12″ Ported Bass Cabinet. Fitting the speaker baffle. Finishing and test.

The baffle, having been cut, prepped and sprayed with primer – still needs a final coat of matt black spray paint to finish it off. Whilst I had the speaker in place for a dry run, test-fit, I noticed that, even though the face of the outside of the speaker cone appeared to sit fully recessed within the baffle board, there was a possibility that forward motion of the speaker cone might just, possibly, lead to the cone edge actually coming into contact with the speaker grille cloth. With my old “rabbit hutch”, 15″ extension cabinet, (where the speaker cone was visible behind it’s chicken-wire grille), I’d been surprised just how much bass cones can jump around. Sure, the holding bolts should compress the gasket down a little, but cone fouling cover still seemed a theoretical possibility, and I don’t know what the result of cone hitting grille cloth might be. I’m guessing it’s not ideal.

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Since I have planned to sit the baffle board well inside the surrounding carcass – there’s plenty of spare space to make the baffle board a little bit deeper – whilst still allowing the finished face to sit within the confines of the cab. In stretching canvases for painting, or for framing – there’s a technique where the material is stretched over a frame which has small, raised area right at the edge. This actually helps stop the underlying frame from showing through as a visible line on the stretched cloth. It occurs to me that I might use a similar technique to add a bit more depth to the front of the baffle, around the edges. That should help stretch the grille fabric, and add a good 5mm, or so, of distance between the front of the baffle board, and the stretched cloth.

I use a thin, softwood moulding with a radiused edge. The moulding is approximately 8mm thick – so routing a 3mm deep channel around the edge of the baffle board will allow me to glue the moulding securely and accurately in place – resulting in the 5mm extra projection. With the channel around the edge routed out, I can use my Morso cutter to mitre the corner joints of the moulding accurately, so that the stretching frame is partly recessed and securely attached to the baffle board with wood glue. With the pieces set in place, I clamp each in turn, until the glue goes off a little, and then continue to apply pressure on the drying joints with pieces of thick paper tape, until the glue has fully dried overnight. A few breezeblocks set on top of a thick, oak board acts as insurance – keeping things firmly in place.

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Next day – with the glue firmly set – I can finish off the corners with a few dabs of filler, before rubbing evrything down ready for a final coat of primer over the new, outside frame edges. The primer is left for 24 hours to fully cure, and then the entire front face of the  baffle is given an all-over coat of matt black car paint. This will prevent any interior details from visually telegraphing through the grille. The black coat gets another 24 hours to fully dry and cure, and then I can do a final check for fit. I need to make sure there’s enough clearance all round to allow for the grille cloth and the piping. I also need to make sure the clearance is consistent all around the enclosure. I don’t want any “gapping”, or tight spots.

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From here on in – everything else gets attached to the baffle board, and as each component goes on, and fits into place, there’s less and less flexibility to go back and re-adjust things or fix things that haven’t worked out. From here on in everything has to go right, or I face having to cut and finish an entirely new baffle. Those Celestion and Eminence specs had better be spot on – and my construction had better be up to the job too.

First to be fixed in place are the 3 inch port tubes. The Eminence specifications suggests a sealed unit similar in size to the one I’m building, (although the actual dimensions are adapted from a Celestion set of plans). The Eminence specs for the Basslite S2012 speaker specify a two port setup, with one port “flush”, and the other tuned to an overall length of 5.843 inches, (148mm). I managed to get hold of a pair of 3 inch tubes, both 150mm long. I trust, “flush” means the back of one tube is cut flush with the back of the board, and that 150mm is close enough to the desired 148mm, (if I’m wrong – I’ll have to find out the hard way). A bit of sharp work with a Dremel cutting wheel takes one tube down to  the correct length – the other, I’ll use straight out of the box.

