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Old 17th August 2008, 09:21 AM   #1
waam68 is offline waam68  United Kingdom
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Default Vintage Full range- Need some cab advice

I rescued an old homebuilt 1950's/60's cab from being dumped a while back. I don't know much about speaker design but it's obviously a Bass reflex designs from looking at my old GA Briggs cabinet books. When I opened up it had a "Grampian Reproducers" 12" driver..from what little I can find about Grampian they were high quality & also made PA /Cinema amplification & speakers.

I hooked up this speaker to my old reconned Heathkit push/pull amp & it sounded lovely...I had a friend round who is big into diy hifi & he was very surprised how good it sounded. Anyway I'm building a 3.5w stereo SET amp at the mo & thought these drivers would be a good match. I managed to score two more Grampian drivers very cheaply on ebay cheaply which look identical but which have black rear covers (I have been advised that they are the same driver). Anyway space is tight & skills are limited...so should I clone the reflex cab I have or build two slightly smaller cabs & if so what type?

The cab is 36"/18"/16" & stinks which I think is the old glue used to glue on the material/wool lining. The WAF factor is quite low as the missus hates the smell & says it looks like a coffin!

All suggestions gratefully received!
Mike

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Old 17th August 2008, 10:51 AM   #2
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Bigger is better, & typically vintage drivers tend to need a bit of cabinet volume. So, lacking any T/S parameters for the drivers, and given that you say you like the sound of the old BR cabinet, I'd suggest you stick with that design & simply build a pair of new ones. Birch ply for preference, if you must use MDF, then use 1 1/4in thickness of the stuff so it's stiff enough.

There are some other ways & means... going by a trad BR formula, a cabinet slightly larger than that (36in x 18in x 19in internal) will be tuned to 60Hz with that driver, assuming the vent = the driver cone area (Sd). You can lower the tuning by an octave by making the vent 1/4 of this size. There are other methods too -really, it depends how much you want to play with them. Haven't run across Grampian before. Shades of The Hound of the Baskervilles.
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Old 17th August 2008, 11:20 AM   #3
waam68 is offline waam68  United Kingdom
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Cheers Scott
I may just clone it as it did sound very good indeed but space is becoming an issue...tall & thin is easier to accomodate than short & fat!
The vent is 8"*1.5"
I don't really know what a T/S parameter is
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Old 17th August 2008, 02:44 PM   #4
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I thought it might be something like that. The box will probably be tuned to the 30Hz regions.

Yes, tall slim boxes usually are somewhat easier to accomodate -however, you have to be careful not to go too far, or you'll end up generating powerful standing waves (resonances) in the box. The ideal dimensions for a bass reflex cabinet follow the golden ratio, which will ensure no standing waves inside the enclosure.

T/S parameter is an abbreviation of Thiele / Small parameter -they were Australian engineers who came up with a number of parameters, some mechanical, some electrical that can be measured from a louspeaker drive unit. The results may then be used in conjunction with a set of algebraical formulas to determine a specific box alignment, tuning frequency & dimensions. Most competant manufacturers now supply the T/S data for their drive units as a matter of course these days (although whether you'd actually trust their claims is another matter of course...) Anyway, their work was done in the 1960s - early '70s, initially by Thiele (based on a paper by another background figure, Novak), and extended by Small some 10 years later, so its rare to have such information for vintage units, unless an enthusiast has measured a pair of their own, & we have to resort to using methods that pre-date Thiele & Small's work to design a cabinet.
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Old 18th August 2008, 04:13 PM   #5
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In the 1962 hi fi year book, there is just one grampian speaker listed
it is a 1255/15

only details on it are..
voice coil 1 3/4"
gap flux 14.500 gauss
total flux 130.500 maxwells
they claimed 35 to 15,000 c/s... l would be suprised if they went that high...

that sort of puts it into the ballpark of a goodmans 51 ( which is a 201 without a whizzer)...excepting for the 15.000c/s top end

thats.. if it is the above...

it will have very little xmas compared to modern speakers
and would just about make a nice bass helper in OB supporting say a fostex.. if the baffle was well planned it should be ok
i would guess that they are quite fast speakers and around 95db efficent..
the more powerful speakers of the time sound a little more real or more 'life' about them.. but they would still sound very good

hope thats some use
do you have the model number

the complete grampian speaker was the CE12 and was 29x18x12
and used the above driver.. but looking at the picture yours is not that cabinet
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Old 18th August 2008, 06:49 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

They might work well in a smaller box, they might not.

Experiment with the current cabinet. Make the port bigger - double it
up - then block one or both - how big a difference does it make ? Try
taking up some space in the box, again how much does it change ?

TBH you would need some idea of the drivers parameters to build
the smallest sensible box. You can change the shape and would
possibly get away with a 33% volume reduction - not guaranteed !

If the box is too big peaky vented bass cannot be avoided, too small
and you will not get the best bass efficiency you need for flea power.

Tune the vent around 40Hz to 50Hz ? higher ?

/sreten.
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Old 19th August 2008, 09:03 PM   #7
waam68 is offline waam68  United Kingdom
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Hi All
thanks ...no model no I'm afraid. One has FH/C written in white on the paper cone front & 23/103/002 on the back & a faint print GR Test 61 on the cast frame so 1961 I guess. Under the cover the magnets look Alnico.
Sensitivity wise I hooked up an 8wpch push pull amp on my mid 60's wharfedales then swapped onto these & they were about twice as loud at thesame volume setting ....so I guess s/b OK for SET without horn loading.
I like these old drivers as they were cheap...free & 20 each respectively & I have a couple of vintage valve amps I've restored all of which mate well to 15ohm drivers. As I have 3 sets of vintage 15ohm speakers I tend to build amps with 15ohm outputs. Quality wise how do these old units compare with the newer smaller drivers?
Mike
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Old 19th August 2008, 09:30 PM   #8
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I'm no expert at older drivers although i have quite a collection of them
i use a couple of goodmans mid 1960's speakers with lowther ex4 in open baffle, all connected full range i cannot tell which speaker is making what sound.. the lowther just goes much higher.. its really used as a tweeter..

so i have no problem in saying that my goodmans match the ex4 very well.. that to me says a alot about the possible quality from them..

looking at the construction of the grampian speakers, i would have thought to go as high as they do, there would be quite a bit of flex in the cone.. so how they would compare side by side with the goodmans i would not like to say.. i have heard quite a few sets of speakers and a would happily say the are not quite the deepest bass i have heard.. but nothing yet has come close to the speed and the lack of colouration as these older drivers in OB

i have a pair of rare corner accoustical ribbon speakers (early quad) and these too are very good
they also use goodmans for the bass drivers

hope thats some use..
i too use SE valve amps of good quality.. all home made.. 2 watts is very very loud...

cheers steve
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