Rehousing a Peavey Speaker

[anrque]

Phil thank you again, for both the knowledge and your patience. I think I missed that we were talking about the added variable of "tuning." I was getting hung up on dimensions first and choice of materials. The 55L at (purple) 55Hz is a fine number to settle on.

When you speak of tuning, this is the job of the port itself, correct? And the length of the port will determine the frequency tune?  (Really the volume of the pipe but if I settle on standard drain pipe 110mm UK or 4" here in USA the length is the only part I vary)

I picked up a 1/2 sheet (30" x 60") birch plywood today. It is being sold here are 1/2" but comes in at 12mm by my caliper measurements.

I'm going to sketch this out on paper in the next few days and see if I can possibly get started by this weekend.

 

[Phil Starr]

That's brilliant, I still love a build. 

 

It would be worth measuring the inside diameter of the drain pipe, It's probably the same as here but we don't want to do a NASA. I still work in mixed units as does most of the building trade. Our sheet materials are 8'x4' and I still have to ask for 4.2metres of 8x2" at my local timber merchant.

 

The tuning is a function of both the cross sectional area and the length, a narrower pipe will be shorter for the same tuning and we can trade off the length with a narrow pipe if we need to. the cost then is the increased risk of turbulence and therefore wind noise in the pipe. 4" should be fine with a single 15 so that is a good starting point. The other material I use for ports are the cardboard tubes that carpets are wrapped around. The warehouses usually are happy to give them away but drain pipe is good and comes in black. I'm getting 7.3cm  as the length for the port. Port velocities are high for full power though, That speaker shifts a lot of air. 

 

I'll examine tuning and port velocities tonight. I set those tunings to bracket the sensible range it may be that tweaking it a few Hz gives us a little extra. 55Hz looks good but I may as well double check before you start cutting.

[Chienmortbb]

On 14/10/2021 at 02:25, anrque said:

Phil thank you again, for both the knowledge and your patience. I think I missed that we were talking about the added variable of "tuning." I was getting hung up on dimensions first and choice of materials. The 55L at (purple) 55Hz is a fine number to settle on.

When you speak of tuning, this is the job of the port itself, correct? And the length of the port will determine the frequency tune?  (Really the volume of the pipe but if I settle on standard drain pipe 110mm UK or 4" here in USA the length is the only part I vary)

I picked up a 1/2 sheet (30" x 60") birch plywood today. It is being sold here are 1/2" but comes in at 12mm by my caliper measurements.

I'm going to sketch this out on paper in the next few days and see if I can possibly get started by this weekend.

 

I think 12mm is within the normal thickness variation for 1/2" ply. I should add that 1/2" is 12.72mm. That equates to approx 2/64" undersize

Edited October 15, 2021 by Chienmortbb

[anrque]

I like the NASA reference. We all groaned that day...

[anrque]

I was hoping to at least get the headshell knocked together today. No such luck. I dragged the table saw out in to the driveway and set up a makeshift feed and outfeed tables with stands/sawhorses. I dug out the zero clearance insert I bought ago and never used to limit tearout on the birch ply. Switched the saw on and slowly raised the blade to cut through the blank zero clearance plate the first time. I was going to use the shallow cut method to score the underside of the ply at less than a mm and then repeat with the blade raised , again to prevent tearout.

I practiced feeding in the half sheet of birch with the blade lowered and the saw off to see how it would work. No matter what I did, I could not get the ply to sit against the table saw fence with the  factory edge. Realized that I was going to ruin this $40 half sheet of ply if I proceeded.

Decided that I am going to reverse the procedure and bring the saw to the plywood. I'm looking at edge clamps and DIY edge guides now on YouTube to use with my circular saw. Time to run to the big box HW store...

I will try to pick this up again on Sunday.

[Phil Starr]

This all sounds very familiar, I use a Sheppach site saw for this sort of job and with the English weather working outside is a problem. My workshop is too small to manoeuvre an 8x4 sheet. Lots of temporary tables to set up. I recently bought a cheap plunge saw with a track and it means I can now cut the board down initially with much better accuracy than with the Skillsaw. It also does a scoring cut which is so successful I've started doing it with the table saw. Someone locally is selling a big industrial panel saw which I'm very tempted by but I've nowhere to set it up at the moment. I'm going to try my next build using just the plunge saw just to see how accurately I can cut with it. It's this sort of thing  YouTubePlunge Saw

Edited October 17, 2021 by Phil Starr

[anrque]

I grabbed a Kreg rip guide last night and I'm going to try this out today. I also bought some solid core foam insulation to use as a sacrificial backer board. Wish me luck!