I have another rough box made up out of some spare bits of plywood, (from another speaker cabinet project I’m currently playing around with), and using this as a support, allows me to sit the baffle level and secure at a comfortable working height, whilst having some of the unwieldy, protruding elements fitted in place. The port tubes fit neatly into their recessed holes, (the hand cut, irregular outer holes a bit of a visual embarrassment – but nevertheless functional). I mix up a bit of Araldite epoxy adhesive, and firmly glue the tubes in place. The epoxy takes around 15 hours to fully go off – so I leave the tubes in place overnight. When I check back the next morning – most of the tube fins, (the inside contact points between tube and baffle board), are well bonded – but a couple appear not to be quite in contact. I can’t afford to have these tubes working loose – so I need to secure any dodgy contact points now. I turn the board over, and add a few extra blobs of epoxy, just to make sure. It’s another 15 hours waiting for everything to dry again – but once the epoxy is fully dry and cured – the port tubes certainly don’t appear to be going anywhere.

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While the additional epoxy dries – it gives me time to sort the speaker fixings. I can’t find suitable T-Bolts anywhere this side of the Atlantic, and all of the standard, M6 size bolts available, have heads that are just too big to sit within the outside channel of the speaker frame. In the end, I source some ordinary, 30mm long, M6 type bolts online – together with some 6mm washers and suitable “Nyloc” nuts. I don’t want the nuts to gradually slacken due to vibration. The Nyloc solution works nicely on motorcycles. Why not here?

A metal cutting wheel fitted to the Dremel, allows me to cut off a couple of small bits from the head of each bolt – so that I’m effectively making my own T-Bolts. It’s a faff, and not exactly the precision work I’d like to be able to achieve – but it gets the job done.

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Once the epoxy on the port tubes is dry, I can turn the baffle board over, and load the speaker from the front. A few bits of cardboard around the outside ensures that the speaker is properly centralised within the recess, and the holes can then be accurately marked with a sharp 6mm, brad-pointed drill bit. The bit fits the securing holes exactly, and has to cut through portions of the cone edging which is left there during manufacture. I can be pretty sure, therefore, that the exact centre of each bolt hole is accurately marked by the sharp tip of the brad point. After double checking that I haven’t located any holes too close to the edge of the baffle board, (where they might run foul the cabinet carcass battens), I remove the speaker temporarily and drill the holes for the securing bolts – trying to drill straight and true. It helps to drill down onto another piece of timber, to stop the back of the plywood from splintering out. When I’m done – most of the holes look good and true, with only two showing slight splintering, and breaking through at the edge of the inside of the opening. Hopefully – there’s still enough wood there to hold firmly onto the bolts. The compression between the speaker frame and the washer should help keep things snug.

With the speaker back in place, I push all of the bolts home from the front, so that the hammerhead shape of each bolt head sits neatly within the channel of the speaker frame. It takes a bit of a wiggle to get all eight properly down into place, but once they’re all seated, I can fit the washers and Nyloc nuts from the back, and then tighten everything up with a socket wrench. Nice and tight. Eight bolts later – I don’t think the speaker is going anywhere in a hurry. The soft rubber gasket has compressed a few millimetres. The speaker is nicely recessed into the face of the baffle board, and I can’t force any play at all between the speaker frame and the baffle. I think it’s fair to say that the speaker is now, well and truly fitted.

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And confidence in the mounting and fit of the speaker and port tubes is vitally important. Since the grille cloth and piping will be applied over the front of the baffle – if I need to disassemble anything, or chase faults later, I’m going to have to rip off the coverings to get to the recessed mounts. Maybe it’s a better idea to rear mount the speaker? – I don’t know. I’m kind of committed now. Sometimes you just have to have a stab, make your plans, and then go with the flow, and see how things work out – learning all the time from the results, and the experience. Just as long as I can trust the quality of the build thus far – I can confidently finish the baffle board with its’ coverings, and then fit the whole shebang together. It will be, what it will be.

