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Topic: Yet another speaker impedance/ amp question |
Lane Gray
From:
Topeka, KS
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Posted 7 Oct 2014 10:09 pm |
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So I have this 135W Twin with 2 12s.
The Internal jack is labelled 4 Ohm minimum.
So is the External speaker jack, and I have a sweet sounding SICA 15, 4 ohm.
At living room levels it sounds über sweet with all 3 cones pushin' air. I assume that it's built to do that, runn8ng 4ohms in each. Am I right?
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2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Jack Stoner
From:
Kansas City, MO
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Posted 8 Oct 2014 2:21 am
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If the two internal speakers are 8 ohms each and wired in parallel that is 4 ohms total. If the external speaker jack is also wired in parallel, that is 4 ohms in parallel with 4 ohms and the total ohms load is 2 ohms. (two resistances in parallel and both the same resistance equals half of one, thus two 4 ohms in parallel is 2 ohms).
There are disagreements on what that 2 ohm load will do to the amp's components, let alone what it could do to the signal. |
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Lane Gray
From:
Topeka, KS
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Posted 8 Oct 2014 2:39 am
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Indeed, Jack.
I didn't take steps (like setting the 15 outside on the porch) to make sure that using the external jack didn't disconnect the pair of 12s.
The label on the back specified impedance per jack, but didn't say "use one or the other, but not both."
The vague ambiguity is why I turned to the room here.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Dick Sexton
From:
Greenville, Ohio
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Posted 8 Oct 2014 4:36 am Speaker Question...
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Lane, Jack is correct of course. But being curious about the subject, I found this: http://fenderguru.com/how-to-select-speakers Lots of info...
4 ohm amps: Champ 1×8″, Vibro Champ 1×8″, Vibrolux Reverb 2×10″, Tremolux 2×12″, Pro Reverb 2×12″, Twin Reverb 2×12″, Bandmaster 2×12″, Bassman 2×12″. According to the rule of thumb these amps handle a speaker load between 2 and 8 ohms. The 4 ohm amps are flexible and supports a lot of extension cabinet applications. A single speaker extension cabinet at 8 ohm will, together with the 4 ohm speaker load, result in 2.67 ohms. Two speaker extension will give you a 2 ohm total speaker impedance. In both cases the power is distributed evenly between all speakers as long as they are 8 ohm each and wired in parallel (which we personally always do). Good extension cabinets with the amps above are 1×12″, 1×15″, 2×10″ or 2×12″. In any single speaker extension cabinet we recommend highly efficient speakers since we want that extra speaker to be heard along with the two built in speakers. It is one against two.
And Twin specific: http://fenderguru.com/amps/twin-reverb |
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Lane Gray
From:
Topeka, KS
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Posted 8 Oct 2014 4:52 am
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Excellent. I tell you, it sounds pretty sweet with all three. And that extension cab is LIGHT.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Tommy Boswell
From:
Virginia, USA
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Posted 8 Oct 2014 5:38 pm
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Lane,
Is your Twin labeled like this? picture here:http://bb.steelguitarforum.com/viewtopic.php?t=220024
According to the schematic, my Bassman 70 external speaker jack is a switching jack. I'm guessing it switches over to a 2-ohm tap on the transformer. But I'm no guru when it comes to reading schematics, so maybe someone else can confirm.
All I know is that the amp will drive two 4-ohm cabinets without breaking a sweat. The first time I plugged into the external jack, the amp hummed for several seconds, like something was waking up that hadn't been used in a long time. After that, it works great every time. |
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Douglas Schuch
From:
Valencia, Philippines
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Posted 8 Oct 2014 5:46 pm
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Lane, I don't know what vintage your amp is, but my Bassman from '78 has the ext speaker jack wired so that it actually switches the connection on the output transformer so that I have 4 ohms through one jack, or 4 ohms through both jacks. You should be able to research that for your amp, or just look inside to see if there is a switch on the ext jack that goes back to the transformer. Oops, I now see that Tommy B. has said the same thing, but will post this as "conformation". Tommy, I believe the way mine works is it uses an 8 ohm output, and the jacks are wired in series. Mine is a later model - the Bassman 135
Doug
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Bringing steel guitar to the bukid of Negros Oriental! |
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Tommy Boswell
From:
Virginia, USA
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Posted 8 Oct 2014 5:54 pm
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| If mine needs two 8-ohm speakers, then I'm doing it wrong. I should also add that I've only tried it a few times, and not a lot of play-time. |
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Tommy Boswell
From:
Virginia, USA
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Posted 8 Oct 2014 6:05 pm
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I found this on another forum, this guy thinks I'm right about the Bassman 70 jacks, for what it's worth:
"This is one of those amps that play with output transformer secondary-winding selection depending on whether or not the extension speaker jack is occupied. When only the main speaker is plugged in, the full winding is used. When two speakers are plugged in, only half the winding is used.
Although the schematic doesn't come right out and say it, it seems clear that each jack wants to see 4 ohms, for a combined impedance of 2 ohms. The main jack is denoted 068270 SPKR JACK FOR 4 OHM MIN. LOAD, and the extension jack is denoted 014143 EXT. SPEAKER JACK FOR 4 OHM MIN. LOAD. That and the split secondary winding tell me it's 4 ohms per jack." |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 9 Oct 2014 4:02 pm
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| The Ultra-Linear Twin Reverb has a tapped 8-ohm transformer, and a switching-jack arrangement. It's looking for a minimum of 4-ohms on each jack, and when both jacks are used, the amp is still seeing a 4-ohm load (provided the extension speaker is also 4-ohms). |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 9 Oct 2014 6:47 pm
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| Donny Hinson wrote: |
| The Ultra-Linear Twin Reverb has a tapped 8-ohm transformer, and a switching-jack arrangement. It's looking for a minimum of 4-ohms on each jack, and when both jacks are used, the amp is still seeing a 4-ohm load (provided the extension speaker is also 4-ohms). |
Not possible... here's the schematic:
http://schems.com/manu/fender/twin_reverb_sf_135_schem.pdf
The speakers are seriesed and put through the 8ohm winding when both are used... with one speaker the 4ohm winding is used. Note that you always have to use the first speaker output before using the second one, this is the case for all Fender tube amps.
For the Vibrosonic, note the different arrangement... the jacks are paralleled into the 4ohm load, and the loads are called out for 8ohms min each. The one internal speaker is 8ohms and gets the 8ohm winding when by itself.
This nomenclature can get you into trouble... you shouldn't run two 16ohm cabinets, for instance... Fenders are normally OK for one step away in impedance, but lower is normally better than higher, since voltages can go too high and arc over your output transformer or tube sockets.
I have a PA 135 and when I finally get that round tuit I'm going to connect the jacks as 4ohm and 8ohm and label them as such. This gives much more flexibility and control... you just have to be sure not to run it into an open load... Fender' stock arrangement makes sure that you run it into a short instead of an open when nothing's connected... the amp lasts a little longer this way, perhaps long enough for you to notice your mistake.
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Too much junk to list... always getting more. |
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