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70’s B-15 external speaker jack wiring

Discussion in 'Amps and Cabs [BG]' started by sandersd, Apr 28, 2018.

  1. sandersd


    Apr 16, 2018
    Greetings to all. I have a mid 70’s B-15 that I purchased years ago for $300 in Austin Tx. I was told the techs checked it out and all was fine. I brought it home, used it a few times and then it sat unused for about ten years. When I decided to use it again it had issues so I replaced the cap can (kit from Fliptops), put in a new power cable, removed the death cap and cleaned it up. Then it sat for a few more years till recently when it quit working all together.

    I took it to a local tech at an amp company who traced the issue to a bad input jack. He replaced it and it works better than it ever has so I’m thankful to him for that. He also said the external speaker jack was wired oddly and he didn’t understand it so he rewired it in “the standard fashion”.

    Now the external speaker jack has no signal when I hook up the killer Ampeg 4x10 that I picked up off Craigslist today for $300. I opened up the chassis and see that he did indeed rewire the jack but incorrectly, so now I want to fix it. The big issue as I see it is he connected the 8ohm tap and the main speaker out to the tip of the ext speaker jack and the 16 ohm tap he wired to R41, the 250 ohm to ground.

    Someone please verify and correct me if I’m wrong but shouldn’t the lead from the main speaker cable go to the tip, the yellow 8ohm from the OT go to the shunt, and the 16ohm tap go to the sleeve with a jumper to R41?
    Last edited: Apr 28, 2018
  2. sandersd


    Apr 16, 2018
    D2786F74-DE59-4B7B-BA99-303FB9F93FC7. Here is a pic I took before he rewired it. I can’t really see the jack wiring because the tip is in the way.
  3. sandersd


    Apr 16, 2018
    A01A0DCA-00E3-4D61-919B-EE06CE13A3E0. Schematic for quick reference:
  4. sandersd


    Apr 16, 2018
    5E9EE8FF-A8E7-4049-B873-7160C8AE10F3. Just as a really cool aside, this amp used to belong to Roscoe Beck...
  5. The External speaker jack must be insulated from the chassis. What in essence happens is that the external speaker is in series with the internal one. The tap on the OPT is switched from the 8Ω tap to the 16Ω tap for correct matching. The amp expects to see an 8Ω external load.

    If your ’tech’ has wired the two sockets in parallel the wiring is completely wrong.
    bobyoung53 and coreyfyfe like this.
  6. Without being able to see where the wires go in the picture of the Jack it's kind of hard to tell what's happening there.
    As far as the schematic goes, it appears that without an ext speaker plugged in the switch on the Jack connects the 8 Ohm tap of the transformer to the internal speaker. Then when you plug an 8 Ohm external speaker into the ext. Jack, the switch on the Jack opens, disconnecting the yellow, 8 Ohm tap wire from the internal speaker. Then the two speakers are in series (16 Ohms) and they are connected to the higher impedance (16 Ohm) tap on the transformer.

    One side of the internal speaker should be grounded.
    The other side should have a (green?) wire going to the ext Jack switch contact that moves when you insert a plug.
    The yellow wire should be on the other side of the ext. Jack switch so that yellow and green are connected by the Jack switch with no external speaker. The sleeve connection on the external Jack should be the other green wire going back to the transformer. The external Jack looks like it should be mounted so it is isolated from the chassis, otherwise the 16 Ohm tap is shorted to ground. That wouldn't be good.

    It looks like he has the yellow and green soldered to the same lug on the ext Jack. I think they need to be on different lugs so they are on either side of the Jack's switch. It might be hard wired at the Jack to operate only in 8 Ohm mode.
  7. sandersd


    Apr 16, 2018
    Yes, I forgot to mention it also looks like one side of the insulation washer for the jack is missing so I need to replace that. Thanks for reminding me. I studied the wiki page that explains how the circuit functions and I understand why it is designed the way it is and how it works, I just need to be sure I’m applying that knowledge correctly when wiring the physical components.

    The lead from the main speaker cable appears in the schematic to go to the tip, the yellow 8ohm from the OT go to the shunt, and the green 16ohm tap go to the sleeve with a jumper to R41. That seems correct since with no plug the tip (speaker) and the shunt (8 ohm tap) are connected. With a plug inserted the shunt is disconnected and the sleeve (16 ohm tap) is connected to the external speaker which returns and is connected to the tip and continues to the main speaker.