So the grille cloth is next to be fitted. The baffle is laid on top of the spare plywood box, and a piece of black and cream, “Salt and Pepper”, Marshall style grille cloth is laid over the board – with enough selvage on the sides to be able to comfortably grab the cloth and stretch it a bit. The cloth is stiff – so stretching it out with your fingers is a bit rough on the knuckles – but the technique is pretty much the same as stretching canvases for painting on. The cloth is stiff enough to take a slight crease – so it helps to gently form the cloth over the baffle first – applying one crease along one of the long edges – just to get things lined up. The important thing is to stretch the cloth evenly – so that the warp and weft of the fabric stay straight, and perpendicular to the sides. This can be tricky – but it helps to take one of the long sides and to secure that first. The fabric is stretched across the front of the baffle, and around the sides, where it is tacked in place with a line of staples from a staple gun. The staples go in, closely spaced, and on the diagonal – so they don’t stress the weave of the fabric too much. Starting from the centre – the fabric is gently stretched towards the sides, and is secured with staples – working outwards towards the corners. If you use one of the fibres of the weave as a sighting line – you can make sure the cloth is applied square to the first side.

I then find it useful to locate one of the opposite corners – stretching the fabric gently,but firmly enough to draw it taught – and so that the weave runs perpendicular, along one short edge, and  away from the side you’ve just stapled. Once you’ve assessed the amount of stretch required to pull the centre of this opposite side into place, you can then secure the centre point of this opposite side. The centres of the two short sides can then be located and secured. The weave from side to side, and through the centre should now show as running straight and perpendicular – parallel to each side in both directions.

Finishing the stretch is then just a case of stretching the fabric equally – gradually working from the centre of each side, outwards. Applying a few staples on one side, and then repeating on the opposite side. It should then be relatively easy to complete the three remaining sides – leaving a perfectly stretched grille. With all four sides now secure, I like to finish at each corner with a couple of perpendicular staples right up against the corner edges – so that the surplus cloth sticks out – sort of like the triangular front of a boat. I also like to run a second row of straight staples along the innermost edge of the baffle board at this stage. This will keep the fabric in place – even if it gets subsequently overstretched for any reason. It’ll also help stop the edges of the fabric from fraying too much.

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With the fabric in place, I can then turn the baffle board onto its’ front, and then use a sharp blade to trim off any excess fabric, flush with the back edge of the baffle. At the corners, I trim off the little boat shaped bits that stick out – still leaving coverage over the front corners of the board, but tucking the loose edges in, and securing with staples. A little tape over the edges stops things from fraying. The fibres in the grille cloth are stiff and quite sharp when freshly cut. It’s easy to get papercuts from them. A run of thick paper tape, or even Gaffa, over the inside cut edges helps keep your fingers safe from accidental damage, and also helps to stop the fabric from fraying.

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White vinyl piping is then applied to the edges – so that the tubular seam sits just in front of the forward edge of the fabric wrapped baffle. It’s gently stretched and held in place with another line of closely spaced staples. This adds yet another line of reinforcement for the grille cloth fixing. At the corners, the piping is bent around the angle, and extra staples go in to hold the corners securely. Obviously – the piping has to have a join somewhere, where it completes the circle – so it helps to locate that where it lies off the natural line of sight. Not dead centre. Not right in a corner. Maybe low down on one side. Once the circle has been completed, the overlap is cut through at the joining point with a single knife cut – so that both end pieces fit snugly together. It can help to slip a little piece of plastic dowel into the rolled edge of the piping – just to keep things in line, bit it’s usually enough to push the ends together, and then to secure both in place with a couple of staples.

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The complete, fitted-out, front baffle assembly can then be checked for final fit in the cabinet, and pushed home, into place. It’s a tight, but snug fit. Everything seems to sit straight and true – so I can begin to fix the baffle into place. It’s fixed with screws from the inside. I’ve already drilled suitable pilot holes in the front reinforcement battens, so that they avoid potential clashes with the screws used in the cabinet construction – so I can lay the cabinet on its’ front, and drive the screws in from inside, at the pre-drilled locations.

