Rehousing a Peavey Speaker

[Phil Starr]

I've just been asked the following question about designing a cab for a Peavey speaker and I'm going to keep this away from the original thread which might be interesting for people to follow as a piece of design. 

 

Copied from the Easy Build 12 thread:

 

Any way to easily adapt this Easy 12" design to a 15"? I have an old Peavey combo I want to rehouse the amp and speaker separately. The reasoning is 3-fold. Reduce the weight as the Peavey TKO 65 is heavy AF for a simple home practice amp. Also having the 15 as a separate cab will allow the use of other amps. Lastly its sort of ugly sitting in the living room.

Speaker is a 15" Scorpion and assuming info from Peavey is correct for entry into WinISD:

Impedance: 8 Ohms
Power capacity: 800 W Peak 400 W Program 200 W Continuous

Znom (ohms) 8
Revc (ohms) 5.54
Sd (Square Meters) 0.084
BL (T/M) 17.10
Fo (Hz) 50.1
Vas (liters) 132.4
Cms (uM/N) 174.5
Mms (gm) 70.30
Qms 6.05
Qes 0.345
Qts 0.326
Xmax (mm) 3.1
Le (mH) 0.3
SPL (1W 1m) 98.7
No (%) 4.70%
Vd (cu. in. / ml) 13.9 / 227
Pmax (Watts pgm.) 400

 

The existing TKO 65 must be completely undersized for typical WinISD 15" driver recommendations, yet I do like the existing sound just fine. I haven't cracked it open yet to measure the existing internal volume. I am hoping to end up with a smaller cabinet design like the one from this thread that I can build up out of poplar and stain to integrate into the household.

I have a woodshop/tools at my disposal and the requisite skills. (I've even done dovetails for a Vibrochamp clone in the past.) So it is really just the design stopping me at this point.

Edited September 26, 2021 by Phil Starr

[Phil Starr]

Hi I've put the parameters you sent me into WinISD and it has come up with a recommended 82litres as an opening option, basically to give you the flattest response in a reasonably sized box. I've also modelled it in a stupidly small box of 30l and a really large one as a way of you seeing the options open to you.

The darker green is the large box and shows you can get an awful lot more bass from a bigger box, not really a surprise  It's also not a flat response.

Light green is the 82 litres which is flat'ish 1db down at 100Hz and then rolls off gracefully with -3db at 70Hz and -10 db at 42Hz roughly the fundamental of bottom E. 

Red is in the 30l box which is really much too small for a 15. It is 3db down at 93Hz and has a hump at 150Hz.

 

This all illustrates the first question you need to answer, how much bass are you willing to compromise for making the cab room friendly. This is the same for all ported speakers, a big box gives you more deep bass and a small box less bass and a more or less pronounced peak ultimately giving you a boomy upper bass sometimes described as one-note bass.

 

FWIW most of my cabs for 15's tend to be around 60l or slightly bigger and when you measure the original cab I'll suspect it will be around that volume, it slims down the cab without losing too much bass or ending up boomy. If you post it up I can look at the figures and put it into WinISD and we can see what Peavey did. You like that sound so its a good starting point.

[anrque]

Phil, thanks again ( I thanked you in the other thread I posted to). I will see if I can't get the original Peavey combo cab measurements tomorrow and post the volume here.

 

For an additional reference, I also saw a video today from a guy that created a sealed 15" (non-ported) that was essentially half of a Fender Showman cabinet. I was pleasantly surprised at how it sounded. I mean it was a Youtube video and their AV compression, yada-yada, but I enjoyed the sound. He was running flats and had his tone 3/4 the way down. I know there a few different 15" Showman cabinets so it looks like their volumes for a single 15" would range between approximately 62L up to 98L.

Video Example:

 

[Phil Starr]

you are going to get bored with graphs

 

this is the Scorpion in the 'recommended' flattish ported cab compared with a slightly smaller sealed cab. You can see that the sealed cab makes a lot less bass. -3db is just above 100Hz so you are missing quite a chunk and there is about a 5db difference all the way down the bottom octave. It'll still sound like a bass but you can see why most designers go for ported cabs. It's a lot of free extra bass. Of course the slow roll off you get with a sealed cab makes them work well with a traditional bass boost but the cab will also affect excursion limits and power handling.

 

On the plus side the Scorpion is a really sensitive unit making lots of sound for a few watts and it is tolerant of a lot of different cab sizes. 

 

Maybe you need to tell me how you intend using it, if it is just for home use them maximum volume is not going to be an issue and the neighbours and family might appreciate a bass light sound.

