Prescription Electronics COB blowing stuff up

[modsquad]

Thought that would get your attention.  I have a friend of my son's that has a COB and he was telling me that when he has the octave turned upped high and pushed the amp, he has blown tubes and had to recone a speaker.  He now keeps it at 12 o'clock or below.  He has a Fender Bassman vintage amp.   

Anybody have any thoughts?   

[Mark Hammer]

Yes.

Normally,all the treble occurs in the first 100msec or so of the note, during the pick attack.  Distortions, and especially octave-up pedals, extend that duration of maximum treble for a considerable period longer.

So, what does this have to do with speakers?

Voice coils are doing double duty as fuses.  Both voice coils and fuses are thin wire that goes "poof" when there is sufficient heat buildup.  In the case of fuses, that is purely heat buildup from passing current.  In the cae of speakers, that is heat buildup from doing work.  When speakers reproduce high frequencies, they are doing more work than when sitting still or reproducing low frequencies, and generally building up more heat.  Why?  Because the gap between the voice-coil and everything that surrounds it is quite narrow, and unless the voice coil acts as a perfect piston, flawlessly maintaining an equal distance between it and the surround no matter how far forward/backward it moves or how often, it will generate heat due to friction.  And as all boy scouts know, if you want to make enough heat to produce glowing embers, you need to rub the sticks fast.  So, treble generates more friction-based heat than bass.  And of course part of why that is so is because the much shorter gaps between successive "rubs" prevent the heat buildup from dissipating.

So, high volume places the voice coil at risk because of current-related heat buildup, but inserting lots of sustained treble into things exacerbates it by increasing friction-based heat buildup.  The result?  Poof!

As for tubes, having used the same tubes for decades, I'm not exactly qualified to speak, but let us just say that sustained notes with sustained treble at maximum volume can be expected to shorten the lifespan of tubes.

EVERYTHING needs a rest from time to time.

[modsquad]

You are the man Mark.  That even made sense to me 

So the lesson is that over time pushing the treble withh the thing cranked something is going to give.