[anrque]

Alright, good thing I planned this job for Saturday, because it took me until end-of-day Monday to get it all cut and dry fit.

The Kreg edge rip guide was the way to go instead of the tablesaw for even this half sheet of birch ply. I also grabbed the 1.5 inch foam insulation to use as a backing board. The rip cut to 11.75" width for the headshell went smoothly.

The first scoring cut and then cutting to full depth also left me with a clean edge and no tear out.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

That was Sunday. I then tried my chop saw for cross cuts and no matter what I did, I could not get it clean and perfectly 90 degrees. So I chopped the head shell top, bottom and sides oversized and got everything dimensioned properly on the tablesaw.

Today (Monday) is my last day of my work sabbatical, so I wanted to get the box joints finished. Setting up the jig and getting bits centered in the router went easy. First cut and I had massive blowout from the 2 blade 1/2" cutter on the inside on the first 2 pins on one of the side pieces. It was nasty but I didn't take any pictures. I was so flummoxed as to what to do, I forgot. I had a spiral cutter that I should have been using, but it was 1/2" and I wanted to use my smaller 1/4" collet trim router. Next time for the speaker box I'll pull out the 1/2" Bosch router and use the spiral bit.

I found some 1/4" MDF and cut a sacrificial face board to put over the plywood. This worked and the rest of the cuts went more or less OK. I did have a scare as I was not checking the tightness of the router bit after every pass. On the 4th or 5th cut the bit must have loosened and my cuts got shallower as it pushed the bit up to the collet. I am really glad I stopped at this point and did not hurt myself with a metal cutter spinning at 20,000 RPM.

I dry fit the headshell box to check all my joints. I'll clean it up and glue it together this weekend.

I also test fit the Peavey combo chassis to get an idea of what it will look like.

Now, I get to go back to the office after the last 10 weeks off. Argh...

 

 

[Phil Starr]

"I did have a scare as I was not checking the tightness of the router bit after every pass. On the 4th or 5th cut the bit must have loosened and my cuts got shallower as it pushed the bit up to the collet. I am really glad I stopped at this point and did not hurt myself with a metal cutter spinning at 20,000 RPM."

 

I did that just once, the bit flew out like a bullet and smashed on the wall opposite bending the metal shaft, you really wouldn't want it hitting something soft

[anrque]

Yeah, I definitely had a moment of "ya dodged a bullet there, fella."

[stevie]

Please run that by me again, as I'm currently doing some routing work (as a complete novice) and am very attached to my fingers. Do you really need to keep tightening the router bit while you are working? It's a bit awkward with my particular router, as you lose the depth setting when you go in with a spanner.

[Phil Starr]

You shouldn't need to retighten the router at all but the collets sometimes aren't all that smooth and mine has a kind of false biting point where you get resistance but it isn't quite tight. It's worth double checking is all. They are one of the more lethal bits of kit, The rotation speed and gyroscope effect gives them a mind of their own and nothing else I have rotates anything like that fast. There's also no safety guard once you've locked in the depth of cut. I much prefer mounting the router and moving the workpiece if i possibly can, I don't enjoy freehand routing. 

[anrque]

What Phil said and additionally I probably "should" have stuck with my larger Bosch router. The collet is bigger (1/2") and I could have used my straight spiral cut bit. Also, it comes with 2 wrenches that you use in opposite directions to tighten the collet. The router I did use was a smaller Dewalt trim router with a 1/4" collet and uses a single wrench to tighten along with a button you depress that "locks" the router spindle from rotating. I think my Dremel also has something like this too, and although convenient, I always feel like I am not getting the bit in tight enough. I think the 2 wrench style is better.