 

[anrque]

Ha, not getting "bored of graphs" at all. I work for a tech company and background/design data is your friend.

 

This will be for practice amp home use. I'm thinking of using poplar and staining it instead of Tolex so it integrates into the living room with the furniture. I found an example online that convinced me that this would look nice.

 

Also, the reason for looking for a smaller cab design like your "Easy 12." I watched the video of you assembling one in a hour last night.

[Phil Starr]

I'm loving the finger joints and the way he's formed the corners in a contrasting wood, lovely attention to detail. 

 

The 'easy build' is really just the adoption of reinforced butt joints and drainpipe ports. I build a lot of cabs and can't stop experimenting so a quick build with no clamping suits me. With the easy-build carcass you can also just screw the back and baffle in place so they can be removed if you are still in the development stage and need to change things. You do get a bit of panel damping from the battens and the extra weight is minimal. 

 

 

[Phil Starr]

OK this is your Peavey in a 30l cab, again compared with it in a 60l ported cab. As you can see it is creating a small peak and pushing the -3db point upwards. My concern though is that you are 7db down at 80Hz. that 80-160Hz is essential to a bassist IMO as it covers the second harmonic for the whole bottom octave of a 4 string. The old guitar cabs often had a strong hump in that area. having said that the roll off of the 15 in a small box is gradual and you could safely apply quite a bit of bass boost with a speaker this size in a home environment. I've never built a speaker with that sort of response so It's an educated guess as to what it would sound like

 

[Phil Starr]

One more, this is a Celestion G12 guitar speaker in the 30l box which I built for my guitarist. this is a more typical old school response, more like what is in your video.

[Phil Starr]

Hi, I've been chilling out after entertaining friends for a couple of days and thought I'd revisit your design. Without exact measurements for your cab I'm thinking it isn't much smaller than 80l. Fiddling around with different sized cabs I think you have a couple of fairly simple choices.

 

The first is that if you keep the Scorpion then it works well in a cab of 60-80l. Any reduction in size loses a bit of bass. Your choice is:

to go for a really small cab like the easy build 30l cab and just accept it isn't going to have good bass output but that's something you can live with. Small size is more important than sound. 

build a cab similar in size to the Peavey cab  but reduced by housing the amp elsewhere. this keeps the sound you have and you could shave a little bit off the size and be OK

Go for a middle option of a roughly 50l cab which loses your bass but keeps good reproduction of the upper bass/second harmonics.

 

There is another option you can consider, if what you want is a living room friendly cab then buy a different speaker that suits the 30l cab. I can give you more or less the same response of your current speaker with a 10" driver. It just won't be as loud but it will be in a smaller cab.

 

I'll put up a couple of graphs from winISD to show you how this looks.

 

 

[Phil Starr]

OK Green and purple you've already seen. Green is your current response with an 80l cab and purple is what your Scorpion would look like in a 30l cab. I wouldn't like to see that much bass missing and It wouldn't give anything like the bass in the video clip.

 

I think Red is a distinct possibility. It's in a 55l cab retuned to give a slight peak at just over 100Hz to warm up the sound and -3db is at 70 Hz so all your second harmonics will be there. I think this will sound great in your living room where deep bass can be just a bit of a nuisance. It was the best compromise I could find that would keep that second harmonic (80Hz -160) rich enough to sound good. The old school sound you like from guitar speakers lacked a lot of deep bass which they compensated for with rolling off the upper frequencies with the tone control on the bass. No guarantees but you might well be able to achieve the sound you want this way. Is 55l small enough for domestic harmony as well?

 

 

[Phil Starr]

If you are prepared to use another speaker to get a more room friendly size this is a 102 driver in a 30l cab compared with your scorpion in an 80l cab. I've pretty carefully matched the frequency resonse down to 50hz so the bass response is going to sound very similar. The scorpion with it's much larger surface area is going to be noticeably louder though .

 

So over to you. New (or used but smaller) driver or if not how much are you prepared to compromise the bass response to get a cab that works for you?

 

 

Edited October 2, 2021 by Phil Starr

[Pea Turgh]

Starr by name, star by nature!

[anrque]

Sorry for the delay in new information or responses. The Peavey combo disassembly is happening in the basement and as always happens in order for me to do one activity I want to do, another activity stands in the way. Anyways, with that over with, the Peavey combo could now be taken apart, measured, weighed etc.

