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Sixties model Portaflex B-15N

Discussion in 'Amps and Cabs [BG]' started by kd5uwl, Aug 24, 2013.

  1. kd5uwl


    Jun 7, 2012
    Bass friends,

    I found my dad's old guitar player who has the bass amp I grew up hearing. I started playing bass about three years ago after picking up dad's old bass. He died in 2001 and this amp has been in the corner of a room at his guitar player's house since the last time they practiced together, probably 25 years ago.

    I haven't seen it in probably 35 years and I am going to pick it up tomorrow, a sixties model B-15N .. not sure exactly what year yet until I see it again so I don't know which exact model of B-15N it is.

    My question: I'd like to take my bass and play with this guy while I am there. I doubt I'll have room to bring my own bass amp into his house. Would I be ok to turn on the old Ampeg and play it? Or should I never turn it on until I recap it?

    Second question: I'm only assuming I'll find some electrolytics in there .. anyone have a schematic?

    Thanks in advance!
  2. If it has been sitting there unused, DO NOT POWER IT UP. Yes there are old electrolytics in there that could very well short and possibly cause expensive damage. They can be powered up by someone with the experience and knowledge by using a variac or current limiters. Can't remember now if a schematic is attached inside or outside the bottom cover but a google search will locate one if missing.
  3. basspipe


    Dec 19, 2004
    That's awesome. Everything you need is online. Just Google it. There are websites dedicated to vintage Ampeg fanatics and even a website that specializes in replacement parts. The owner of that site is very knowledgable and once assisted me in making my '67 B-15N sound as good as new. I would recommend that you find yourself a tube amp tech in your area. Chances are something will fail at some point, but don't let that discourage you and do not make the mistake of selling it.
  4. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Schematics for B-15's aren't hard to find, and usually the schematic is on the chassis tray under the head, but there were several different early models so you have to determine what you have first. A Google search should provide you with some schematics should yours be missing.
  5. kd5uwl


    Jun 7, 2012
    Thanks everyone. Looking at the schematics for the various versions in that era (thanks basspipe) it looks like there are only a very few electrolytics.

    I successfully re-capped my 1982 Peavy 400 bass head (was also my dad's) which now rivals my 2012 H-SVT, in some ways.

    I _do_ know about the high voltage in tube amps ... I am a ham and have all-tube linear amps for that. So I will be careful.

    One question - and you don't have to answer if I'm boring you with all this stuff:

    I remember dad talking about feeling voltage to his hands through his bass sometimes .. seems like he flipped the on/off switch to the opposite polarity to fix it. But I was only a child then, not sure I remember clearly.

    Anyone know about that issue with this amp?

    I see the schematic only shows two blades on the power plug ...

    Again, sorry if I'm wearing you out with this. As you may be able to tell I'm quite excited about finding this old amp and getting it back. It's my memories of the way this thing sounded in our house that made me pick up dad's old bass and learn to play long after he was gone.
  6. WretchedExcess


    Jul 29, 2013
    sounds like the first thing you should do is to replace the 2-prong cord on the amp with a grounded plug for safety.
  7. kd5uwl


    Jun 7, 2012
    Ummm, well it sounds good to me, WretchedExpress. Has anyone here done that? How did you do it?
  8. kd5uwl


    Jun 7, 2012
    WretchedExcess, I meant .. sorry.
  9. basspipe


    Dec 19, 2004
    Yes, updating the plug is a necessity and most technicians are obligated to recommend if not out right require it before going further with any other work. It was the very first thing I had done with mine. Replacements can be found at the website provided above.
  10. kd5uwl


    Jun 7, 2012
    Thanks for all your help!

    Got the amp .. serial number 052557 .. 1967? Been stored in an air conditioned room with a cover over it since the last time it was used which was 1997.

    Any suggestions for cleaning the exterior? Can I use something like 409?

  11. Mild soap and water, dry and use like an automotive vinyl cleaner/protector that detailers use for dashes/seat vinyl.
  12. kd5uwl


    Jun 7, 2012
    Thanks, B-string. Thanks to everyone for the help and pointers to Ampeg restoration websites.

    This one is in great shape without all the tears I've seen on many of the ones posted on the web. The black covering is still shiny. It was still working and sounding great the last time dad turned it off.

    I'm going to install a grounded power cord and cap kit. After that, I truly believe I can adjust the tube bias myself ...simply a mater of adjusting the voltage at the "B" points on the schematic, if I'm not mistaken.

    If I can't find the recommended bias voltage at that point for 6L6GC tubes I may ask you all here.

    Otherwise, I'll post again with final pictures once I get it going and am playing it!

    Thanks again for all your help.

  13. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    I think there are several iterations of B-15 schematic floating around TB in the Portaflex thread. I believe several included the proper voltages when run off of a 110V line. I'd expect the voltages you see in the amp to be a little higher due to line voltages being a bit higher in most places today.
  14. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    This is the redrawn version of the schematic. I know there are some original versions floating around that have voltages and such on them, but this one lacks them and the old unofficial ampeg page is down (over bandwidth). I'm pretty sure I have a few other schematics on my computer at home, if I find one with voltages I'll post it up. I'm sure someone else has them though (David or Mark most likely).
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Mark has a nice set available on his vintage-blue web site.

    Here are a couple more. Both are what they call NF revisions but are very different. The first one is the later revision with the tone switches.



  16. kd5uwl


    Jun 7, 2012
    Thanks to everyone for the help.

    Was able to get inside the amp this weekend and here is the schematic for this one. Very similar to the second one you posted, beans-on-toast, but I note this version is dated 10/66.
    Two questions:

    - When updating to 3-prong cord, disconnect the "Ground Switch"? That capacitor on the Ground Switch to ground will be out of the circuit too, correct?

    - Any recommended links on how to properly bias this fixed bias amp you could share? I'm assuming its adjusting one or two of the fixed resistors. I know to shut down and bleed off any DC voltages before making changes like that, but if you guys have a good link on this for the B-15NF I'd sure appreciate it.

    I'm installing the cap kit and 3-prong cord before doing any re-biasing, of course.

    Thanks in advance,

    Attached Files:

  17. I'm going to offer the somewhat heretical viewpoint on biasing: replace the bias caps and leave the bias set at exactly the voltage the factory designed it for.

    Yes, I know, many folks insist on putting a potentiometer in there and setting bias to some magical number, but there's a million opinions on what that number should be or how to measure it. The factory bias worked quite well so why mess with it? :bag:

    Yes you will remove that cap to ground, that's where the "tingle" of line voltage was coming through.
  18. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Agreed with Bill. I always just throw power tubes into mine without regard for bias and it works fine every time.
  19. kd5uwl


    Jun 7, 2012
    Thanks, nashvillebill and jimmyM .. that's actually great news. Sounds fine to me! Caps are on the way.

    Thanks so much for your help.