B15N style cab build FINISHED!

[rubis]

I forgot to ask, one last question! 

I used these roundheaded cabinet nails to fix the corner pieces, I got them from Modulus, but they didn't have enough and their Email is crap, I can't find them anywhere else, anyone know where I can get these things?

https://modulusamplification.com/Stud-Nail-Silver-Colour-for-metal-corners-Single-part-P1989503.aspx

[Jus Lukin]

-

Edited March 4, 2022 by Jus Lukin

[rubis]

Thank you very much Jus Lukin

[SpondonBassed]

Your diligence has paid off.  That cab looks solid.

[rubis]

Thank you kindly 

[rubis]

Thank you for the interest, encouragement and help, without it this would have taken me far longer and not been half the fun.
Here is a family photo with the Ampeg PF20t and the '64 style P bass I made this cab to partner up with, I think they're made for eachother.

[mikebass456]

Classy looking rig you got there now!😀👍😎

[stevie]

On 08/03/2020 at 14:17, rubis said:

Stevie I have a bit left over from this, it's about 150x140cm, you're welcome to it if you want it?

PM me your address and I will post it to you. 

That's very generous of you @rubis. I eventually decided to order a roll - so I'm now well stocked. I didn't appreciate just how big a roll is, but hey, it'll come in handy. 😁

[Stub Mandrel]

You have done a great job there.

I'm assuming the double baffle is spaced to create porting but I'm not sure how it works can you point me at an explanation?

[rubis]

3 hours ago, Stub Mandrel said:

You have done a great job there.

I'm assuming the double baffle is spaced to create porting but I'm not sure how it works can you point me at an explanation?

Thank you Stub Mandrel

If you look at this photo of a pukka fliptop, taken from above with the lid removed. 

The innermost panel, with the slotted ports has the speaker attached to it. If you look at the circular speaker cut-out, you can see the gap between this panel, which on any other cab would be the solid, fixed front panel of a cube really. 

                                                                                                                                                                                                                                                                        

The difference with this cab is that it has another removable front panel, which has the speaker grille cloth attached to it, and is bolted to the inner panel. There are spacer blocks with bolts through them to keep the gap even all  the way round. 

These two diagrams are like a cross section of the cab, as if it were cut in half horizontally. The turquoise part being the outside grille baffle/panel.

Here's an exploded view of a whole fliptop cab.

The cab I made is simpler, with a solid top and a removable back panel, quite conventional in that respect, but it's the same dimensions and has the same baffle arrangement. There is an American company called Fliptops who make replacement B15 cabs and also do an extension cab, just like the one I made. 

I have a proper set of dimensioned plans for this, if anyone wants a copy then please PM me.

 

Edited March 12, 2020 by rubis

[Maude]

That looks superb. Fantastic job. 👍

[Si600]

18 minutes ago, rubis said:

The difference with this cab is that it has another removable front panel, which has the speaker grille cloth attached to it, and is bolted to the inner panel. There are spacer blocks with bolts through them to keep the gap even all  the way round. 

Sorry if I've missed the explanation, but why does it have another front panel? Is it just a different way of hanging the grill cloth?

[Stub Mandrel]

That's fascinating; I wonder if the very varied path-lengths for the porting give it a distinctive frequency response/sound?

[rubis]

17 minutes ago, Si600 said:

Sorry if I've missed the explanation, but why does it have another front panel? Is it just a different way of hanging the grill cloth?

 

16 minutes ago, Stub Mandrel said:

That's fascinating; I wonder if the very varied path-lengths for the porting give it a distinctive frequency response/sound?

Here is a diagram from one of the early brochures or sales literature, it seems to me as if the various parts act together, to get the desired effect.

I also found this description, which explains it far more elegantly than I could! 

The first B-15s generally housed a Jensen P15N speaker, brilliantly set in Jess Oliver’s patented double-baffle design. The original design placed the speaker between two baffles with 15-inch ports, separated by ⅜ of an inch. The outer baffle had a 15-inch circular cut out and a bar across the center of the speaker cone. The speaker was mounted to the inner baffle, which was fixed to the cabinet. This inner baffle featured eight oblong ports (two above, two below, two on each side) that passed the speaker’s rear wave output through the ⅜ of an inch space and through the outer baffle’s 15-inch opening, where this rear wave meets with the front wave completely in-phase.

Here's a link to the full article   https://reverb.com/news/the-golden-age-of-the-ampeg-b-15-1960-1980

 

And here is a link to a portaflex thread on Talkbass, with more than enough information on the various versions of the design

https://www.talkbass.com/wiki/technical-speaker-cabinet/#2-double-baffle-cabinet

Hope this is helpful gents, but I must confess I don't completely understand the physics involved.

I don't know whether it would work at higher volumes, the original B15's were, I think only about 25 watts, but I have read somewhere recently of folks using a PF20t or PF50t and then slaving it up with something like a Baby Sumo to get a 'turbocharged' B15! It's probably a good thing that I don't play in a band, as I'd be after something like that!

