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Porting questions/problem

Discussion in 'Amps and Cabs [BG]' started by fuzzy beard, Jul 9, 2012.


  1. Ok i have a 1983 Fender bassman 20. It is a 18 watt tube combo amp with a single 15. I have come to the conclusion that at some time someone put 2 20oz pop cap sized ports in the cab.

    So tonight i pluged them with pop caps;) And here are a few things i noticed.

    Amp settings are volume 5, treble 1, middle 6, bass 4. Bass wa max volume and tone was rolled off.

    Tone was tighter not as airy.

    Seemed to have more volume at same settings.

    Tone was more even across all te strings. Not as much touch was needed with the high strings to sound even.

    So what happen? 2 small holes can change things that much?

    What should i do?

    If i want to leave them pluged what is the best way?

    Also how do you clean dust from speaker?
     
  2. I use a good horse hair paint brush to carefully brush dust off drivers and you can buy port plugs - ask JohnK 10, he can tell you more about them - their effects, where to get them and etc.

    EDIT: Correction - the brush I use is a Kelly-Moore Gunfighter, 3½” made of a nylon/polyester blend. It’s not like a cheap hardware paint brush. And I’ve found it works better than compressed air cans for getting dust buildup out of the cone rings - just be careful around the center dust cap and etc. It also works well for dusting off amp faceplates, effects, mixers, etc.
     
  3. Beard bump!
     
  4. I would think compressed air would be a good start? As far as blocking the ports, if your method is working, there isnt anything wrong with it.
     
  5. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Search Google for "rubber expansion plug", "rubber freeze plug" and "rubber freeze-out plug". They're a quick fix for a leaking automotive engine block plugs and come in many sizes. Here's an example of a 2" plug. Tighten the nut and the rubber expands making a nice air-tight seal. You can try your local auto parts store too. Might be over-engineered for what you need but they work really well.

    http://www.ebay.com/itm/2-RUBBER-EXPANSION-PLUG-LOT-FREEZE-4-219-4010-/310413119676?pt=Motors_Car_Truck_Parts_Accessories&hash=item48461050bc&vxp=mtr
     
  6. I thought about that. But didn't think they made them that small.
     
  7. craig.p

    craig.p

    Sep 28, 2008
    New Hampshire
    Yes sir, they can make a real mess of things. Here's what I'd get if I were to drill a pair of 2" holes in the baffle board of my MB15 combo amp. Boom Box City.

    [​IMG]
     
  8. Hmm so it scooped your mids?
     
  9. will33

    will33

    May 22, 2006
    austin,tx
    It adds a big response hump in the midbass, around 100-120hz. Makes it sound like less mids / boomy by comparison.

    Having a too high port tuning like that also causes the box to "lose control" of the cone at low frequencies. Excursion (moving back and forth) goes dangerously high and can blow drivers.

    Probably not an issue with an 18 watt amp but can lead to trouble with bigger amps. Plugging your holes just moved the bass response back down where it should be. More "even" with the rest of the sound.

    Cool little amp BTW. I like that one.
     
  10. Well took some mesurments. The cab is 15 1/4 inchs wide, 15 3/4 inchs tall, and 9 1/4 inchs deep. All inside mesurments. So a 1.28 cubic foot box. The home made port holes are 1 inch wide in the baffle the baffle is made of 3/4 ply. I dont know who made the speaker so i guess alot of this info is pointless. But thought i put it out there!
     
  11. Anyone else? BFM? Duke?
     
  12. Beard bump!
     
  13. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yep. Looks like there was a bigger port in your cab at one time and it was blocked from the inside. That would probably work fine for the smaller ones as well. While you're at it you could post a pic of the driver.;)
     
  14. Without knowing the driver installed, this is all guess work.
    You can download the free version of True RTA from www.trueaudio.com and do some adhoc testing.

    Your computer sound card can drive the cab to 1w without a problem.
    Place the cab on its back and use a few grains of rice to monitor the cone motion.
    Use the RTA to pick single sine wave notes until you see the least motion of the rice.
    Your ear will hear a resonant note from the ports.
    This will tell you where it is tuned.

    Bass output falls off very quickly below the tuning frequency.
    If you are tuned at 85 Hz, you can expect a big hump in the midbass as shown above, and no bottom.
    This is perfectly normal for a JBL D140F, but without knowing the driver installed, it is guesswork without an RTA measurement.
     
  15. There was never a bigger port. The big hole passes into the chassis compartment. Also I have found others who own this amp and there's are sealed.
     
  16. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yep, should be sealed if factory stock. Cool amp, too bad someone hacked it. That happened to a lot of the Rivera era Fenders though.

    Blocking the holes with a couple of little pieces of 1/2" ply would probably be my first choice.
     
  17. LowEZ

    LowEZ Supporting Member

    Mar 29, 2011
    Central NJ
    Speaker is a Pyle from 1982. Right, that top hole is inconsequential but I don't think it's supposed to be there. I can confirm the Bassman 20 was a sealed cab.
     
  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Fuzzy, I find that a shaving brush like this one works well for cleaning speaker cones. It is also great for cleaning amps and instruments. It doesn't scratch and gets into all the corners.


    [​IMG]


    As for plugging the holes in the baffle. Depending on the size of the hole, you can use a good quality hole saw to cut a plug out of a piece of plywood. Then glue it in place. You can get ones without the drill bit in the center, they cost a little more. Small holes can be filled with an epoxy putty.


    [​IMG]

     

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