Converting closed-back to open-back

Re: Converting closed-back to open-back

Postby Chuckles » Thu Apr 22, 2010 11:37 am

He certainly didn't address the specifics of the three-speaker thing I've put together, but I am still a huge fan of Butler's designs. After this Earth Day gig Saturday, I'll probably take the K120 and put it in one of his TubeDriver amps that I've had for about 25 years and see how that sounds in the hopes of having a lightweight combo for practices, etc...
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Re: Converting closed-back to open-back

Postby tigerstrat » Thu Apr 22, 2010 11:46 am

I really don't get the significance that seems to be placed on using three speakers. What about all the years that Jerry used 4? or 2? or 8? or 20?
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Re: Converting closed-back to open-back

Postby JonnyBoy » Thu Apr 22, 2010 11:57 am

Strummin', I know that message didn't make sense. He (Butler) obviously didn't read my whole message. I have done some dealings with BK concerning modding/fixing my Mosvalve, and wound up getting his email address through Ebay (Tube driver ask a question) at one point in the past.

As always, I am only trying to help, give my opinion, or learn from other like minded people here. Since I came aboard here I have learned more than I could ever give back, and my guitar recently sound has proved that. Plus, my wallet is much thinner :lol: . I agree, his answer was lazy at best, if not just plain bullshit. I stated he is not my "Bud", but he does answer questions about his gear for his customers. I got the same kind of responses about doing mods. He probably didn't want to be liable for anything frying in writing.

The conclusion was both Chuck and I agreed that his answer was worthless, and the best bet was to do a ohms check and make sure its over 4 under 8.

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Re: Converting closed-back to open-back

Postby Chuckles » Thu Apr 22, 2010 12:24 pm

tigerstrat wrote:I really don't get the significance that seems to be placed on using three speakers. What about all the years that Jerry used 4? or 2? or 8? or 20?

It's all I got/can afford at the moment!
:lol:
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Re: Converting closed-back to open-back

Postby ugly rumor » Sun Apr 25, 2010 10:07 am

I started to answer this post when it was first made, thinking more qualified people than I would answer. Your amplifier doesn't know how many speakers you have. All it sees is a load (impedance, resistance). So long as you do not exceed the minimum load, your amplifier will be fine. I consider going below a rated 4 ohm load to be amplifier abuse. A rated load, as has been pointed out, is usually more than the actual load; in other words, a 4 ohm rating is probably closer to 3 ohms. An amplifier rated to handle a 2 ohm load, as some are, I consider to be an exercise in planned obsolescence. 4 to 8 ohms is ideal, in my opinion. Higher than 8 ohms results in a diminishing return. Jerry and anyone else drove higher numbers of speakers using a combination of series-parallel wiring and/or slave amplifiers. Even then, especially with the wall of sound, blown speakers were a problem, primarily because of surges in voltage overcoming the resistance. Think of a water hose trying to wash off stuborn dirt, with no luck. If you place your thumb across the opening, you get a surge of power that may overcome the resistance of the dirt, and it washes away. With a speaker bank, the resistance is the speaker impedance, and a surge can blow it if it is strong enough. That's why smart giggers gig with surge protection. Who knows what's going on in some of these dives electrically, especially the ungrounded ones.

Hope that helps a little. Also, with your cabinet, there is no real reason to open the back or leave it sealed for a guitar. The frequencies of the instrument are very directional, and short enough wavelength to not matter. I am surprised that it had insulation in the cabinet. That seems to be more like a bass cabinet, which needs to absorb waves especially in a sealed cabinet, as it makes the cabinet seem larger to the speaker trying to produce long bass frequency waves against the air resistance of the out-of-phase cycle of the speaker excursion.

Good luck!
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Re: Converting closed-back to open-back

Postby ugly rumor » Sun Apr 25, 2010 10:13 am

One or two more things...you don't especially want to mix speaker impedances. If you use more than one speaker through the same amplifier, the amplifier will send twice as much power to the low impedance speaker, so you will get an uneven power delivery. That's if you have two speakers using two outputs of the amp. Also, you want to be sure the voice coil is large enough to handle the power you deliver to it. A 15 watt speaker is not good in a 50 watt amplifier.
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Re: Converting closed-back to open-back

Postby nabco » Sun Apr 25, 2010 10:43 am

Thank you for the info UGLY RUMOR!........... that was very clearly stated also.
You have got me thinking though,
Would it be safe to only run one speaker (8 ohms), on a Fender amp that has a 4 ohm output?
The amp has two 8 ohm speakers wired for 4 ohms right now, and i would love to make a new cab with only one speaker w/out having to buy a new 4 ohm speaker.
Am I going to kill my amp if I do this?
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Re: Converting closed-back to open-back

Postby nabco » Sun Apr 25, 2010 10:48 am

One more thing, Is there any drawbacks to having a 10" speaker and a 12" speaker in the same cab(JBL E110 & JBL E120)? Thanks
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Re: Converting closed-back to open-back

Postby Chuckles » Sun Apr 25, 2010 11:06 am

Thanks, indeed, UR!

