Marshall 1931 1x12 cab - buzz

[guitarman555]

Hi, my 2 cabs (1x12) buzz when I put them through Marshall PA 20. Both are equipped with 1xG12 H - T 1217 from 1972.
The cabs are
Marshall 1931 A - original cab in good condition, from cca 1990
Marshall 1931 copy

Both of them have buzz when the volume on Marshall PA 20 is around half way and higher. Anybody has an idea how to solve it? Or can it be the speaker (speakers looks in great condition for their age, almost intact)....?

Note:
Sound si perfect, buzz is not loud, but you can hear them when the long notes are dying out (especially empty strings or power chords on the bottom). it is definetely not a big issue, but if I could reduce the vibrations just a bit, woud be cool.
When I play under half way volume, there are no vibrations. But I wanna play even louder!

 

[Decadent Dan]

Did you check for voice coil rub?

 

[guitarman555]

How can I check or repair voice coil rub? I never did it...

Sound si perfect, vibrations are not loud, but you can hear them when the long notes are dying out (especially empty strings or power chords on the bottom). it is definetely not a big issue, but if I could reduce the vibrations just a bit, woud be cool.
When I play under half way volume, there are no vibrations. But I wanna play even louder!

 

[Decadent Dan]

Check the mounting screws too. Even tension. Not too loose and vibrating on the baffle or too tight and pulling it out of round.

 

[guitarman555]

Check the mounting screws too. Even tension. Not too loose and vibrating on the baffle or too tight and pulling it out of round.

This is something I already took carefully for, yes well tighten but not too much. I even put two shims ( one normal and one plastic) it didn't have any effect on the buzz.

 

[Go Nigel Go]

Buzzes and rattles can be super difficult to locate and diagnose, and that is the first step to fixing anything.

I would use a stethoscope to try and localize the source while someone strums the guitar. The rattle will get louder the closer you get to it. For a "poor man's" stethoscope you can use an old mechanics trick of holding a screwdriver to your ear while touching various places on the car. Obviously it doesn't have to be a screwdriver, a wooden dowel for example can work just fine. You make a sort of "cup" with your hand while holding one end of the device (such as the screwdriver handle) and place the cupped hand over your ear so the end of the device is close to the ear inside the cup. Cover your other ear to block as much room sound as possible and touch the other end of the device to different places and see if the rattle gets louder. When you are "on top of it" the rattle will either be the loudest part of the sound getting transferred up the stick or the rattle will go away entirely if touching it dampens it's movement. Either way you will know you found something.

Obviously you don't want to be touching speaker cones in this way, but you could touch various places on the basket and magnet areas if you can get at them (like with an open back cab). The easiest way to check the speaker is usually to swap with a known good one if you happen to have one you can use or borrow.

Before anything else I would try moving to a different room just to make sure the rattle is unchanged and definitely coming from the amp/cab. I know it sounds silly if you are already pretty sure, but sound (and rattles) can be tricky as hell. Not always what seems blatantly obvious. Ask me how I know...