First line of business was how to get inside. The amp portion had 4 machine screws with the typical metal straps used by Fender/Peavey/etc on top to attach the amp chassis to the cabinet. Unseen until later were 4 wood screws along the back edge of the amp chassis. With these removed and the combo face down, I could push the amp from the front side out the rear opening. The speaker to amp connection is some proprietary plastic Molex type connector that I had to be careful with so the 35 year old plastic did not crumble.

 

With the amp out, I could figure out how to get inside the speaker box. It seems that Peaveys of this vintage use some super Velcro type hook and loop fastener in the speaker grill corners. I had to get behind the wood grill frame with a putty knife the first time to apply even pressure so nothing broke. With the grill off and the cabinet laid face up, I was surprised with a angle rounded triangular shaped port in the right lower corner.

 

Next line of business was to get the Scorpion out so I could take measurements.

[anrque]

I took the 15" Scorpion out and first thing I did was to check the condition of the foam dust cap on the rear of the magnet assembly (motor). As many online forums indicate, the foam was ready to disintegrate at the slightest touch. As I did not want this in the voice coil, I disassembled the motor from the basket and cleaned out the foam. Then cleaned the voice coil gap with masking tape as Peavey recommends in their field replaceable basket documents and reassembled.

Using a kitchen scale, I got these weights:

Magnet assembly only (motor): 4.15 lbs

Fully assembled speaker (motor and basket): 8.15 lbs.

Moving to a digital scale for humans I got the cabinet weight:

Empty cabinet: 39 lbs

Empty cabinet plus grill cover: 41.2 lbs

The cabinet is indeed particle board and probably the reason the 24"h x 20.5"w x 12"d cabinet weighs as much as it does.

 

With the speaker removed, internal cabinet dimensions are:

Height: 18.75" (47.625cm)

Width: 19" (48.26cm)

Depth: 8.25" (20.955cm)

Volume: 1.7 cu ft (~48L)

 

The triangular shaped port was difficult to get an exact measurement on, since the corners are rounded. If they were not rounded and each corner was taken all the way out to the corner inline with the sides they would be a ~4" x ~4" x ~5.66". As it is the matching sides of the isosceles triangle measure ~3" were they transition into a 3/4" rounded corner into the hypotenuse of approximately 4.5" until it transitions into the corners. Lastly, measuring with the side of the ruler flat on a side, the measurement come out somewhere in the middle at ~3.75" x ~3.75" x ~5.25".

I'll use this last one for an approximate port size of 7.03 sq inches (45.35 sq cm).

 

 

 

[anrque]

I contacted the guy in the video about the actual dimensions of his sealed "half" Fender Dual Showman style cabinet. He provided internal dimensions:

58,41 cm high x 43,18 cm wide, 26,10 cm deep (23"Hx17"Wx10.25"D)

My math puts this just under 66L. (2.32 cu ft)

 

Here are the existing Peavey TKO 65 combo dimensions again:

18.75" (47.625cm) high x 19" (48.26cm) wide x 8.25" (20.955cm) deep

Volume: 1.7 cu ft (~48L)

 

Again, it has that triangular port approximately 7 inches squared. Since this port has no extension inside the cabinet except for the depth represented by the thickness of the 3/4" baffle thickness, I'm not even sure how to determine "how" this cabinet is tuned.

[anrque]

I have also grabbed the old Electro-Voice TL606 and TL806 plans floating around the net. This TKO65 cabinet and that quasi-half Showman cabinet volumes fall right in the middle of TL cabs. From some additional Google-fu I have found comments on other forums that indicate the Scorpion benefits from a smaller box volume and only treating them as a mid-low speaker.

 

"...The specific construction tends to favor (or be optimized for) the range above about 70Hz as they were also used in smaller PA speakers, stage monitors, etc."

 

So maybe I just "pick a size" and start experimenting. Somewhere between the original combo 48L and 66L, maybe ~57L. Funny, because 57.8L is the internal volume for the "entire" TKO65 combo if you count the space occupied by the amp...

Edited October 3, 2021 by anrque
adding an additional comment

[Phil Starr]

Hi I'm also getting a feel for your level of competence and you don't need very much support from me, just someone to bounce ideas off and maybe help with a few decisions. As you see I came up with a 55l 'solution'  as a bit of a sweet spot with a -3db point of 70Hz. Doesn't look like Peavey did get it wrong  Don't worry about the triangular port, they aren't wrong but we can do better and we'll recalculate for whatever cab you decide to build anyway. I wouldn't say the Scorpion 'benefits' from a smaller cab. If you go back to my first post you can see what changing cab size does to the response on the graph. It is however remarkably tolerant of different cab sizes. Anyway all you have read ties in pretty much with what WinISD is telling me.

