Cab Conversion, Radio Shack Project Rebuild

[Thor74]

This is my second post here, so by way of quick
introduction (to define the level of ‘Help!’ I may
need), I am a long time musician and bass player. The
introduction thread has some additional background,
but the project I am working on is a rebuild or
upgrade of the Radio Shack ‘Building Speaker
Enclosures’ project, a musical instrument speaker
that I built in 1982. Subsequent improvements in
design technology lead me to believe this can be
upgraded with significant improvements in performance
at reasonable cost. I would like to use it as a Bass
cabinet.

I will post some pics shortly of the project, but essentially,
it is a 2X12” unvented box with 3 piezo horns, presented
at 4 ohms. The piezo line has a 10-ohm resistor in series.
I am going to leave that part alone and only focus on the
subwoofers.

While Weems provided specs on some drivers in his
long out of print book, none are available on this driver,
Radio Shack Cat No. 40-1314. The driver has a silver
dust-cap, I am told from a source at Bose Corp,
(err, he maintains the building and bathrooms) that
this was typical of Altec-Lansing. Confirmation, denial or
discussion of that would help. I have searched the web
extensively looking for specs on these drivers to no avail, if
anyone knows where I could find them, please let me know.

With the specs in hand, I was considering porting the
bass cabinet. I would prefer to do this without a shelf
if at all possible, because of the cab construction, it has
a removable panel at the rear. It would be quite easy to
port the front once the parameters are available.

I did acquire a recent copy of Dickason’s
Loudspeaker Design Cookbook to broaden my
understanding of ported box design.

The ID of the cab is roughly
11.5 x 16.5 x 34.5 = 6546 cui. in and deducting
the volume removed by the piezo tweeter shelf
I come up with an estimate of 6486 or 3.75 cu. ft
for the internal box volume.

Plan A would be to port and dampen the cab.

Plan B would be to replace the drivers with other drivers suitable for my needs, and port the cab if called for.

Just to define my needs more clearly, I would like
to use this as a bass cabinet, and reproduce down to
Low E 41 hertz without farting, I do not play a 5 string
with a low B 30z, so rolloff in that area is not an issue.
I don’t need a lot of real high end performance, needless to say.

I use an Eden WT300 amplifier, with an internal tube
preamp. It will deliver 300 watts @ 4 ohms and 220
watts at 8 ohms. So it should have plenty of power to
punch those silly little chromedomes. I have a
bigger rig as well, an Acoustic 140 head with a 106
bottom, with 2 x 15 drivers circa 1975. My back is
aching looking at it, and it no longer fits in my car,
so this upgrade is beginning to look really interesting
to me.

I can buy a new 2X12 Eden, Aguilar or Euphonics cab
for a little under USD 700 and pickup a used one for USD 400
so that is the upper cost limit.

So, I am at an impasse with the specs here, and welcome
your input and ideas.

Thanks,
Thor

[simon5]

400$ USD and you already have an amplifier? You could start from stratch my friend!

You also only need 40 Hz performance. Easy.
100$ for wood and 300$ for a nice 12, 15 or 18 incher.
300W into 4 ohms is plenty.

To upgrade your old cab, it might be harder to do it correctly.
It's at least 3/4 inch thick? It's well braced? It's totally 100% air sealed? Then you need to change drivers... Give us more info please.

You could keep the piezo part or redo that part too.

If you like my idea, tell me, I'll try to help you. You could even sell that old vintage cab to add more money on the project!!!

[Thor74]

Originally posted by simon5
400$ USD and you already have an amplifier? You could start from stratch my friend!

I would prefer not to start from scratch, and redo this existing
box as my shop access is rather limited, I built this
when I had a full shop in my basement, but have moved
3 times since.

I have a scratch project in the works now,
the 'Compact Bass Speaker Cabinet' by Len
Moskowitz, found here. and I don't want to get into two, but
rework this existing cab and upgrade it.

You also only need 40 Hz performance. Easy.
100$ for wood and 300$ for a nice 12, 15 or 18 incher.
300W into 4 ohms is plenty.

