Ive been fooling around with reverb tanks again lately ,
using REW to view the FFT ,
I think theres some new functionalty in REW V5.3 relating to RT-60 time measurements ,
using it to test reverb tanks is the obvious next logical step ,
Viewing the output of the spring on the FFT panel something becomes very clear ,
the settling time for the spring at low audio frequencies takes very long ,
like 15-20 seconds or more , any physical vibrations the tank is subjected to create a powerful low end hump , which doesnt die away for ages .
Heres the type of tank I'm looking at just now ,
Only one spring is driven , the recovery is taken from the far end of the Z shaped arrangement ,
Output transducer measures 370 ohms with the DMM ,
with the LCR
380 ohms @100hz
470 ohms @1khz
14Kohms @7.8khz
Ls-385mH
Worth noting the 4 springs mounting the sub tray have a small piece of felt pushed into them ,
thats extremely effective at damping large low frequency wobbles due to vibration ,
I might try the same principle with the main springs ,
Lining the sub tray with felt is another trick I do , it prevents the springs ever touching down on metal and in the event the artist kicks the amp over at the end of the gig it doesnt create that horrific tearing iron sound associated with spring tanks .
Hmmm a thought just came mind ,
instead of felt , the loop side of self adhesive velrco tape might make a good crash landing pad for the springs , I might have to try that .
often in guitar amps the reverb channel return passes through a tiny coupling cap , maybe as low as 500 or 1000pF with 100kohms load following at the grid , so some HPF is probably a good idea.
Another thing is magneticaly damping the springs , a bit like a guitar pickup pole piece very close to the strings does .
my tests show a small low powered magnet within a few mm of the spring exerts enough force to give good damping in the subsonic range , Ive yet to audition the effect with my ears , but visually I can see the spring acts less like a slinky with the signal rebounding endlessly back and forth , it comes to a rest very much faster in the LF region .
A small thumbscrew which can raise and lower a magnet to the spring would be perfect ,
its a bit like a continously variable HPF/damping , without any electronic circuitry .
I should be back with the preliminary listening test results later today ,
The pic below shows the positions of the magnets a few mm below the rivet that joins the two spring sections .
Will be interesting to note any variation in impedence on the LCR with the magnets in place compared to without as well as how things look in REW/FFT.