    I’d love to find a clear pic or the wiring in an unaltered amp to verify.
  8. sandersd


    Apr 16, 2018
    I was thinking about this last night and it occurred to me that the positive lead (tip) is usually connected to the + terminal on the speaker and the negative lead (sleeve) is connected to the - speaker terminal.

    The schematic shows the 16 ohm output connected to the sleeve, going through the speaker and then to the tip where it continues to the main speaker. If I plug into an extension cab wired in typical fashion as described above wouldn’t the speakers be out of phase with the main speaker?
  9. Yes.
  10. sandersd


    Apr 16, 2018
    Why would Ampeg design a circuit that a standard extension cab could not be plugged in and function with the speakers in phase. Are extension cabinets typically wired in reverse?

    Perhaps I’m missing something.
  11. sandersd


    Apr 16, 2018
    I just checked the B-15 main cab and what Ampeg did (or someone) was to wire the main speaker backwards so when a normally wired extension cab was plugged in the speakers are in phase

    Sneaky, sneaky, sneaky...
    Old Garage-Bander likes this.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You aren't missing anything.

    Ampeg did some strance things with their wiring. The B18 was worse, it came with either an 8 or a 16 ohms speaker.

    Just make sure that you perform a battery test to ensure that both speakers are working in phase.
    Old Garage-Bander likes this.
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Attached are two images of a 73 with original speaker wiring. Hope that it is clear enough for you to make it out.

    Follow the speaker jack wiring color codes as shown in the wiki. The confusing part is that terminals can be used as tie points. In some models you'll see the feedback resistor R40 going to the jack terminal for instance. The layout is not always exactly the same as the schematic but it is equivalent.

    R41 should be wired between the 16 ohm tap and ground.

    If the phase of the extension cab is out, you can reverse it at the speaker terminals or at the four-pin cabinet connector. Changing the speaker wiring is the easiest.

    As was mentioned, it's important that the EXT AMP jack is isolated from the chassis. Otherwise there will be a ground loop hum. I would replace both, not just the one missing. The Switchcraft part numbers are S1028 and S1029. I usually buy them from mouser. I recall seeing them at Antique Electronics (tubesandmore). With the rewiring, this could be why the jack is not working. You can try removing the jack from the chassis and then plug in the cab and see.

    Your input jack issue could have been oxitation on the inside of the chassis. Check this carefully, the contact has to be clean. The lock washer should be clean. Deoxit can help remove any oxidation. You have to unscrew the nut and push the jack out to properly inspect the area.

    B15 73 1.

    B-15N 73 2.
    Old Garage-Bander likes this.
  14. sandersd


    Apr 16, 2018

    This is what I did, I think it’s right although as we observed in the pics the actual connections can’t be seen. Speaker goes to tip, 8 ohm to shunt, 16 ohm to sleeve with a jumper to a connection between R40 and R41 (41 is the big grey one in pic).

    I ordered the washers earlier today from Digi-key. They had both in stock and ready to ship.

    I’ve been all through the wiki page btw and it’s a fantastic resource. I sent a link to it to the tech when he was working on the amp although it didn’t seem to do him any good, he still messed it up. Kudos to all who contributed to it.

    The attached scan is something I did when I was considering building a clone. I wanted to understand what was happening so I traced the circuit out on the board. In the end I decided not to pursue it so I didn’t complete the entire circuit through the jacks, pots, and tone circuits but maybe it’ll be helpful to someone.

    Attached Files:

  15. sandersd


    Apr 16, 2018

    The replaced jack can be seen in this pic. Something else I find odd is the ground wire from the channel 2 (normal) jack. The jacks are grounded through the chassis so why is it even there? If it’s needed or useful why doesn’t channel 1 have one? (There wasn’t one on ch 1 before the jack was replaced)
  16. Could it be that the one ground wire provides the ground connection to the panel via the Jack? And then the other jacks are grounded via the panel?
  17. sandersd


    Apr 16, 2018
    The chassis is already grounded. The wire is redundant.
  18. Ah! OK.
    The other way around then?
    Providing ground to the circuit board?
  19. sandersd


    Apr 16, 2018
    If you’re seriously trying to help then fine, I appreciate it and all the others who responded. It can be difficult sometimes to tell via posts if someone is only responding to be a smart ass. If so please don’t reply to this thread as such comments are not appreciated. If you are genuinely trying to be helpful please accept my sincere apology for suggesting otherwise.
  20. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    Must be new around here, eh? (Smart ASS response follows ;)). Perhaps the ground wire was added as many times in old amps (yours for example) chassis grounded jacks start to lose good connections resulting in poor ground connections.

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