Rule of thumb seems to be that with a tube power section, you don't want to go over the specified ohm rating, but with a solid state power section you shouldn't go under.

Fwiw, under gig conditions yesterday the three speaker rig sounded great, but: the 2x12 E's wired in series were noticeably not as loud as the 1 K (which makes sense given they were in series at 16ohms and rated rated for 100w more power. I'm thinking either the single K or a single E in the new cab will be the go-to rig for most dates, depending on room size and how much power I need to draw from the MosValve to be as loud as I might need to be. Just don't need anything more than that, really. The K seems much warmer than the E, as it is easier to drive a little harder. So, it'll be 250w through either the 200-rated K or the 300-rated E, or 500w through a single E. The days of hauling the 2x12 are over except in the most extreme outdoors or huge room situation (and that doesn't happen too often :roll: ).

The new cab that came with the K, btw, was most definitely designed as a bass cab I think... there are ports on the front which have been sealed with additional birch 13-ply screwed into place. Opening the back definitely took some of the bassiness out of it - plus, our drummer could hear it clearly! 8)
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Re: Converting closed-back to open-back

Postby JonnyBoy » Sun Apr 25, 2010 12:46 pm

ugly rumor wrote:I started to answer this post when it was first made, thinking more qualified people than I would answer. Your amplifier doesn't know how many speakers you have. All it sees is a load (impedance, resistance). So long as you do not exceed the minimum load, your amplifier will be fine. I consider going below a rated 4 ohm load to be amplifier abuse. A rated load, as has been pointed out, is usually more than the actual load; in other words, a 4 ohm rating is probably closer to 3 ohms. An amplifier rated to handle a 2 ohm load, as some are, I consider to be an exercise in planned obsolescence. 4 to 8 ohms is ideal, in my opinion. Higher than 8 ohms results in a diminishing return. Jerry and anyone else drove higher numbers of speakers using a combination of series-parallel wiring and/or slave amplifiers. Even then, especially with the wall of sound, blown speakers were a problem, primarily because of surges in voltage overcoming the resistance. Think of a water hose trying to wash off stuborn dirt, with no luck. If you place your thumb across the opening, you get a surge of power that may overcome the resistance of the dirt, and it washes away. With a speaker bank, the resistance is the speaker impedance, and a surge can blow it if it is strong enough. That's why smart giggers gig with surge protection. Who knows what's going on in some of these dives electrically, especially the ungrounded ones.

Hope that helps a little. Also, with your cabinet, there is no real reason to open the back or leave it sealed for a guitar. The frequencies of the instrument are very directional, and short enough wavelength to not matter. I am surprised that it had insulation in the cabinet. That seems to be more like a bass cabinet, which needs to absorb waves especially in a sealed cabinet, as it makes the cabinet seem larger to the speaker trying to produce long bass frequency waves against the air resistance of the out-of-phase cycle of the speaker excursion.

Good luck!




Great advise and good info for people to know in the future around here.

On the talk of Surge Protection, Be aware A LOT of power strips have little if NO surge protection. The Dollar store or cheaper Wallmart "powerstrips" are only that, more outlets with no protection or a little bit that can be blown through. I was watching something on the weather channel about," how to protect computers and stuff from the #1 storm related insurance claim in the US", fried electronics due to little or no surge protection. Most people think the power strip is a surge protector. Some even try to make them look like real surge protectors by putting worthless button switches that can't handle a huge jolt.
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Re: Converting closed-back to open-back

Postby ugly rumor » Sun Apr 25, 2010 5:25 pm

nabco wrote:Thank you for the info UGLY RUMOR!........... that was very clearly stated also.
You have got me thinking though,
Would it be safe to only run one speaker (8 ohms), on a Fender amp that has a 4 ohm output?
The amp has two 8 ohm speakers wired for 4 ohms right now, and i would love to make a new cab with only one speaker w/out having to buy a new 4 ohm speaker.
Am I going to kill my amp if I do this?


You can do this safely, but why not just get a 4 ohm speaker. You'll get about 3db increase in volume with 4 ohms over 8. So long as the voice coil can handle the power, though, you won't hurt your amp.

nabco wrote:One more thing, Is there any drawbacks to having a 10" speaker and a 12" speaker in the same cab(JBL E110 & JBL E120)? Thanks


No. This is done all the time. again, make sure polarity is correct (the same for both) and impedance is matched.

