John d is going off what my amp tech told me. My amp tech said there was a unused 4 ohm tap on the OPT.
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Will a Ampeg sb-12 run at 4 ohms?
- Thread starter fuzzy beard
- Start date
Sorry for the confusion, Paul. As I understand the conversation, Fuzzy Beard started out by asking about using a four-ohm load with the SB-12. However, we morphed into using two [DEL]16-ohm [/DEL] 8-ohnm cabs in series, and wire them to the 16-ohm tap.
Fuzzy,
If you are serious about using two PF115 cabinets with your SB-12, here is a way you can do it without any permanent modifications.
Using the drawing shown below, first rewire your "Instrument" jack to the 16-ohm tap on the transformer. Be careful with your wire routing to ensure that you do not introduce any unwanted noise into the system; you may want to use shielded wire. If shielded wire is used, only ground one end of the shield. David (Beans on Toast) suggested using an isolated jack, so you may want to include that. I would take the negative wire for the isolated jack back to the same ground point that the internal speaker ground uses.
Next, get a electronics project box (something from Radio Shack should work well) and wire it as shown in the drawing below (inside dotted lines), using three standard 1/4 inch jacks. Make sure to use isolated jacks, if you use a metal project box.
Finally, simply hook up the speakers to the two-project box, output jacks. Then, connect the output from the SB-12 jack to the input jack on the project box. (I would get something to plug the Amp's "Instrument" jack when not in use. This would prevent anyone form accidently plugging an instrument in this jack, when using the internal speaker.)
I am not sure if you can run an 8-ohm load off of the 8-ohm tap, while simultaneously running a 16-ohm load off of the 16-ohm tap. You probably wouldn't want to anyway; since I would think, the PF cabinets would over power the internal SB-12 Speaker. You may want to get other opinions about this mod or its usage, before you try it. If you try this, let me know how it works out for you.
John
If you are serious about using two PF115 cabinets with your SB-12, here is a way you can do it without any permanent modifications.
Using the drawing shown below, first rewire your "Instrument" jack to the 16-ohm tap on the transformer. Be careful with your wire routing to ensure that you do not introduce any unwanted noise into the system; you may want to use shielded wire. If shielded wire is used, only ground one end of the shield. David (Beans on Toast) suggested using an isolated jack, so you may want to include that. I would take the negative wire for the isolated jack back to the same ground point that the internal speaker ground uses.
Next, get a electronics project box (something from Radio Shack should work well) and wire it as shown in the drawing below (inside dotted lines), using three standard 1/4 inch jacks. Make sure to use isolated jacks, if you use a metal project box.
Finally, simply hook up the speakers to the two-project box, output jacks. Then, connect the output from the SB-12 jack to the input jack on the project box. (I would get something to plug the Amp's "Instrument" jack when not in use. This would prevent anyone form accidently plugging an instrument in this jack, when using the internal speaker.)
I am not sure if you can run an 8-ohm load off of the 8-ohm tap, while simultaneously running a 16-ohm load off of the 16-ohm tap. You probably wouldn't want to anyway; since I would think, the PF cabinets would over power the internal SB-12 Speaker. You may want to get other opinions about this mod or its usage, before you try it. If you try this, let me know how it works out for you.
John
Putting a 32Ω load onto a 16Ω tap is inadvisable for a tube amp. 4Ω load on the 8Ω tap would be better but would be something I'd avoid on a "Classic" amplifier. These amps were designed with one or two 8Ω cabinets in mind and that's the way I would use them.
My B15N never leaves the house. Same for my '63 Precision. Too old and too valuable to risk transportation.
Putting a 32Ω load onto a 16Ω tap is inadvisable for a tube amp. 4Ω load on the 8Ω tap would be better but would be something I'd avoid on a "Classic" amplifier. These amps were designed with one or two 8Ω cabinets in mind and that's the way I would use them.
My B15N never leaves the house. Same for my '63 Precision. Too old and too valuable to risk transportation.
