Amp building idea.......

Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

I can get Will Dykes (Armadillo Amp Works in Austin) to build me a cab for probably about $300.00.

I have access to 4-JBL 12s that won't cost me anything.

All I need is a Twin Reverb chassis (usually pretty cheap on eBay).

My question: (I never was good at ohms) how do I wire the 4 JBLs at 8ohm to create a 4ohm load?

I know, it's a Quad Reverb, and very heavy....I'm a young man that can lift it with no problem.

Thanks folks.

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Rod's Place

stingray · From: maplewood ohio

Hello!

if your speakers are 8 ohm..you want to wire them in parallel.. two speakers only !!this will put your total imp. to 4 ohms NOTE !

don't add ext cab! this will lower imp!

[This message was edited by stingray on 25 October 2001 at 06:18 PM.]

Donny Hinson · From: Glen Burnie, Md. U.S.A.

You can't! You can't wire four 8-ohm speakers, and come out with 4 ohms.

[This message was edited by Donny Hinson on 25 October 2001 at 06:03 PM.]

stingray · From: maplewood ohio

Sorry there! I seen you talking about a twin
reverb & didn't see that you were using 4 speakers!! you can only use 2 speakers
and as some of the other posts..they must be in proper phase!

KEEP PICKING !

John Paul Jones · From: San Diego

oops!

[This message was edited by John Paul Jones on 25 October 2001 at 07:30 PM.]

Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

You guys are rainin' on my parade.

I reckon I need 4-16ohm speakers. I suppose it's either that, find a real Quad (with a 2ohm tranny) or change the tranny in a Twin.....then it ain't so cheap anymore.

Oh well, it was just an idea.

Chris Lucker · From: Los Angeles, California USA

You can use your four speakers if you series them and then and parallel them IF you switch out the Twin Reverb output transformer for a Single Showman 8 Ohm output tranny.
Chris

Bill Terry · From: Bastrop, TX

Hey Rodney... so about "those 4 JBL-12s that won't cost me anything"... And you happen to be coming my way Sunday... Hmmm..

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Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

Well Bill, 'ol buddy, 'ol pal......I won't have 'em fer a couple of weeks. I wouldn't be interested in sellin' 'em though. Holler at me before Sunday. Rod

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Rod's Place

Jerry Hedge · From: Norwood Ohio U.S.A.

Rodney, you shouldn't have any trouble running the 4 J.B.L.s with the Twin. Fender output trannys are usually tolerant of the type of mismatch you're running into. It would be like running the Twin and a Bassman cab, a setup many loud guitarists have used for years. If you want to change the output tranny to something that will match your speaker load better, look for a "Hammond" brand transformer. They are a heavy-duty unit, with more iron. A 100 watt Hammond is about twice the size of the stock Fender. The amp will have better bottom end and a better clean tone for steel.

Bill Llewellyn · From: San Jose, CA

You can wire the speakers in parallel in two pairs, then put those two pairs in series. This will give you 8 ohms. I'd guess the Twin would be fine with that. Here's how to wire it to avoid phase cancellation:

Bobby Lee · From: Cloverdale, California, USA

You can get 3.2 ohms out of four 8 ohm speakers, but I think a Twin Reverb would run pretty hot that way:

     _____     _____


     \___/     \___/


      + -_______+ -


      |           |


      |   _____   |


      |   \___/   |


      |____+ -____|


      |           |


      |   _____   |


      |   \___/   |


      |____+ -____|


      |           |


      |           |


      +    amp    -

Bill Llewellyn · From: San Jose, CA

Bob,

I like your ASCII art.

Nice work.

The top two speakers in your diagram would each deliver only 1/4 the power of each of the bottom two. (P = E^2/R, and each top speaker gets 1/2 E.) For example, if the bottom two have 20W each, the top two would have 5W each. That may be ok if getting as close to 4 ohms as possible is a key issue. If equal power per speaker is more important, then the circuit needs to be configured to deliver equal voltage to each speaker.

One thing about my 8 ohm configuration above.... it won't get as loud at peak amplifier output voltage as b0b's arrangement.

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Bill (steel player impersonator) | MSA Classic U12 | Email | My music | Steeler birthdays | Over 50?

