C38 Port tubes

[JBLOG]

I have seen several C38 cabs on ebay with port tubes but I haven't seen any documentation in the library about this. The port in my C38 (sans tube is 4"), whilest one ebay auction describes 5". ?? Has anyone ever ever 'played' with these? Are these legit? Any experience with tuning a C38 cab?

http://cgi.ebay.com/JBL-BASS-TUNED-PORT-PAIR-C36-C-36-SPEAKER-ENCLOSURE-C38_W0QQitemZ180138669669QQihZ008QQcategoryZ50597Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://http://cgi.ebay.com/JBL-TUNE-PORTS-from-C38-closures-5-diameter_W0QQitemZ300129993627QQihZ020QQcategoryZ5 0597QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[Zilch]

C38 was a cabinet, not a system.

The port length (and diameter, perhaps) should vary according to the driver configuration loaded therein and the appropriate tuning for it. Longer = lower for the same diameter vent.... :yes:

[JBLOG]

Zilch,

What would be the port tube length of a 4 inch diameter port tube to optimize the tuning of a c38 cabinet utilizing a speaker system 030 ( D130, 075 N2600 crossover). The port tube installed on the left?

[Zilch]

Please provide the interior dimensions of your C38 cabinet to the nearest 1/16".

Mine are clones, not originals, so I can't model using them with any great degree of confidence.

I have previously calculated C38 internal volume as "Somewhere between 2.5 and 3.0 cuft...." :p

[JBLOG]

they are probably closer to 2! give me a few minutes to unscrew the several hundred screws...:banghead:.

OK I'm back...

11 7/8 " deep
17 7/8" high
22 1/4 " wide

This measurement is particle board to partical board....sans insulation.

I'm getting somewhere near....2.73 cubic feet....

Thanks Zilch

[Zilch]

BB6P suggests tuning to 55 Hz, which is a 4" duct 0.654" long.

To tune to 40 Hz, it's 4.75" long.

I have a problem modeling D130, because the published Xmax is only 0.762 mm, and BB6P says you're out of cone excursion at about 10 watts. It's pretty hard to compare different tunings.

In any case, the driver is very efficient, as you know, and that'll produce ~112 dB SPL.

Tuned to 40 Hz, F10, representing usable bass, is 51.3 Hz, which is the consequence of the tradeoff for high efficiency; there's little extended bass response. You need a sub or a different driver set for that.

To answer your original question, both tunings are valid. The 4" hole in the cabinet will let it play louder with less bass (F10 = 52 Hz). Adding a duct lets it play lower (by very little,) but limits the power handling capacity:

[JBLOG]

Zilch thanks for modeling this! Not much bang on the buck by adding a duct.

[JBLOG]

I'm out of cone excursion @ 10 watts!

Should I be using a 10 watt amp?

What is the relationship between cone excursion limit and amplififer matching?

[Baron030]

Hi JBLOG
While it is true that at very low frequencies the D130 will run out of linear cone excursion at only 10 watts. You must remember that statement only applies to low frequencies. The thermal power rating on an original D130 voice coil is 75 watts. So, the driver can handle a lot of power in the mid-bass and higher frequencies bands where cone excursion is not so much an issue. Over the years, I have used several different amplifiers with my old 030 system, with power ratings ranging from 13 up to 350 watts per channel. Admitted, a crown K1 with 350 watts per channel was defiantly over kill, but more power is always better. Clipping Kills Speakers! So, I would personally recommend about a 100 watts per channel for a 030 system.
And there is one little trick you can do to improve the low bass response on the 030 system. Place your C38 enclosures in the corners of the room. The resulting wall reflections can improve the bottom end by several db. Also, toe the speakers in, so that 075s are pointing straight at your favorite listening position, as 075 bullets are very directional.
Enjoy your 030 system; I know I enjoyed mine for 32 years, until I handed it down to my nephew and made a wee little up grade.
Baron030 :)

[JBLOG]

Currently I'm using a 75 watt amp with a separate pre-amp. The bass although not deep, is well controlled. I'm thinking of adding a subwoofer (Hsu) to the mix for some added depth. The 030 system is really fun listening.. but it gets way too loud with the components I have.
They get 'stupid loud' with the preamp volume nob at only 8 o'clock. I recently swapped in a pair of modded dynaco MarkIIIs which I believe are rated at 50 watts. They sound warm and sweet, taming the 075 a bit.....but still way too much gain. I'm thinking about trying some lower watt amps ~8-10 watts. With the effiency of these speakers, I don't think it sould be a problem.
Am I going in the right direction? Or should I be thinking about more signal attenuation in the pre-amp??

[JBLOG]

Grumpy,

Are you using a commercial in-line attenuator or a DIY?

[Baron030]

Since, you say you are using Dynaco tube amps.
I am going to assume that the input impedance for this amplifier is about 47,000 ohms.
And this is a fairly common “high impedance” input value for most tube amps.

Using an online T-Pad/H-pad Calculator, I have up with some recommendations for you.
Link: http://www.icycolors.com/nu9n/tpad-calculator.html

Try a 10db T-Pad attenuator between the preamp and the Dynaco power amps.

In using these online calculators, you have to play around with them for a while.
Before they generate anything that comes even close to what can be considered as "Stock" part values.
Fortunately, for your application, you don't have to be super exact.

So, in the circuit pictured below, the R1 and R3 values should be 27,000 ohm resistors and the R2 value
should be a 33,000 ohm resistor. And these are "Stock" resistor values, so you should not have too much
trouble finding them. And a 1/4 to 1/2 watt resistor power rating is all you need.
Parts Express sells these values in packs of 10 in the 1/2 watt size.

And if by chance that find 10db to be too attenuation, then a 5 db attenuator can be made with R1 and R3
values of 13,000 ohms and the R2 value of 75,000 ohms.

Baron030 :)

[JBLOG]

Thanks Baron!

I'll give this a try and fiddle with the dbs.


Grumpy Thanks Too! what happened to your posts??:blink:

[grumpy]

FWIW, I try to delete redundant info. Looks like you're good to go :)