That was a typo, Paul. Should have been two 8-ohm cabs in series connected to the 16-ohm tap. Thanks for the correction.
Putting a 32Ω load onto a 16Ω tap is inadvisable for a tube amp. 4Ω load on the 8Ω tap would be better but would be something I'd avoid on a "Classic" amplifier. These amps were designed with one or two 8Ω cabinets in mind and that's the way I would use them.
My B15N never leaves the house. Same for my '63 Precision. Too old and too valuable to risk transportation.
Really? I did that with a Rickenbacker 360 that I bought when I was 17 by working at this miserable car wash. I scratched it once with a pocket knife clipped to my belt and didn't do much but look at it for nearly five years. Now it's behind me but not getting much action. We almost exclusively own tube amps here and most (3) of the (4) guitar amps are down and out. Plus I'm house bassist.
Putting a 32Ω load onto a 16Ω tap is inadvisable for a tube amp. 4Ω load on the 8Ω tap would be better but would be something I'd avoid on a "Classic" amplifier. These amps were designed with one or two 8Ω cabinets in mind and that's the way I would use them.
My B15N never leaves the house. Same for my '63 Precision. Too old and too valuable to risk transportation.
To each his own. Almost every gig I ever played in my twenty-five year career was played on a '62 Fender Jazz bass. Musical insturments are made to be played. Pictures are for hanging on the wall. YMMV
Agreed to a point. My Precision has had it's frets dressed quite a few times. Next will be a re-fret and I simply don't want it to have that done. I have been playing six string basses exclusively for the last fifteen years and I'm more comfortable with those. BTW I've owned the P since January 1968.
Regarding the difference between the SB-12 and the B15 - my opinion as to the suitability is merely based on the size of the transformer. The B15 just seems considerably more robust.
I use old amps exclusively, but I think a large part of my positive experience and lack of reliability issues with them is due to not stressing them. I don't doubt that the SB-12 is capable of driving a pair of 15"s, but if you're in a live situation playing with drums regularly. I believe you're going to be exceeding its happy zone.
I use old amps exclusively, but I think a large part of my positive experience and lack of reliability issues with them is due to not stressing them. I don't doubt that the SB-12 is capable of driving a pair of 15"s, but if you're in a live situation playing with drums regularly. I believe you're going to be exceeding its happy zone.
Regarding the difference between the SB-12 and the B15 - my opinion as to the suitability is merely based on the size of the transformer. The B15 just seems considerably more robust.
I use old amps exclusively, but I think a large part of my positive experience and lack of reliability issues with them is due to not stressing them. I don't doubt that the SB-12 is capable of driving a pair of 15"s, but if you're in a live situation playing with drums regularly. I believe you're going to be exceeding its happy zone.
+1
The beauty of the B-15N's and SB-12's is that they sound wonderful with the original cabinets designed to match their respective heads. When Ampeg built the B-15NC and the B15ND they used the B-18 head because it would power that system at a higher RMS wattage. IMHO, IME running the cabinets with higher powered heads works fine, in fact I used a pair of B-15N cabinets (with JBL E140's) driven by my first Walter Wood's MI-100-8 (100 watts) and they sounded great.
Going the opposite direction and trying to drive 2x15's with the SB-12 head doesn't quite make sense to me. Just my opinion though. Granted, Ampeg did put extension speaker jacks on the back of the head but running them at 4 ohms is going to put more stress on the output transformer, especially if it's around 40 years old.
If I wanted to get a SB-12 at higher volumes I'd stick a mic in front of the cab like Jimmy does and leave it at that.
Ric
I'm more than a little confused here. My memory, poor as it is, says that the SB12 is a porta-flex unit fitted with a 12" 8Ω driver. Is the OP wanting to use the amplifier section with two single 1x15 cabinets? If so I don't see what the problem is.
Please clue this aged old fart into what is needed.
Please clue this aged old fart into what is needed.
Yes that was the jist of my origenal post. The problem was that 2 Ampeg pf115he cabs will give me 4 ohms. And the amp wants to see 8 ohms. I dont want to mod the amp! So my other option is just get a 8 ohm 2x15 cab.
