Fuzzy, I was trying to keep my SB-12 original, too. However, I have been trying to chase down a problem where mine was periodically distorting. I finally found the problem last night, and it turns out that one of the latches was not making good electrical contact. The distortion got to the point of cutting out completely, before I found the problem. I noticed that the output transformer was getting hotter than usual, as I think it should with no load on the amp. I've decided to bite the bullet and install 1/4 in jacks on mine to protect the amp if nothing else.
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Will a Ampeg sb-12 run at 4 ohms?
- Thread starter fuzzy beard
- Start date
Fuzzy, I was trying to keep my SB-12 original, too. However, I have been trying to chase down a problem where mine was periodically distorting. I finally found the problem last night, and it turns out that one of the latches was not making good electrical contact. The distortion got to the point of cutting out completely, before I found the problem. I noticed that the output transformer was getting hotter than usual, as I think it should with no load on the amp. I've decided to bite the bullet and install 1/4 in jacks on mine to protect the amp if nothing else.
christw said:
Problem is I have no way to switch between taps with out modifying the amp.
Also john d I didn't want to clutter up the other thread. I find some times questions get lost in that thread with all the traffic it sees. Also if you replace your lid gasket you would get better contact. Also I thought about taking light gauge copper and bending it around the lugs to make better contact.
The custom head case and a cab from LDS is the easier and cheaper way to go. But jimmy m rig looks so good. Then I could use my head with stock cab or latch to base and use with the 2x15.
Also john d if you feel you have to do a speaker out don't drill any holes! Disconnect the instrument in and wire that as your speaker out. You can always change it back later. Hate to see you put a hole on some thing that has been around this long.
Yeah, I know what you mean. I haven't loaded the drill yet. I had already planned to put weather stripping under the lid to seal it better, and I understand that this helps tighten the lid. I already replaced one of the latches, since one was missing when I got the amp. I guess I should replace both, before I get carried away. I'm still hoping David or some of the other experts will chime in about the wiring diagram I posted. I don't know, I still may mount the jacks. That's a good idea about using the instrument jack. I could leave the main speaker connected through the latches and use the instrument jack for the extension speaker. Would have to change to an isolated, switched jack, but I could still make that work.
It's a low power amp and you could do what you want with the addition of an outboard matching transformer. It will be fairly heavy and a bit pricey but better than risking the OPT in the Ampeg.
John D said:
I must have read wrong o thought you where going to a speaker out for just the sb-12. Not to add an extension cab. I don't know if it would work like that. I thought you where going to disconnect the latch wires and wire a speaker out.
Paul, I don't know how long you have been following this thread, but some of the SB-12 amps already have the external speaker jack. It is switched and it connects the output to the 16 ohm tap the same as a B-15, so there should be no danger to the OT. That is, unless they used a better transformer in the amps with the ext jack. Am I missing something? Also, do you know the purpose of the 250 ohm resistor across the OT secondary of the amps with the ext spkr jack?
Actually, I was trying to do both. I just think it would be cool to have a mini JimmyM stack.
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?
John, the purpose of the 250 ohm 10W resistor is to offer some protection for the amp in the event that the amp is turned on without the speaker connected. It is a good thing to do and doesn't affect the tone of the amp. The resistor will not act as a dummy load over the long term but is intended to offer protection in the short term. It is a good safety measure to add to an amp as it could save the power tubes and output transformer from being damaged if the amp is run without a speaker connected. Not all amps have this sort of protection.
I like to isolate all output jacks from the chassis. This avoids currents through the chassis which could cause noise. This is my personal preference. Normally the external speaker is isolated on these amps as you have indicated and your drawing looks correct.
As far as I know, the 6L6GC models and 7868 both use the OT-215 output transformer.
The feedback resistors are different, in part, because the phase inverter circuits are not the same.
Thanks, David. This is all as I thought, so it shouldn't hurt the early SB-12 amps to make this modification. Now, to find time to do it.
FWIW, I'm not a fan of running a 4 ohm load on an 8 ohm tap. It puts a strain on the tubes and the old output transformer. It also changes the tone of the amp. This is just my preference. I figure that it is better to be safe than sorry.
I'm not aware of anyone trying this with an SB-12.
In general, it has to do with the design specifications of the SB-12 output transformer, the operating voltages within the amp, the design of the power stage, and even the speaker. Headroom and distortion are affected. Depending where everything falls, some amps can tolerate operating at a mismatched impedance, others can't. Even if you had all the necessary info to calculate things out, it usually comes down to trying it and seeing what happens.
I'm not saying that it won't work. Try it carefully at a lower volume, monitor the heat being given off by the transformer and check if the tubes are not running too hot. If things seem fine, try it at progressively higher volume levels and see.