 

I'll have a look later at what effect differing panel materials will have on the weight of the cab but it looks like you are accepting a 50-60litre cab as a good compromise. It'll give you plenty of output in the 80-160Hz range and enough below 80Hz not to be disappointing. If you like the showman sound then it's going to fine for your needs.

 

Before you build let the software do the experimenting for you, you don't need to build multiple cabs nowadays. I think most of your decisions are made.

[anrque]

Thanks for all of the great information and thank you for the encouragement, Phil.

I have attempted to enter the Scorpion details into WinISD but not sure if I have done it correctly. The first graph it returned for me was very optimal for bass to 50Hz but the defaulted cabinet size was gigantic. I downloaded and installed WinISD 0.7.0.950 and created the .wdr file for the Peavey speaker since included by default.

Can you share your .wdr file for the Scorpion so I can see if mine is correct?

 

I was aiming at using a solid wood like poplar or the clear pine that I have built a Fender-style modified vibro-champ from in the past, so I might do some nice joinery and stain. Unfortunately, it seems that both the dimensioned pine and poplar boards available stop at 12" widths. If I wanted to make a cabinet deeper than this to still have the requisite 60ish L it would have to be taller and wider and therefore less compact. Compact was one of my goals. I could achieve a depth greater than 12" by laminating boards, but I don't think it will look as nice. Also, I did not want to go to the lengths of purchasing thicker 8/4 poplar, splitting them and book matching. So I need to give my materials and eventual covering some more thought. Black tolex is boring, but I guess it is pretty classic.

[Phil Starr]

Peavey Scorpion 15.wdr

 

I really liked the cab you showed with contrasting timbers forming the corners. My 15"cabs are only 350mm deep and using something 25mm or so all round each panel in a contrasting timber could look quite good. That would enable you to have a really big radius on the edges too if you wanted. It looks like that is how they made the cab you have shown. I know it means extra work but if you have a biscuit jointer  it shouldn't be too hard to form. I don't know where you live but round here you can buy bigger timbers and even get them to make up larger jointed boards at a price. Alternatively I've used veneered panels with timber edges to make up cupboards and they can look quite good.

 

All this looks expensive though, unless you have a supply of well priced timber or have a lot of off-cuts in stock. Are you sure this is the best place to start? If I was going to invest maybe £100 on timber and quite a few hours work on the perfect cab I'd want the perfect speaker and I'm not sure the Scorpion in a 50l cab is that.

 

 

[Phil Starr]

Here we go, I'm doing this at coffee breaks which gives an idea of how quick WinISD (and other modelling programs) allow you to work. For this I've done my usual routine of making a basic assumption and running through the options I know are most likely to work. For this I've assumed you want it as small as practicable and to me that says 55litres. This graph looks at tuning and I've tried 40Hz,55Hz and 70Hz (green,purple,red)

 

That doesn't tell you how it will sound, but i'm specifically looking at two things here. How quickly the deep bass rolls off (below 80Hz) but more specifically how the cab shapes the response above 80Hz. I find it most useful to look at the area of the graph rather than any specific point on the graphs but then ask myself a few specific questions. As you can see the higher the tuning the more bass you get in the 80-160hz octave, that's counter intuitive but you can see you lose out at 50Hz so you are trading rich harmonics for weaker fundamental.

 

I can see other people are reading this thread so if you feel I'm being over pedantic, well I can't forget I used to be a science teacher

 

Edited October 5, 2021 by Phil Starr

[Phil Starr]

Another coffee break, so looking at the graph above a 1db difference is only just going to be noticeable but there is a 2db peak  with the highest tuned cab which is really going to warm up the bass, pschoacoustically we hear this area as bass and this is the sort of response a lot of the old 60's cabs often displayed. Quite a few modern cabs too, if you like MarkBass with the tweeter disconnected this might be for you. the problem might be with EQ. If you have a traditional bass controlthen boosting it to get a few extra db at 50hz might well give you problems at pushing that 120Hz peak even higher. At the other extreme the lowest tuning has a slow roll off which starts quite high and that will probably sound quite good with some gentle bass boost.

 

The middle option looks interesting, just over a db more at 80Hz up to 120Hz and the same output at 50hz. there is a hump but at less than 1dB it isn't going to be significant.

 

Of course this is only frequency response, if you want to use this cab at gigs we need to look at how tuning affects power handling.

