Weird ring/thump noise when I hit my speaker cable

Good question Thsalmon. Flicking speaker cables isn't exactly a habit of mine. Actually, I first noticed the sound when I was fiddling around with power cords on my CD player and the cord bumping against the the equipment rack produced the sound. I then went around rapping my knuckles everywhere - the shelves, the pillars, the amp, the CD player, all of which produced the sound. But I found that the sound was loudest when I flicked the left speaker cable with my finger.

If it is happening with you "tapping" on various components physically / electrically tied to your system, you can bet it is happenind at all times when you listen to music. As Gthrush mentioned, this is called microphony and will vary with the intensity of vibrations applied to the components. In such cases, the end effect is smeared sound which gets worse as volume is increased. Due to the increased volume, you've now got more vibrations filling the air, which further modulates the microphonic devices harder.

As Marakanetz suggested, start by checking your tubes. I would first start off by making sure that they are all properly seated before doing anything though. Then again, if you can "flick" your speaker cables and the system picks that up, touching anything in the system may set the problem off. As such, you could be in for some work tracking this problem down. If you can substitute another amp in the system and see if it goes away, you'll know it is something in the amp and can go from there. Sean
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Even perfectly normal tubes display some degree of microphony, which varies widely depending on the particular tube design. Tap on the glass envelope of any tube with your equipment powered up and you will hear it. Some hardly at all, some a whole lot. Given that music is nothing but vibration, this is one of those things that it's best not to think about too much if you own tube gear. Best to just enjoy the extra ambience it provides...:) But yes, check that everything is seated and working properly, and swap tubes if you need to.

Karls, tube microphony issues were resolved in late 40's. Now if tube has a microphony its a bad tube or will blow soon.

Marakanetz, have you ever actually TRIED IT? Not just on one particular tube, but on many different ones? I'm not claiming that they all ring like a bell, only that they do in fact have SOME microphony and that it varies a lot depending on the type of tube. As you would expect, it is usually most obvious in high-gain small-signal applications (phono stages, preamps, input tubes) rather than big output tubes. And if this issue has been fully resolved, why in the world would anyone want tube dampers or fancy platforms to set their tube preamps on? It would appear that you have unwittingly already taken my advice to heart:)