One option would be to have both 16 and 8 ohm speaker out jacks on the amp connected to the appropriate taps on the output transformer. If the 215 cab has two 8 ohm speakers, wire them in series so it is a 16 ohm cab. With a configuration like this, you can't run both a 16 ohm cab and an 8 ohm cab at the same time. Only one or the other.
For 2 15 8 ohm cabs, they can be wired to switch to the 16 ohm tap as in the later versions of the amp.
I'm not aware of anyone trying this with an SB-12.
In general, it has to do with the design specifications of the SB-12 output transformer, the operating voltages within the amp, the design of the power stage, and even the speaker. Headroom and distortion are affected. Depending where everything falls, some amps can tolerate operating at a mismatched impedance, others can't. Even if you had all the necessary info to calculate things out, it usually comes down to trying it and seeing what happens.
I'm not saying that it won't work. Try it carefully at a lower volume, monitor the heat being given off by the transformer and check if the tubes are not running too hot. If things seem fine, try it at progressively higher volume levels and see.
One option would be to have both 16 and 8 ohm speaker out jacks on the amp connected to the appropriate taps on the output transformer. If the 215 cab has two 8 ohm speakers, wire them in series so it is a 16 ohm cab. With a configuration like this, you can't run both a 16 ohm cab and an 8 ohm cab at the same time. Only one or the other.
For 2 15 8 ohm cabs, they can be wired to switch to the 16 ohm tap as in the later versions of the amp.
You're right, Bob. 25 watts is more like it:I know I'm typically the one to say you don't need as much power as everyone says you do, but I've got an SB-12 and although I love it, I think you need a little more gas to push a pair of 15"s properly. It's a 23 watt amp.
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You're right, Bob. 25 watts is more like it:
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Yeah, you and your two stinking extra watts.
That's a bad rig, Jimmy.
Can't complainYeah, you and your two stinking extra watts.
Using my regular Vintage Blue and Fliptops double baffle cabs for my Bowzer tour all this weekend and Tuesday, but the PF cabs with the B-15 are a nice change of pace. Different but still very much the same kind of sound overall.
Hmm jimmy if yours is a b-15nc isnt it 50 watts? I thought you had just a b-15n but checked your profile i seen different. Also i got all my info from Portaflex amplifiers
- Feb 13, 2008
- 1,247
- 1,916
- Disclosures
- Owner, Vintage Blue (repro cabinets)
The B15NC designation tends to be a bit confusing because it can refer to two completely different amps. Ampeg originally used the letter after the 'N' to designate the circuit revision of the amp. In the case of Jimmy's B15NC that dates it very roughly to around 1963-1964. The last lettered circuit revision was the B15NF, roughly dated to 1966. However, the confusion arises because they also came out with a B15NC and B15ND around 1967. In this case though the 'C' and the 'D' referred to the speaker configuration rather than the circuit revision. The 'C' had a 2x15 column-type cabinet and the 'D' was a standard single baffle caninet with a separate 1x15 extension cabinet. Both of these B15s used the circuitry normally found in the B18N which is why these two B15 versions were actually 50 watts.
So jimmy m b-15nc is 25 watts. My sb-12 is 25 watts. I am sure they both were conservitive ratings. So other than jimmy having a few more options than mine. There is no reason to think mine wont push a 2x15 of proper ohm rating. Also other than a few options are the b-15 and sb-12 much different?
I dont want to give up on my sb-12 cab so changing the ohm tap of my head is out. I just want to be able to hook it up to the 2x15 as needed or wanted. So like i said before a custom 8 ohm cab from LDS. And making a head box/base wired through latchs is the cheaper and easeir way to go. I'll have over 800 just in the 2 Ampeg pf115he. My quote from LDS for a old school 2x15 at 8 ohms was alot less than 800.
Also thanks Vintage blue for clearing that up for me!
I dont want to give up on my sb-12 cab so changing the ohm tap of my head is out. I just want to be able to hook it up to the 2x15 as needed or wanted. So like i said before a custom 8 ohm cab from LDS. And making a head box/base wired through latchs is the cheaper and easeir way to go. I'll have over 800 just in the 2 Ampeg pf115he. My quote from LDS for a old school 2x15 at 8 ohms was alot less than 800.
Also thanks Vintage blue for clearing that up for me!
Yes I know but you were asking about using a 4Ω cabinet were you not? If you have 8Ω and 16Ω taps you need to artificially create a 4Ω tap. This can only be done with an external transformer. This can be a simple outotransformer or even an output transformer with the primary leads insulated.
BOT has answered your question about the 250Ω loading resistor.
