Aria 5120 feedback

mr-mac

Ok, obtained one of these hollow body bases silly cheap for our open mic/band jam. The neck is lovely with very low action and it sounds superb. Apart from being a silly good deal guy who comes and plays bass is older and in a mobility scooter and wanted something nice and light for him as often also gets pain in hips and shoulder. Think I made right choice as he loves it...

however, here comes the age old feedback question, its very old and wood has matured nicely giving a very nice acoustic tone as well as lovely amplified tone. But as you guess it feedbacks like a bitch (not so good as the lovely auld goat also needs to hear himself which means having his amp up enough for his old ears to hear).

so first things first I need to tighten the jack so wiring will be coming out. While I am there I intend to wax pot the pickups (which I assume is a no brainer for such an old set and feedback reduction) not a heavy wax pot, just enough to hold windings better (30sec to 60 secs I guess). So hopefully it will reduce the chance of loose winding feedback but not totally kill the character of the pickups.

Next I don't want to stuff whole bass full but thinking I can at least fill it from neck join area to almost bridge through pickup holes. So what's best? I can use foam but guess that only stops sound travelling so well under pickups but wouldn't really tamp the front from vibrating much.  I could carefully build up some expanding foam along the centre line pickup width to bridge area which would reduce resonant space and once dry should offer slightly more damping/stiffness to centre area where pickups and bridge are. Yes it will change unamped tone and amped tone a bit but should still be between hollow and tone block style. My last fought is material/cloth of some sort packed into pickup holes along same centre line area (so should damp acoustic sound getting to back of pickups and reduce vibration in top wood in those areas).

So which would we recommend would be best option for filling/damping centre area from neck to near bridge. All are reverseable as a bent wire coat hanger should easily remove the expanding foam. Tbh I am vearing to expanding leaving sides hollow and rear of bridge hollow so like i say I damp slightly and reduce hollow area but not as severe as a total fill or a solid wood tone block but effectively same idea. If after that it needs a wee tweak then can make some plugs for f holes too.

Would love some comments and advice on my plan from some of you experienced guys and any suggestions I haven't thought of. Thanks loads...

Shark_Eyes

I think @ICBM might be best to talk to here. I remember he had a horror story involving a hollowbody and expanding foam, so he might have a simple solution for you.

mr-mac

Cool will give him a shout, plan to almost put a single line round where I want it to be, then build it up one small line at a time... If I don't firstly probs keep slowly expanding till fills bloody thing, 2nd it could, as it dries start expand in other direction and even split bass. By doing it one line at a time it should prevent any issues like that

ICBM

Yes, I had to fix an Epiphone Sheraton that someone had filled with expanding foam to 'stop feedback' - it certainly did, because it penetrated into the controls and also emits a corrosive vapour (acetic acid) as it cures, so it had wrecked the wiring too and it didn't work at all.

Luckily it hadn't got right up into the pickup cavities, so by cutting the pickup wires off close to them and adding new ones I was able to fix it "just" with a full new wiring loom. The real difficulty was clearing out enough space inside to get the old parts out and the new ones in - working through the f-hole with violin-maker's chisels. It was a lot of time-consuming work though - in the end it was almost as expensive as buying another second hand Sheraton…

So if you're going to try this, you really need to be very careful that you protect the wiring area in some way to stop the foam getting that far - not sure if building up a 'wall' of it will work, but I do have an idea - if you get a polythene bag, put a balloon inside it and arrange it so that with the bag inside the guitar and the end of the balloon outside, blowing the balloon up expands it to fill the control area tightly, that should work! The foam shouldn't stick to polythene so you'll be able to pull the bag out afterwards.

I think…

mr-mac

It was cheap, it doesn't have the damper bridge bit and outside has odd bash etc so its not an uber clean thing that would be a sin to alter to male it just what's needed for our jam. Light and not too feedbacky...

if it was in better condition I would defo be going along a route that is temp like cloth/foam/hole blockers etc

mr-mac

Icbm is going to have a wee look, he just had to fix the mess but didn't create it

ICBM

I don't actually think that it forcing the bass body apart is a serious risk, in fact - it doesn't seem to apply a lot of force as it expands, just keeps on bubbling until it has filled the space available… it's very penetrating into small spaces but I think that's because it 'wicks' in as a liquid and then expands, rather than by 'forcing' in.

