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Will a Ampeg sb-12 run at 4 ohms?

fuzzy beard

fuzzy beard

Mar 8, 2011
2,747
31
Aurora, Indiana
#1
I know it is a 8 ohm head. And i know that the Ampeg b15 will run at4 ohms. And that Jess Oliver often replaced 8 ohm speakers with 4 ohm speakers on the b15. But will the sb-12 handle 4 ohm load?

I have kind of been looking for a 8 ohm 2x15 bass cab. There not very common! And then i seen Jimmy m b15 with 2 Ampeg pf115he cabs and fell in love! I like the ideal of having 2 1x15 cabs better than 1 2x15 cab. But cant do it if the head wont handle 4 ohms:(
 
#2
It'll be fine most likely. According to the expert folk, tube amps do better running at lower impedances than higher. It'll sound a little tighter than usual, but that's usually OK with most bass players.
 
#3
The amp tech called me back today i had left a message with the same question. But his response doesnt make much sense to me. He said there is a 4 ohm tap in the amp if i wanted to do a speaker out we could. But if i didnt want to put holes in the chassy he would recommend me running a 16 ohm load not a 4 ohm load. That is the part that threw me for a loop!

His reasoning is that the ohm rating is not a constant it varies with frequency respone acrooss the range. And that a 2x15 wired to 16 ohms will be in the range of 12ohms most often. And that running a 4 ohm load woul make it run to hot out of the 8 ohm tap.

Does this make sense to anyone? Kind of goes against every thing i have read about tube amps! Also if this theory does check out is there a way to run 2 ampeg pf115he cabs at 16 ohms?
 
#4
Speakers are reactive loads. As he was saying, the actual impedance varies with frequency range, thus a speaker rated for 8 ohms nominally isn't 8 ohms at every frequency. Your output transformer matches the high impedance output of the tubes to the low impedance load of the speaker, and responds to this reactive load in doing so, which is supposed to be part of what gives tube amps (or at least amps with output transformers) their tone. That's also why it's important to match speaker loads, as there is already built in variance to the speaker load, and having a mismatch may swing your tubes/transformer too far into the danger zone.

Like I said in the portaflex thread, the schematics for your SB-12 show 8 and 16 ohm taps on the transformer, but it's possible they used a transformer with a 4 ohm winding also available and never hooked it up.

I'm fairly certain the outputs on these amps were all in series, at least that's how it was with the B-12/15, so if you're going to add an output, I'd get rid of the latch connection, and have 8 and 16 ohm taps. Then you could run a single 8 ohm cab or two 8 ohm cabs. It would also require adding a 1/4" jack to run the SB-12 cab. It's less than ideal because you would be adding a jack to the head, but it would give you what you want.
 
#6
His suggestion is that in order to best match the load of the amp to the load of two 15" 8 ohm speakers, he suggests that you wire them in series to "16 ohms" (which is about 12-13 ohms in reality) and pair that to the 16 ohm tap. You could easily create a series speaker cable to run the two 115's that way.

Not quite sure why the 4 ohm tap is a worse idea though?

Edited. Missed a detail.
 
#7
christw said:
His suggestion is that in order to best match the load of the amp to the load of two 15" 8 ohm speakers, he suggests that you wire them in series to "16 ohms" (which is about 12-13 ohms in reality) and pair that to the 16 ohm tap. You could easily create a series speaker cable to run the two 115's that way.

Not quite sure why the 4 ohm tap is a worse idea though?

Edited. Missed a detail.
Click to expand...

That would require bypassing the internal speaker otherwise he would present a 24 ohm load since the speaker connections
are in series. The whole bit the tech said seems a bit odd to me. There is no reason to consider a 16 ohm load as anything but, unless he was going to measure the specific impedance range of those cabs. Unless the tech was referring to running a 4 ohm cab in the ext spot on addition to the internal speaker which would present a 12 ohm load on the 16 ohm winding.
 