To upgrade your old cab, it might be harder to do it correctly.
It's at least 3/4 inch thick? It's well braced? It's totally 100% air sealed? Then you need to change drivers... Give us more info please.

Lower than 40 would be better, but I was trying to define
what my needs were, so that adequate choices could be
made.

As far as $300, believe me I would be a lot happier if
I could port the cab and spend 10 bucks. If I could
find drivers for 60 - 80 each, that would be great.
400 will buy me a slightly used but nice Eden, Aguilar
or Avatar cab 2x12. At those numbers I would be better
off selling this and getting new stuff, ie that is the threshold
where it no longer makes sense. I still have one year
of collge tution to pay for, so money is flowing out a
lot faster than coming in.

You could keep the piezo part or redo that part too.

If you like my idea, tell me, I'll try to help you. You could even sell that old vintage cab to add more money on the project!!!

Yes, I suspect that I may have to change drivers, though
these are perfectly fine, I think. I can't find the specs and
don't have a sine wave generator to check them to find
Fs and derive the Q.

The box is braced, and caulked. It is made of 3/4"
particle board, and covered with tolex. The back panel,
which screws on, will probably need a 1/4 by 3/8 weather
strip to be called TOTALLY airtight. But it screws into
wooden cleats set into the back, and is reasonably
airtight now.

So, the project is to maximize the performance of this cab,
and minimize the investment, basically.

This allows me to produce those low notes you guys so
love to reproduce.

Yeah, a bass player is not much good without less than
300 watts. Generally, pro players today are using amps
that put out 750 to 1000 watts , such as the
Gallien-Krueger 1000, Eden World Traveler 1000, etc.
We generally consider 300 a practice or small
gig/coffehouse amp. It is nice to be able to
shut down some of the big egos with the Marshall stacks.

I took pictures last night, and once I get them up
on the computer, will get them on here.

Once they are up, tell me how best to proceed ...

[simon5]

Ok, since you want to stay at minimal cost if possible...
You don't have parameters so you have to go blind.

You got 3.75 cu.ft internal.

To tune that box at 30 Hz, get a 4 inch diameter dual flared port if possible and also a 7 inches long 4 inch diameter ABS pipe for testing. Make the ABS pipe 7 inches long. Don't glue it permanently, try to find something to hold it temporarly in place and seal it as good as you can for good testing.

Play with your bass. Not loud enough? Cut the pipe shorter. Try 5.5 inches long for a 32.5 Hz tuning.

Play with your bass. Not loud enough? Cut the pipe shorter. Try 4.5 inches long for a 35 Hz tuning.

Play with your bass. Not loud enough? Cut the pipe shorter. Try 3.5 inches long for a 37.5 Hz tuning.

Play with your bass. Not loud enough? Cut the pipe shorter. Try 2.75 inches long for a 40 Hz tuning.

Write down your impressions, then cut the dual flared port to the required length and glue it finally in place. You can do smaller steps if you want to fine tune, maybe using a 2nd ABS test pipe.

Good luck!

[Thor74]

I suppose that is the only way, barring testing the
drivers.

If I had the drivers Fs, Q and Vas, and could
run the rest of the equations, what would be a
reasonable tuning Frequency for the box?

[simon5]

If you had the Fs, Vas and Q, then I would use WinISD Pro or WinISD online to tell you the right tuning frequency.

It depends on what do you want... maximum output or maximum flatness or a compromise in between...

[Thor74]

I fired up the cab after rewiring and punched it pretty hard with the Eden. Though I don’t have the Thiele/Small parameters, it is obvious to me that the RS drivers do not have enough low end to handle the bass produced by the passive/active P/J 81 Ibanez Roadster and the Eden wt300.

Accugroove makes Whappo jr. cabinet , and it has been mentioned to me that they have one 12” that functions as a subwoofer, and another 12” configured as a mid-range driver. This suggests a low cost solution of finding a suitable 12” subwoofer replacement and designing a crossover circuit to limit the low end of the RS driver.

Does anyone have a suggestion for a suitable subwoofer? Anything for under a $100?

Any other and all suggestions are welcome.

Here are the pics as promised:

[simon5]

A 12" under 100$ ? I would look at the Dayton lineup at PartsExpress.com