My question also brought up other questions by other sb-12 owners and we are all kind of talking about all of it. I see how it can get confusing!
My question also brought up other questions by other sb-12 owners and we are all kind of talking about all of it. I see how it can get confusing!
fuzzy, are your plans now to just leave it attached on the lid and to have a custom made 2x15 @ 8 ohms that latches to it?
I've been struggling with what to do with mine, including nothing. I think by the time my B15N is healthy and I'm back in a band, I'll realize that I have ample Ampeg rigs of all sizes.
I've been struggling with what to do with mine, including nothing. I think by the time my B15N is healthy and I'm back in a band, I'll realize that I have ample Ampeg rigs of all sizes.
I'm more than a little confused here. My memory, poor as it is, says that the SB12 is a porta-flex unit fitted with a 12" 8Ω driver. Is the OP wanting to use the amplifier section with two single 1x15 cabinets? If so I don't see what the problem is.
Please clue this aged old fart into what is needed.
Paul,
You've got it right, and I'm with you on using two 8-ohm cabs. The problem is that some models of SB-12 don't have any output jacks. Instead, they use the head latches to make the amp to driver connection. However, they do still have the 16-ohm tap on the transformer. The amp would have to be rewired to access this tap.
John
I think you are all set with Ampeg gear!LOL
My other ideal if i couldnt do a rig like jimmy was. Have made or find a 2x15 8 ohm cab. Then i could make a base that my sb-12 lid will latch to. And run a output fom the base. So speaker signal would go from head, through lugs, though the latches, through the wires in the base, and into the 1/4 inch plug. Then i could plug into any 8 ohm cab.
My other ideal if i couldnt do a rig like jimmy was. Have made or find a 2x15 8 ohm cab. Then i could make a base that my sb-12 lid will latch to. And run a output fom the base. So speaker signal would go from head, through lugs, though the latches, through the wires in the base, and into the 1/4 inch plug. Then i could plug into any 8 ohm cab.
I agree that I have plenty [when I'm being logical.]
You can probably create a latching mechanism to mate the old lid to a single PF115HE lid but without some sort of way to change taps, we're still stuck at 8 ohms. I support a custom lid to mate these heads to. I also support a custom cab but the lid wins at versatility.
You can probably create a latching mechanism to mate the old lid to a single PF115HE lid but without some sort of way to change taps, we're still stuck at 8 ohms. I support a custom lid to mate these heads to. I also support a custom cab but the lid wins at versatility.
New Question!
I was told a 2x15 8ohm cab would make the head work to hard. That doesnt make any sense to me it should work no harder no matter how many speakers there are right? It should take the 25 watts and split it among the speakers evenly. And with the cab being 8 ohms it would be seeing the resistance it wants.
I really dont think this is true i once hooked up a valve jr. to a 8x10!
I was told a 2x15 8ohm cab would make the head work to hard. That doesnt make any sense to me it should work no harder no matter how many speakers there are right? It should take the 25 watts and split it among the speakers evenly. And with the cab being 8 ohms it would be seeing the resistance it wants.
I really dont think this is true i once hooked up a valve jr. to a 8x10!
If it's an 8 ohm box, just do it and see if you're happy with the results. If you're running the volume at 3 o'clock or better, it'll let you know in fairly short order whether its happy with that situation or not. 
The head sees whatever load the cab presents it. It doesn't care whether it's a 215, 410, 810, 1610, or 2017 as long as it's kosher with the impedance.
Fuzzy,
IMO, running your SB-12 into an 8-ohm cab is not as risky as Jimmy running his B-15 into a 4-ohm 810. You will only have a problem if, as nysbob said, you try to crank it beyond its power capabilities. If you are concerned, run it with the cage removed, and check the transformers periodically. If they start getting hot, you'd better back off. Out of curiosity, I've been running mine hard this past week, using the SB-12 cab, and the transformers never got past slightly warm to the touch. The OT didn't get warm at all. YMMV.
JD
