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Ampeg SB-12 1969 Modification Question

Discussion in 'Amps and Cabs [BG]' started by John Ferguson, Apr 21, 2018.


  1. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    An Ampeg SB-12 came my way with no cabinet, so I got one from the folks at flip tops. Turns out the amp is modified and I think it's overpowering the standard speaker. from the photos here, can anyone hazard a guess about the modifications? Two extra caps and a very large transformer on top - so tall the cage needs risers to fit over it. I'm hoping maybe this is/was a common mod you might know about. Would like to know how much power the amp is putting out. Thanks! IMG_6879.JPG IMG_8510.JPG
     
  2. Well, sorry to not answer your question Just from the pics though, it looks like the gemlins have been busy.
     
  3. Vintage-Blue

    Vintage-Blue Gold Supporting Member Commercial User

    Feb 13, 2008
    Cincinnati, OH
    Owner, Vintage Blue (repro cabinets)
    I'm sure more knowledgeable people will chime in but my first impression is that the amp had power supply issues at some point. The transformer that was replaced is the power transformer, and for some reason what looks to be a B15 transformer was used as the replacement, resulting in the added height. The original transformer would not have been potted like the one shown. My guess is that the capacitors are replacement filter caps for the power supply. Again, I'm sure somebody will come along with a more detailed, knowledgeable reply.
     
  4. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    Thank you for this. It looks like someone went to a lot of trouble to mount the larger transformer.
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    What was done to your amp is not a mod that I've seen before.

    To calculate the power output you will need to take measurements with a dummy load and volt meter connected to the output, a signal generator at the input. Or a piece of equipment that measures output power directly. This is something a tech can do for you.

    How much power is being delivered depends on the modifications that were performed and the output transformer. With a B-15 transformer, if the amp has a fixed bias circuit and if the amp was wired like a B-15, it could be putting out as much as 30 watts.
     
  6. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    @JimmyM Is this a mod you're familiar with?
     
  7. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    No sir. Sorry.
     
  8. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    Another question on this (5 months later...) currently the cage is not grounded due to the wooden risers it sits on. I'm guessing a grounded cage helps with noise suppression, but I don't see the cage represented on the schematic. I get lots of hum when I'm near the amp with my MIM Jazz bass that's been shielded inside. Any thoughts or good advice on this?
     
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The cage is not normally grounded. It only serves to protect the tubes.

    Does your amp sit on a tray? Some of these amps did, some didn't. The tray sould have an aluminum sheet on it, this serves as a shield over the bottom of the chassis.

    Your bass is grounded through the shield of your instrument cable. Check inside the chassis where the input jacks are in contact with the chassis. Look for any oxidization. This is common and can result in a bad ground. THis contact needs to be clean and tight.
     
  10. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    Thank you so much! I don't think there's a tray but I'll look with fresh eyes tomorrow. Would the tray be on the wooden flip top? Not sure what that would look like. I can add one though - I presume it's a grounded plate that covers the footprint of the amp chassis. I'll check out the input jacks as well.

    I'm looking forward to quieting this rig!
     
  11. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    There is a plate (tray?) under the amp. The two large mounting bolts go through holes on either end, however it's not obvious to me that the plate is grounded or connected to anything.

    I cleaned the input jacks mounting hardware with Deoxit5. There were star washers between the jack an the inside of the mounting plate, so it looks like that was done to assure continuity.
    IMG_9630.
     
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The aluminum sheet acts as a shield. It is in contact with the metal chassis. It doesn’t have to be grounded to be effective. The amp will be less prone to picking up noise when installed against the sheet. Many amps use this type of shielding.

    Those star washers really have a way of oxidizing. They also help lock the jack in place.
     
  13. John Ferguson

    John Ferguson Supporting Member

    Nov 19, 2017
    Oakland County
    Thanks again. Do you suggest I remove the star washers? I can see how they may just have more surface area to oxidize. I'm glad to know the shield is effective.
     
    beans-on-toast likes this.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The star washers are in every vintage Ampeg amp. I would leave them in. They help keep the jack tight against the chassis which is a good thing for the ground. These washers do oxidize but not that quickly that you have to be overly concerned.

    I normally carefully pull the jacks back a bit to clean the oxidization. The washers are available at home depot, I often replace the washers rather than clean them. In doing so, you have to be careful not to damage the wire going to the jack. They are brittle and can break.

    As for the hum issue when you get close, try not to get close then. With single coul pickups, The direction that you are facing can make a difference. Use an instument cable with a good shield. It doesn’t have to be expensive. One with Nutrik 1/4” connectors is good.

    Make sure that the jack nut on your bass is tight. Use your fingers to check it. During regular maintenance, deoxit can also be used on the instrument jack contacts and cable plugs.
     
    John Ferguson and JimmyM like this.

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