It makes sense to strip out the jack plug assembly and the handles while the securing screws go in. There’s not much room in there as it is – and the projecting handles only make things harder. Things are doubly critical since I really don’t want to accidentally damage the back of the speaker cone. The reinforcement battens are softwood – so to stop the securing screws from driving in too far, I use cup washers similar to those used on the back of the cabinet. I can then use the ratchet drive on a handheld drill/driver to drive the screws into place. There’s enough torque to screw into the plywood baffle edges, and also pull the board back snugly into position. Once again, I work from the centres outwards. 30 screws in all – so the board is held firmly in place.

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While the jack plug socket assembly is out – I take the opportunity to attach some 6mm female spade end connectors, to the bare wire ends, as supplied. The wire ends are already tinned, so I trim them to length and then secure them in place by crimping over the folding ends. A little solder flowed down into the fold secures the wires, and a conductivity check with a multimeter shows the wires are properly connected. With the little insulating boots slipped over the spade ends – the assembly can be reseated into the cabinet, and secured in place.

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Before the jack wires are fitted to the speaker, and before the handles are reseated in place – I also take the time to run a bead of silicon around the inside join between the baffle and the reinforcement battens. The battens are already sealed air-tight with silicon sealant on the insides. Running this extra bead around the inside will complete the air tight seal, so the speaker can work at maximum efficiency. I had, originally, planned to run a gasket around the front opening before fitting the baffle, similar to the gasket on the back, removeable, panel – but adding the extra depth for the baffle edging removed any tolerance I had to compress the gasket. Running a sealing bead around the inside offers just as good a seal – if not better – although one which won’t win any awards for elegance of application.

The silicon dries overnight, and then the handles can be fitted back into place. The last thing to do, before fitting the back panel, is to gently hoover out the enclosure to pick up any loose debris – staples, wood splinters, bits of dried sealant, biscuit crumbs etc., and then to fit the jack wires onto the correct poles of the speaker. One last check over, and the back can be fitted, and the securing screws tightened up. The cabinet is complete.

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Only one way to find out if it works. The cabinet is connected up to the correct, 8 ohm, ouput jack on the tube amplifier – bass plugged in, levels down – power on – levels up and…

Nice 🙂

Obviously – a full test and proper playing-in is required, but on first listen, the speaker cabinet sounds natural and open. There’s plenty of bottom end, (and a suprising amount of volume from a meagre 30watts), – but then I’ll need a solid floor and a bigger room to really test it out properly – not a vibrating old worktop in the workshop – but even there, there’s plenty of definition and, most importantly, no honking, farting or flapping, which might indicate a badly tuned port tube. Also – no apparent distortion or buzzing or rattling or anything like that, which might indicate a loose speaker mounting, or dodgy cabinet assembly. The only minor thing of concern is just how noisy my bass guitar circuit wiring is in the workshop. I’m picking up plenty of earth hum, radio interference, and one of the pots has gone a bit scratchy. It’s the fluorescent lights – but there’s another little job for the winter.

Of course – this is the first opportunity I have to really get to know the amplifier too – so there’s a lot to play around with, and plenty to twiddle. I need to try the setup out with my SansAmp bass driver DI too. That will add a shaped preamp stage, which I can then run into the all tube amplifier via the “active” input. This is all, of course, going to take time – but for a first test, I’m hugely pleased with the result. Chalk up another success for the DIY ethos. The cabinet sounds good, pairs reasonably well with with the amp styling, and the whole footprint fits into a cosy domestic setting which will offer plenty of distraction on the coming, colder winter evenings. It also offers a compact and practical, portable solution for, (small), gigging opportunities. 30 watts and one, single 12 inch speaker. Sometimes, it’s all you need. No, seriously!

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