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Modding an Ampeg B328...

Discussion in 'Amps and Cabs [BG]' started by christopherogut, Dec 26, 2010.

  1. Hello All,

    So, I recently picked up an Ampeg B328 combo amp (150W RMS B3 head, 2x8" speakers and a tweeter rated for 150W RMS also) for dirt cheap... It's in absolutely beautiful condition except for the fact that the output transistors are literally blown apart. The cabinet itself is ported and seems to be built like a brick sh!thouse, so I'd like to put as much of it as possible to good use. The way I see it, I have several options:

    1. order some replacement Darlington transistors (2x TIP142/TIP147) and repair the head. This (with any luck) gives me a nice 150W Ampeg combo for cheap.

    2. swap out the B3 head for another Ampeg head, like a 200T/350H/450H... there's no room for tubes/transformers so it needs to be a solid-state head. Problem: any of these heads pumps out more juice than the two 8" speakers can handle, plus they're at 4ohm total load... Does it make sense to put some beefier Alpha or Beta 8A's in there? I could make the cabinet 8ohm with two new speakers and then have the option of stacking this on another Ampeg cab for a cool little rig.

    3. Swap out the B3 head for something totally different... Trace Elliot SMX head, Gibson GB440, etc... swap out the speakers, too, and end up with an interesting franken-combo.

    4. sell it, part it out, etc.

    Any thoughts or advice?

    - Chris
  2. I wouldn't replace the speakers. You may end up with a worse sounding cab and be out the cost.

    I also think you should figure out why the transistors blew up before replacing them.

    Replacing with a different amp makes more sense, although you'd want to find it dirt cheap to keep this thing a bargain. Sometimes these "amazing deals" can end up costing more than buying a new combo.

    If it were me, I would just use it as a practice cab with a separate head for a while. Keep an eye open for cheap parts, or maybe find this same combo except with the speakers blown!

    Whatever you decide, have fun with it!
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    if you replace transistors, (better done by a tech) you also need to replace what blew them, and the otehr things that got blown at same time.........

    Good news is that those TIP parts are cheap. (low cost ;) )
  4. KramerBassFan


    Jan 3, 2009

    More often than not, there are other problems that caused the transistors to blow. Now a days, its just cheaper to get a new amp than get it fixed often.
  5. The guy I got the combo from said that his son was trying to "hook it up" when he wasn't around and blew the thing. When I went to go look at it, there was a wimpy 1/4" cable from head to cabinet and a cable labeled 'Speaker' plugged in to the input! My guess is that someone ran it either unplugged or with a dead-shorted cable or something. Such a shame. The head itself (front panel, sliders, knobs) is dead mint.

    I took a good look at the board and found that two of the TIP142's were blown to the point that there were chunks of the plastic casing loose inside the head. Two normal transistors looked a bit scorched and would need to be replaced also. A bit of inspection with a multimeter showed that all of the resistors and diodes around this area of the board are still fine. I think I might get 5 or 10 of each transistor and have a go at it.
  6. Leitchy


    Dec 26, 2010

    Todays throw away society enforces this quite a lot. But if you have the soldering skills, equipment and experience then give it a shot. But yer these components were most likely blown from something else in the board. If you have a multimetre AND KNOW ABOUT THE DANGERS OF WORKING WITH HIGH VOLTAGE, then I would be checking the voltage levels that are going into transistors themselves, the output of the voltage regulator IC's and possibly any other components that are ascociated with the darlington. Your other problem is going to be finding a schematic, which would help this process a lot.

    The guy that sold it to you may have tried using an extension cabinet, which in that case could have done damage. Have fun:hyper:
  7. Interestingly enough, there is a standalone Ampeg B3 head on Long Island craigslist right now in decent shape for $150. If I could get it for less, I'd swap the power section into my head and have a really nice combo right away. Then I could get the power transistors replaced on my blown board, swap it into the other head (which looks to be in decent condition) and keep or sell it. Decisions, decisions...
  8. I picked up a b3158 a month or so back. It had the same blown transitors. I bought replacements and some other parts from digikey. I have been messing with it in my spare time but as stated above something else is probably wrong. Just replacing the tip 142 and tip 147 transitors didn't fix mine. I have the B3 schematic that ampeg sent me. Pm me if you want it emailed to ya or if ya have any questions about the power amp layout.
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I also have the schematics if that is wanted..... PDF.

    I'd be pretty surprised if the transistors were blown apart, and there was no damage to the 0.33 ohms, or to Q3, Q4, and related parts. I guess it can happen, but.......... Usually the protection is toasted when a really catastrophic failure occurs.
  10. Everyone, thanks for the info... I figured it wasn't going to be a super-easy fix, but like it's been pointed out, if I need to spend time troubleshooting and replacing a dozen or more components (beyond just the power transistors) to get the head working again I guess it really isn't worth it. To me at least.

    I'd really like to toss in two 4ohm Eminence Beta-8A's (225W RMS each) in series and a SVT-450H head. 275W combo with the ability to stack it on another 8ohm Ampeg cabinet. I guess I should work out the volume of the cabinet and see if it's appropriate for those speakers.

    Jerrold, I see in your signature that you used to work for Ampeg... might you know any specs about these stock speakers? They're marked SLM Electronics, 86-155-08. A bit of snooping online found that they're stock drivers in the SWR Henry the 8x8.
  11. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Per repairs, the total cost should be low, there simply are no expensive parts in there. If your time isn't worth it, well...... ok.

    As for speakers, if anyone else uses them, that's basically a coincidence...... we'd ask Eminence for a design to do "X", and they'd provide it. if they provided it to someone else, also, well, neither of us would probably ever know that the other was using an identical speaker.

    And, they might LOOK the same without BEING the same, actually. There is more to the speaker than the looks and size.

    Specs.... might have, might not. That's also not really "service" info, and I'll think about whether that level of somewhat proprietary detail is appropriate to give out. Ampeg is still a going brand, after all. If they were totally defunct, I'd have no concern. Schematics etc is no issue, that was handed out freely.
  12. Leitchy


    Dec 26, 2010
    I don't mean to hijack, but Jerrold do you happen to have the schematics to a ampeg SVTII-P. I had one sent to me from a random guy, but it is so small that it is unusable.

  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I may, it will probably NOT be PDF, though. I'll look.

    if you don't hear back, PM me.
  14. dmpfund


    Jan 8, 2011

    I am repairing an Ampeg B3 head, but about half of the output stage is missing from the schematic I have and I am having a horrible time finding the issue without a complete schematic. Does anyone have one or know where to find one??

    I appreciate the help
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    yes, PM an email to me
  16. ingbojorquez


    Jun 29, 2011
    hi guys
    I have a B3158 that blow up the output transistors and I need the schematic, you can help me with that?

    replacing blown transistors but after 5 minutes the ruined again :S
  17. jongor

    jongor Supporting Member

    Jan 11, 2003
    Can someone forward the schematic to me please?
  18. BassFiddle63


    Oct 4, 2002
    Anybody know where I can get a control knob for a B-328 Combo? Thanks!
  19. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Why is it tha folk start a thread, such as this one, and not follow through to the resolution??? We just get left in mid air wondering if that amp was repaired, rebuilt or the whole idea was scrapped. Come on guys! fill us in!!

    It's like watching a show you're really in to only to get the dreaded "To be continued" at the end! :)