 

The more I think about it, these dovetail jigs are flawed in that they require you to make the complete pin or tail cut in one go. Normally, when I route out a guitar body, I either cut away or hog out as much wood with a saw or Forstner bit. By the time I start routing with a template, I am only removing small amounts of material and lowering the bit until i get to the final depth. I do not like a 2 cutter dovetail bit biting full depth into a cabinet side. The is now doubly so with routing this birch ply. The Fender-style cabinet I did half-blind dovetails in solid pine was not the endurance test this Peavey headshell was. Retrospectively, since these were straight cuts for box joints instead of an angled dovetail cut, I guess I could have removed material in smaller bites.

 

I still need to work out final dimensions for the speaker cabinet, but I realized my dovetail jig is only good for 12" width boards. This headshell basically maxed it out. The speaker cab with bee deeper, so I think I am limited to rabbit joints with the tablesaw.

[anrque]

No progress on the head shell, too much going on. I did verify that the Scorpion is still operational after cleaning out the old foam dust protector that disintegrated on me. I had disassembled the magnet a month ago and cleaned all of this out as well as using tape to fish any garbage out of the voice coil gap. I reassembled the magnet to the basket but never had any way to verify the speaker was still OK.

 

Well this weekend, I popped the speaker back in the old Peavey cabinet with a new speaker wire pigtail terminating in a 1/4" jack. I plugged this into the amplifier out on my Orange Terror Stamp pedal board amp and it tested out fine. The bonus was learning that the Terror Stamp made a fun little bass practice amp as well.

 

Glad I verified this before spending time constructing the cabinet only to find I had messed up the speaker.

Edited October 25, 2021 by anrque
more info

[Phil Starr]

If anything happens to the speaker the cab is still a good potential home for quite a few 15's out there with only a bit of tweaking o the port length. Still it's great to know you have a working Scorpion.

[Hacksawbob]

Hi Following with interest, I bought a Peavey TNT 160 Which has the same Scorpion 15" speaker. I bought it as a fixer upper but it didn't need a great deal doing to it, mostly cosmetic and a new jack socket which is all sorted now.  After having played it a couple of weeks the sound is really growing on me, but the the weight of the thing,  it has it's own event horizon! Definitely a younger fitter persons amp. I am like yourself in that I just need something to play around the house and can be moved out of the way if necessary. All these graphs mean nothing to me though. I am fairly handy with tools so I think making a box is within my skill set, probably not a pretty one though!

 

I'm be interested in further details the dust cover you mentioned on the speaker, I took it out briefly to check for damage but I didn't see anything that resembled 30 year old foam.

Edited November 10, 2021 by Hacksawbob

[Phil Starr]

I'm not familiar with the Scorpion itself, all my Peavey experience is with the Black Widow speaker which looks to be similar. The dust cap which you get on every speaker (just about) is usually just a fabric or paper dome stuck down onto the middle of the cone , primarily to stop dust and crud getting into the magnet/coil. The foam is/was found inside the back of the magnet if I remember. I haven't had that problem. Like the BW you can remove the magnet off the back of the scorpion and see inside and clean out any debris. If your speaker sounds good and doesn't make any 'off' sounds then you probably don't have a problem to solve.

 

So I'm recommending a 55litre cab with a 4" soil pipe forming a port, you only need 7.3cm length though. Are you confident enough to work out the panel sizes yourself? 

 

This might be useful it has instructions for taking the magnet off. Be gentle with the voice coil though, it's easy to damage with a big heavy magnet.

Edited November 10, 2021 by Phil Starr

[Hacksawbob]

That's great thanks, I think I can work it out OK, ideally i'd reuse the amp in a separate unit so that defines the width and depth to some degree. the height would hopefully make up the remainder.  I take it you remove the volume of the speaker itself? some sort of cone pi calculation which I probably missed in an 80's maths lesson    I could have it cut out of ply at local timber merchants/ B&Q, I can cut the holes with a jigsaw. I'm OK with  soldering electrics so long as I have a schematic to work to.

 

I could 3D print the port, is there any benefit to having it a straight cylinder (soil pipe!) or is there something more efficient, rolled edges/ vortex like/ bends? seeing as I am handy with Fusion 360 I may as well use it. But I am really in the dark as to what it all does, so following you blindly!

 

Any benefit to adding a tweeter like on the 12 build? I dare say this is a whole other can of worms but your advice is greatly appreciated.