Joe Donahue · From: Blackwood, NJ, USA

Bill Llewellyn -

Are there any ill effects to adding an 8 OHM dummy load in parallel with your arrangement to bring the impedance down to 4 OHMs net?

Bobby Lee · From: Cloverdale, California, USA

It would be a waste of half of the amp's power. What's the advantage of running at 4 ohms anyway?

Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

Twins have a 4ohm tranny bOb; if you added a dummy load to bring it down to a 4ohm load, would you have the amp operating where it's supposed to be?

My question is (and was addressed by Jerry up there); what's wrong with running say 2 of the speakers out of the speaker jack and two out of the ext. speaker jack........wasn't the Twin Reverb made to operate okay that way?

But my concerns may be somewhat unfounded anyhow......all the speakers are in our mid cabs right now.....after asking the singer what the load was, he didn't know. It may very well be that the cabs are running at 8ohms each which would make my speakers 16ohms each (since there's 2 in each cab) and then getting the 4ohm load would be no problem anyhow.

It only came to me as an after-thought that most cabs are wired to run at 8ohms with all components......whether it be 2-12s/1-12/2-15s/1-15 and with or without a horn..if that be the case here.........I'm worried about nothing anyhow.

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Rod's Place

ajm · From: Los Angeles

I'll need to get some input from others to verify things here, but here are a few thoughts off the cuff.....

I believe that the ExtSpkr jack is just another jack wired in parallel with the main jack. Plugging into it would be the same as wiring up your speakers externally.

Most tube amps can tolerate impedance mismatches like we're talking here. The main thing you'll notice will be a loss of power. I'd probably wire them up for 8 ohms. 60 watts is still pretty loud; it might be enough for you. You could also suggest that everybody else turn down a notch, and it might make the band sound better as a whole, but that is kind of sacreligious to most musicians so it probably won't happen.

Sometimes transformers have multiple taps for different impedances but the manufacturer just tapes them off and doesn't use them. If that's the case with the chassis you end up with then your problem may be solved.

I'd put the speakers in either two or four separate cabinets. It'll be easier to haul around, and you have some more options if you ever want to spread the sound out and/or run stereo in the future.

Bobby Lee · From: Cloverdale, California, USA

The traditional black face Twin Reverb ran at 4 ohms, and had an external speaker jack to add another 8 ohm speaker in parallel to that load. That would give you 2 2/3 ohms.

It runs hot, but the amp was designed to handle a load that's less than 4 ohms. I'm not sure that it would be safe to run it down to 2 ohms, though (all 4 speakers in parallel).

The configuration I outlined above delivers considerably less power to the two speakers that are in series, compared to the other two. You'd probably get a better sound with 3 speakers wired in parallel because the load would be more evenly distributed.

Here's a link to the black face Twin's specs and schematic.

Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

Well, I have one problem knocked out......after gettin' our new mains, we looked in the cabs at the JBLs and they're 16ohms.......the load should be no problem......if I decide to pursue this idea, I'll let y'all know how it comes out.

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Rod's Place

Steve B · From: Garland Texas

Rodney - If you are having Armadillo Amp Works build a combo for you here is something to think about. The guitar player in my band had them build a 4x10 combo for his black face Bandmaster head. He even left the head with them so that they could make sure it would all fit right. He brought it home to put the speakers in and guess what? The top left (if looking at it from behind) speaker will not fit because the transformer touches the speaker magnet. I would advise leaving your speakers & head (chasis) with them when you make the order.

Other than the speaker problem, their work looks great. The construction & covering are all put together perfectly.

Chris DeBarge · From: Boston, Mass

Rodney, invest in a forklift too.

Jim Bob Sedgwick · From: Clinton, Missouri USA

Or a Hernia belt.

Rodney Shuffler · From: Montgomery, Texas USA (Home of the Bears)

I ain't scared. I carry my Pro Reverb in an oversize roadcase (had it made to fit any Fender amp). That Pro is loaded with old Altecs with magnets that look like syrup buckets and it's heavy.....I don't have any problems with it. It's all about the tone, I carry my stuff about a minute tops.....I have to listen to it all night long.

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Rod's Place

Chuck Smith · From: Crockett, Texas ,U.S.A.

Hey Rod, You can always just borrow my old Session 500!!!!! (;

Later Bro....

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Chuck.. ."Every day's a Holiday and every night's a Party";)

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