[Stub Mandrel]

10 minutes ago, rubis said:

This inner baffle featured eight oblong ports (two above, two below, two on each side) that passed the speaker’s rear wave output through the ⅜ of an inch space and through the outer baffle’s 15-inch opening, where this rear wave meets with the front wave completely in-phase.

Even with simple ports the rear wave and front wave only meet 'completely in phase' at one frequency.

With porting with such a big difference in path lengths my guess is that rather than 'completely in phase' what you get is rather than a relatively sharp resonance is one less strong but spread over a larger range of frequencies, possibly approaching a whole octave.

It seems the classic cabs are given a frequency range of 50Hz-17Khz which isn't particularly low so I am fairly confident in my guess this gives them a characteristic sound rather than some miraculous low-frequency response  -not that this is a bad thing!

I would be interested in @Bill Fitzmaurice's comments on this design, especially the comments at your link!

Edited March 12, 2020 by Stub Mandrel

[rubis]

Yes, I think that's a fair assumption 

[Phil Starr]

15 hours ago, stevie said:

That's very generous of you @rubis. I eventually decided to order a roll - so I'm now well stocked. I didn't appreciate just how big a roll is, but hey, it'll come in handy. 😁

I'm still finishing off a roll I bought back in the 70's

[Phil Starr]

8 hours ago, Stub Mandrel said:

Even with simple ports the rear wave and front wave only meet 'completely in phase' at one frequency.

With porting with such a big difference in path lengths my guess is that rather than 'completely in phase' what you get is rather than a relatively sharp resonance is one less strong but spread over a larger range of frequencies, possibly approaching a whole octave.

It seems the classic cabs are given a frequency range of 50Hz-17Khz which isn't particularly low so I am fairly confident in my guess this gives them a characteristic sound rather than some miraculous low-frequency response  -not that this is a bad thing!

I would be interested in @Bill Fitzmaurice's comments on this design, especially the comments at your link!

Spot on about the only being completely in phase at one frequency.

These were designed in the 1960's way before Thiele and  Small came along in the 70's even than their academic paper took a while to permeate into the design of instrument speakers. I was designing and building speakers in the early/mid 70's and it was more craft than science. Development involved a lot of listening and testing and even the tuning of the port often involved a lot of trial and error to get right despite Helmholtz resonators being fully understood.

My best selling design (mainly I did custom builds) came about completely by accident. I was attempting to build a small (by 1970's standards) cab with rear horns when I needed a couple of cabs for a gig quickly. I bodged together the half finished cabs with a simple straight horn rather than the folded horn I'd been intending. It sounded great with a nice smooth punchy bass ideal for disco and was way quicker to build than the 'proper' design I'd intended. W designs were very popular at the time which was probably why it sold. I surmised that it had a broader flatter tuning than a straight reflex port but I never investigated. Hell I had a nice sounding easy to build design people liked.

There were many attempts to get a broader output form the ports at the time, changing the Q of the cab. This may have been at the back of the mind of the designer but personally I think it's just a support for the grille and it sounded good so they kept the design. A bit of me still misses the hours of fiddling with a cab and hours of listening while you tried to get the 'best' sound you could. Not very scientific but it might be why so many people like vintage gear.

Anyway that's a lovely build rubis, I hope you get a lot of pleasure from it.

 

[Stub Mandrel]

3 hours ago, Phil Starr said:

I'm still finishing off a roll I bought back in the 70's

If only everyone was like you there wouldn't be this panic buying problem. 🤣

[Phil Starr]

4 hours ago, Stub Mandrel said:

If only everyone was like you there wouldn't be this panic buying problem. 🤣

Don't you remember the great vinyl crisis during the three day week? I reckon my current stocks of loo roll will last until 2045

 

[rubis]

On 13/03/2020 at 07:30, Phil Starr said:

 

Anyway that's a lovely build rubis, I hope you get a lot of pleasure from it.

 

Thank you very kindly Phil, praise indeed from a man who knows about these things 

[JStrater]

remarkable project.

in the 70's (and through to today)  I played with a friend and any of his B12 B15 B18 trio was a treat. Usual thing was to run the output signal from the B-XX into a KUSTOM 200 head and into a pair of KUSTOM 2-15 cabs.

I NEVER knew about the version with the double baffle till today. what hit me instantly is that it's the same idea as the Fender Tone Ring cabinet, but not infringing patents and not needing a special-stamped-metal part (said Tone Ring) .

It's not so much a bass reflex (at ALL!) but a tuned port that opens into the front of the speaker driver: What you;re doing inside that sandwich is creating a volume of air that affects the low-end and cutoff frequencies. It likely has a bit better bass but the response just CLIFFS at some pitch (hopefull below the instruments lowest)

 

Great thread!