[Phil Starr]

Here we go. It's pretty clear that the higher tuning improves power handling at all but the lowest frequencies.  At 80hz the power handling of the 40hz tuning is reduced to 100W, but at 50Hz only the 70Hz tuning is incapable of handling the full 200W thermal rating.

 

[anrque]

Thank you for posting the wdr file for the Scorpion 15. The numbers in the one I created are vastly different, so not sure I entered them correctly. It is also difficult to get ALL of teh Scorpion 15 specs especially since this one is from the 1980's and even the most recent incarnation is EoL.

I am not 100% sure what that final graph is telling me, but I see the big scoop in the green line. It looks like the red line for the 70Hz tuning in a 55L cabinet is the best compromise. Again, this is a cabinet to have around the house and not intended for playing out. Really just an attempt to reuse what I already have, make it usable across different amps if needed be and reduce the weight from this massive Peavey MDF monster.

I think I am going to shy away from solid wood at the moment and use 1/2" (12mm) Baltic birch ply instead. I'll start with rehousing the amp portion of the combo first since it "should" be the easier of the 2 jobs.

[Phil Starr]

Ok I suppose I'm trying to feel out what you want from your build and to give you a chance to comment (if you want) on a design. I think you want it small but you don't want it to sound OK and not to sacrifice too much bass sacrifice too much bass, so somewhere about 55 litres is your sweet spot??

 

So making that assumption, and that you want to use this speaker and want a fairly mellow old school sound the only other real decision is tuning the cab. If you go up 4 posts you can see what 40/55/70 Hz tunings do to the response. None of those tunings are ridiculous but you'd hear a difference. If you tune it higher than 70hz you'd get an even bigger peak and even less bass, personally I wouldn't go that far but you can also go for a frequency between these figures. you can't change tunings though without affecting other things.

 

The latest graph with the dips shows the 'cost' of higher tunings. As  you move away from the natural resonance of the speaker in a 55l cab the cab loses control of the speaker and excursion gets too high at high powers. The cone moves so far that the coil exits the magnet and it distorts and can eventually damage the speaker at high power. The dips show where this happens. So the red line shows that the over excursion at 100hz is fairly limited but below 57Hz it if over excursion all the way. At 50hz power handling is only 65W and at bottom E (41hz) it is only 25W.

 

Look at the other extreme, the green line; the over excursion is wide and deep and at 75hz power handling is reduced by half to 100W but from 50hz to 30Hz the 40Hz tuning port takes control of the speaker and power handling is restored to 200W as the port not the speaker does most of the bass making job.

 

So to unpick all this all I'm saying is that you can't have everything. If you chose the best bass response green line you'd tune low, and that looks good for eq too, but you'd lose power handling at 80Hz. If you choose the best power handling at critical frequencies between 80-160hz with the red line tuning then your power handling low down is shot and you'd need to think about an HPF at 50hz if you wanted to crank up the power. if you are only ever going to use this at home though the 25W of bass power where you want it might be perfectly OK. Or you might want to avoid neighbour annoying rumble going through the floor. Only you can decide that. The point is that at this stage speaker design is like squeezing a balloon: every time you squeeze a bit more performance somewhere you have to look to see where the balloon has popped up elsewhere. Once you have the figures in WinISD (other software is available) you can squeeze away if you want  and home in what would be the best compromise for you.

 

For me I'd look at this and think the purple line looks tempting, You sacrifice a little bass, but only below 70hz and only a couple of db at 40Hz. You lose a little power handling at 90hz but it'll still handle 140W. You'll never use it at home at that level anyway. In fact I think we are at the point where I'd say a 55l cab tuned to somewhere around 55hz would be about as good as you could get as a compact cab with this driver.

 

 

 

 

 

 

 

 

 

[Phil Starr]

OK two more graphs with the same figures,the first just shows the cone excursion at 200W, literally how far the cone moves. The horizontal line is Xmax where the cone starts to leave the magnetic field. The second one shows the effect this all has on the actual volume you can achieve. My rule of thumb is that 120db will let you play with a very loud drummer, (and cause permanent hearing loss in a few minutes.) 100db is going to be very loud in your home and will probably drown out your hi fi. 

 

I'm happy to go on helping but at this stage I think it's probably better if you ask questions and I'll do my best to answer. Building the cab in 12mm ply is going to save you a lot of weight. if you are happy with the 55l and 55ish Hz tuning then I'm happy to suggest port dimensions and so on. If you make the port out of a standard 110mm plastic pipe then you can cut three lengths and try all the tunings, and any others if you want something in between but over to you