[Phil Starr]

You should add maybe 8 litres for the speaker and port. if you can flare the port at both ends it reduces the sudden pressure changes and ultimately the turbulence you get which causes port noises at high levels. However it will also mean that if the tuning is out you can't easily change the length of the port to re-tune. I suppose one answer will be to make the flares so that they are designed to push fit into the end of a standard soil pipe. That would be quite neat and there might even be a small market for them

 

However I've never had a problem with port noises in a band setting, even with smaller ports than these which should be better than the original Peavey ports anyway, if it's important to you and you have expertise then give it a go but it's a luxury touch not essential IMO.

 

Adding a tweeter will change the sound of the speaker. That's complex because of the way we hear sound and because the changes will depend upon the exact characteristics of both the speaker and the compression driver/horn that you choose. If you like the sound of what you have then do you want to change the sound. Crossover design is complex and still requires a lot of trial and error and ideally proper measuring equipment. You can buy a generic crossover but it will only be very approximate. I can talk you through some of the design problems but I'm not really an expert.

 

To be fair a lot of commercial cabs with horns don't have well designed crossovers so you can get something that 'works' from a generic crossover and you might like the sound. It doesn't have to be 'honest' to sound good.

 

There are pluses though in having a horn. A 15 is very directional in the mids and tops so what you here and what the audience hear wont be the same. The upper range is all coming with the cone under break-up conditions when the cone is no longer a pure piston and as such is distorted and often slightly louder on axis than the bass. It's part of what gives 15's their lovely old school sound. Get the horn and crossover right and you'll get a much more modern flat response. More like a bass played through headphones. It's up to you really but you could just build the cab but leave space on the baffle (front board) to retro fit a horn if you later decide it's a must have.

[Hacksawbob]

Thnaks for that, great information. I've had a quick look at cross over design but it seems like just this part of the build would take a lifetime of research so I'll leave the option for future addition. If you know of any resources that can explain it clearly I'd appreciate it.

 

I had a tinker in  in Fusion today and came up with this, an adjustable port 4 inch internal diameter (internal) which can adjust from about 40-80mm length with threaded tubes. It's quite a long print, probably take a couple of days so I might need to optimise it, but you get the gist. I might look at converting an existing 4 inch pipe with an interferance fit (can be glued once you find your sweet spot). But if I know the range of port lengths for various bass frequencies it might reduce the variables and therefore the print times/materials.

[Phil Starr]

Hmm it's 50 years since I did any fluid mechanics but there's not enough of a radius on that design to make any difference to the air flow inside that port so the only advantage IMO is that the length is continuously variable. though the lip covering any problems in cutting a hole in the baffle will improve the finish.

 

To be honest it's a lot quicker to cut lengths of pipe

[Chienmortbb]

22 hours ago, Phil Starr said:

You should add maybe 8 litres for the speaker and port. if you can flare the port at both ends it reduces the sudden pressure changes and ultimately the turbulence you get which causes port noises at high levels. However it will also mean that if the tuning is out you can't easily change the length of the port to re-tune. I suppose one answer will be to make the flares so that they are designed to push fit into the end of a standard soil pipe. That would be quite neat and there might even be a small market for them

Precision Ports are probably the leaders in flared port technology. A quick look at their website will give you an idea os the flare required.

[Hacksawbob]

Maybe something more like this...

[Phil Starr]

That looks a lot better, you'd need to think about how the pipe butts up against the flare as that joint would need to be as smooth as possible.  Think as if you were designing the aerodynamics of a car, Any sharp joins are going to cause wind noise and drag. Maybe a collar running outside the flare and the pipe?

 

You'd also want to slim down the front flange unless you intend recessing it into the baffle.

 

This is now looking like a really interesting project.

[Phil Starr]

On 11/11/2021 at 23:16, Hacksawbob said:

Thnaks for that, great information. I've had a quick look at cross over design but it seems like just this part of the build would take a lifetime of research so I'll leave the option for future addition. If you know of any resources that can explain it clearly I'd appreciate it.

 

 

There's loads of resources out there but as a practical, working source of advice The Loudspeaker Design Cookbook by Vance Dickason is as good as it gets, TBH nowadays it's my bible, I read a lot of the more technical stuff years ago but don't go into that level of detail in building and designing speakers. It's my go to when I want to check advice I give here is correct as far as it goes.