Chuckles wrote:Thanks, indeed, UR!

Rule of thumb seems to be that with a tube power section, you don't want to go over the specified ohm rating, but with a solid state power section you shouldn't go under.

Fwiw, under gig conditions yesterday the three speaker rig sounded great, but: the 2x12 E's wired in series were noticeably not as loud as the 1 K (which makes sense given they were in series at 16ohms and rated rated for 100w more power. I'm thinking either the single K or a single E in the new cab will be the go-to rig for most dates, depending on room size and how much power I need to draw from the MosValve to be as loud as I might need to be. Just don't need anything more than that, really. The K seems much warmer than the E, as it is easier to drive a little harder. So, it'll be 250w through either the 200-rated K or the 300-rated E, or 500w through a single E. The days of hauling the 2x12 are over except in the most extreme outdoors or huge room situation (and that doesn't happen too often :roll: ).

The new cab that came with the K, btw, was most definitely designed as a bass cab I think... there are ports on the front which have been sealed with additional birch 13-ply screwed into place. Opening the back definitely took some of the bassiness out of it - plus, our drummer could hear it clearly! 8)



Just rewire the speakers in parallel for a 4 ohm load. Volume will be sustantially better, and you won't sound muted.
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Re: Converting closed-back to open-back

Postby waldo041 » Sun Apr 25, 2010 7:28 pm

ugly rumor wrote:Also, with your cabinet, there is no real reason to open the back or leave it sealed for a guitar. The frequencies of the instrument are very directional, and short enough wavelength to not matter.


it does matter whether a cab is open or closed back in it's release of those frequencies as they are departing from the front AND the rear, out of phase with each other. a closed back cabinet does just that, closes off the rear frequencies from meeting the frequencies coming out of the front of the speaker. this produces more bass from the cabinet as the rear leaving frequencies have no where to go or cannot meet the front leaving frequencies. this makes the box release them in the form of a lower frequency. with the cabinet completely open there is no hindrance of the rear frequencies meeting the frequencies of the front ones. this is what tames the bass frequencies or boominess of a cabinet. partially closed cabinets will do a combination of the two extremes and can differ in just how the back is constructed. too much of a partially closed back will produce slightly more bass then one that has a smaller area covered. also placement of the partial back can also effect it's frequency response. so while they are directional frequencies coming out, they are coming out from both side in reverse phase of each other and that is why there is a difference.

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Re: Converting closed-back to open-back

Postby Chuckles » Mon Apr 26, 2010 12:21 am

Made a HUGE difference in tone cutting out the back! Ain't the purtiest thing, but it gets the job done... wish I'd remembered to turn the damned D50 on... :?

Just rewire the speakers in parallel for a 4 ohm load. Volume will be sustantially better, and you won't sound muted.
Oh, yeah.. for sure they were quieter because they are rated for higher power. My question is: if the 2x12 is wired in series as a 16ohm cab and the K is at 8... do they draw different amounts of power, or are they all part of one big 5.66 ohm circuit? I'd suspect the latter, but I know squat about electronics (maybe I picked the wrong hobby! :lol: ). Anyway, the K was much louder. And - whether the K or the E - I don't see a whole lot of need to go back to a 2x12 (or 3 12's for that matter) ever again... so long as it's either a normal room or if an outdoor show it's miked and sent through the PA properly (we don't exactly have a lot of people dancing on the rails, so that front row air movement isn't all that important...).
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Re: Converting closed-back to open-back

Postby ugly rumor » Mon Apr 26, 2010 6:45 am

The K at 8 ohms will draw twice the power of the 16 ohm load. That's assuming you have these wired in parallel. Pretty don't count...sound do!

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Re: Converting closed-back to open-back

Postby KCJones » Mon May 03, 2010 9:05 am

Here is a recent project where I converted my circa 1969 2x12 Fender Bandmaster cabinet from closed back to open back. As far as port/hole size, I basically measured a 2x12 Tone Tubby cabinet which is just about the same size, and copied it. The wood is birch ply. The cabinet has 2 x16 ohm P12Ns (non-vintage) wired in parallel for an 8 ohm cab. I also recently converted both this and my 1x12 Tone Tubby (8 ohm) cab to Speakon. When I need the additional live volume I use the 3 x 12 @ 4 ohm setup as pictured. Usually I just need my 1x12. My main amp pictured is a Fuchs ODS 50 amp.
Image
Image
Image

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