the power line transformer, although the basic principle is very much the same: to transform one signal form to another.

The tube power amp output Z is not good for the speaker, so this big amount of iron and copper changes the output Z to 2/4/8/16/32 ohm output. It is often said, that the tube amp output Z has to be exactly the same as the speaker input Z. This is especially important if the power amp is driven close to its maximum. Otherwise this Z mismatch may kill the output transformer: when the mismatch heats the coils, they may eventually melt.

Output transformer is one big component that costs quite some. Melting may also kill other parts from the power amp. Expensive failure.

On the other hand, if the power levels are like playing at home (1 - 2 watts, not that maximum of 100 W or whatever it is), I can not see any restrictions why you could not use the set. Impedance itself is not one constant, as it changes over the frequency range. The given number is just an estimation and the transformer can understand some mismatch with low power levels that is probably 1/10 of the max output power. A rough estimation: if you can play with a band, find a matching cab.

Another thing is that using an amp without any load is far more dangerous than with any load.

[nikon F]

thanks itu . so it could be any number of things ,,,that 'll teach me to not get distracted whist playing around with different cab combinations.   ta