I think it shouldn't even produce enough force to collapse a well-blown-up balloon, but it may be worth an experiment if you have a reasonable amount of it - try filling a small cardboard box with it, with half the box taken up with a balloon. If it doesn't crush the balloon or split the box you'll be fine.

mr-mac

Brill advice, thank you, a single line shouldn't expand far and once dry should stay put, the wiring is all coming out so I can check it over, tighten or replace jack and wax pot the pickups. Had considered tubes for pickup wire so you have just decided that one for me.  Going to Put in some tubes from pickup cavity area to near controls so can feed wiring through after foam done and pickups have a light wax pot.

only going to start filling area with foam once I have a dry top to bottom barrier at perimeter of area I want foam in, then build it up one line at a time so its not going to create pressure onto the outer area. As defo want to keep cavities under f holes and round bottom rear of bridge so retain as much character as I can while improving resistance to feedback.
such a small amount of foam each time it just doesn't have the volume of expansion to go that far from where I put it. Will leave tubes long till done then trim to size.

mr-mac

When expanding it can only force something out of way if there is nowhere for it to go (is fill a wall and cover with cardboard when it gets to point there is no space left to fill it then will very effectively push things out way as creates a pressure that needs to go somewhere. Should be a foam (if use stuff I do) almost as soon as out of nozzle so can only force through to other areas if enough volume applied to make if fill top to bottom then keep trying to go. As I intend to do a small single line, and maybe another and let dry it can't expand into those areas... Once have a dry foam barrier top to bottom I then build up a line at time back towards pickup holes (advantage here is if put in too much the already dry line of foam means it will only expand to path of least resistance which is back towards pickup hole). And easy enough to cut end recreate hole if make that mistake (but won't). Tbh the the chemical will probs kill a balloon anyway but won't be needed if carefully and slowly build up the barrier/perimiter and let it dry for 24hr

mr-mac

However if you try to do to big an area and outside dries first (or something without f holes it can apply a hell of a force.... Just look at what it can do to a speed camera

mr-mac

Will take some photos of what I do and how, maybe find a mirror with led lights that I an stick in to see what I am doing

ICBM

Yes, if it's inside an airtight space the gas pressure it will produce will be substantial.

I was assuming you wouldn't be using it in a sealed space, if the bass has various holes to the outside…

mr-mac

Exactly and its why if I create a full dried barrier with careful use of smaller quantities of foam then fill gradually back to pickup holes it should expand back towards pickup hole rather than fill guitar....

like I say will do best to get some photos and do a big step by step description with some lines on a photo showing where I put stuff inside. 

will be done with loom out and as I say some tubes for wiring to be slid through the foam once its dry.

Winny_Pooh

Will the foam not destroy the feel? personally I would track down some thin laminate, paint it black and glue it to the inside of the F holes using neo magnets placed through the pickup cavities to hold it. 

mr-mac

What feel? Blocking holes may help but think its far too resonant a body, sounds almost full acoustic bass, its not in good enough cond to worry about it being an interesting vintage instrument.... For moment I've wax potted and stuffed centre section below pickups with old towel as tight I can but even then a tap on body let's strings sound loudly for quite a while... Think thin resonant laminate used and is now very dry and old so even more resonant (only bracing I can see if a small post nearish bridge but not under it) tbh was built like a proper thin acoustic archtop.... may be lovely for recording but think they'd be a nightmare live...

Won't kill the feel of instrument as it would still be very resonant. It'll move it from acoustic sounding to 335 style (but as foam not as dense as a block of wood it should sit somewhere between the two styles). For a band jam if it sounds decent but is low fuss for feedback etc then its perfect. Will see how material and wax works for a starter for 10.

Rocker

Why not setup a single headphone (for the bassist) from the amp and move the amp as far away from the player as possible.  The whole idea of putting *anything* inside an acoustic or semi acoustic bass or guitar makes me feel nervous and concerned.....

mr-mac

Can't see why it make anyone nervous... Surely putting something inside a hollow thin archtop is exactly what Gibson did when they came up with the 335... Regular guy who plays bass is old and does it old fashioned way and tbh are we play in is very tight so even if i moved amp turned down and gave him headphones monitor speakers would still be an issue . Don't worry no gonna kill it, did a wax pot and used towel to stuff middle bit just now to see what it does

mr-mac

Well, feedback sorted not even a hint with a quick dip wax potting and stuffing some old towel in centre.  bridge pickup seems a bit iffy though, doubt i'll find a replacement, size seems unusual too so may need to make do with neck only for now