#9
christw said:
Edit... I think I missed the point.
Click to expand...

I think I'm missing something too. Fuzzy are you talking about running a 4 ohm load on the extension alone or two 8 ohm 1x15 cabs using the two outputs (I've been assuming the whole time you've got the latch connecting version of the sb12, please correct me if I'm wrong)? Or a single 4 ohm load on the main output?
 
#10
I'd put my SB12 to a punk band through my '72 810 if it had an extension jack or some better form of speaker connection than the lid clips... :rolleyes:

That said, it'd be at its wits end the whole time if I did so best to not. :D

So are we shooting to bypass the internal speaker and outputs and run off of the 4 ohm or 16 ohm tap to an external speaker jack?? Or does your not have one? Mine doesn't.
 
#13
So what about those of us with the later versions? Mine is a 68 and has a ext speaker out (8 ohm). I'm not looking to get SVT volumes from it obviously, but wonder what it would sound like through my Avatar 212 Neo (4ohm). So long as it's safe for the amp, would I be able to get small/medium gig volume from it? It would be nice if it were possible so I wouldn't have to look for another all tube head that I don't have money for at the moment...
 
#14
Jess Oliver recommended to use a 4 ohm speaker to upgrade the B15, so if that carries to the sb12 it should work on the main output. Most tube amp transformers can handle a 100% mismatch and ampeg is known for using fairly robust transformers so it should be ok, but you will work the tubes harder and they will fail sooner. I wouldn't recommend it, but if history is any indication it should work alright.
 
#17
Ok let me try to clear this up. I have 1965 sb-12 so mine connects through the latches. Mine also does not have a speaker out. And I rather not install one! I was looking for a 8 ohm 2x15 until I saw jimmy m b15 and 2 pf115 and fell in love with it. And if the b15 will drive 2x15 good enough for jimmy I think the sb-12 with only five less watts would do just fine. Problem is my amp wants 8 ohms and if I run 2 1x15 I will get 4 ohms. I am trying to figure out of it would handle it.

Also if i went with the pf115 i would attach to the lid of pf115 and wire the latches. If i find a 8 ohm 2x15 or have LDS make me one i have another plan. Make a base that my lid would latch to and wire the latch to a 1/4 plug so i would have my sb-12 on its origonal lid latched to a base that could even be a head case and run the wires on the base from the latches to the 1/4 plug. Does that make sense i can see it in my head!
 
#19
You could try to find a way to mount the SB12 lid to the PF115HE lid and wire a 1/4" off of that, like you mentioned. However, then you'd want to run the 4 ohm tap to the latches. A head case isn't a bad idea either. You could put 1/4" outs for each tap! In new stock Ampeg blue checker, tasty!
 
#20
Great thread guys. Earlier today, I was thinking about rewiring my SB-12 to have one main output jack and one switched external output jack similar to the B-15. So far, I have found four different SB-12 schematics: two for the 6L6 version, and two for the 7868 version. The 6L6 version already has the main and extension output jacks as described above, so my first thought is that there would be no problem rewiring my 7868 amp to match. However, my concern is whether Ampeg used a different output transformer for the two versions. They both have 8-ohm and 16-ohm taps, however I can't help but wonder if the 6L6 version has a heaver duty transformer. The only other differences I can find are the resistor values in the negative feedback loop, and that the 6L6 version has a 250 ohm, 10-watt resistor across the output transformer secondary.
My output transformer is labeled (PN) OT - 215 and 682601 (Electric Windings, first week of 1966). Does anyone know the part number of the OT in the 6L6 versions? What is the purpose of the 250-ohm, 10-watt resistor? Should that be added to the SB-12, 7868 version if it is modified to add the output jacks? Are the jacks wired correctly in the attached drawing? Should the external output jack be isolated form the chassis as shown in my drawing?

FB, do you think this thread would get more replies if it were moved over to the portaflex club?
SB-12-2.gif
 

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