Official Ampeg Portaflex Club

Discussion in 'Amps and Cabs [BG]' started by JimmyM, May 25, 2008.

  1. sjrash


    Nov 30, 2007
    Tampa, FL
    I bought a 1973 B-15N in March. After looking inside the chassis, I found a lot of original parts inside. Knowing it was in need of a good look over and most likely a few parts replaced, I decided to learn how to service the amp myself. I have a little experience with electronics, but I received a lot advice from beans-on-toast. It is evident how willing he is to help fellow TBer's with technical advice.

    I added an adjustable bias circuit to my amp. The circuit board is mounted by removing the 47k bias resistor (R34) from the original board, then soldering two lengths of buss wire where the bias resistor was. The buss wire is soldered to the new bias circuit, and the other side is supported by two 4-40 nylon screws that go into two nylon standoffs. The standoffs are siliconed to the original board. This is a completely reversible mod.

    The parts I used on the board are:

    (2) 75k ohms 1/4 watt metal film resistors
    (1) 20k ohm 5% wire wound Bourns Trimmer resistor

    The two 75k ohm resistors in parallel result in 37.5k ohms. With the 20k ohm trimmer, the adjustability is +/-10k ohms from the original 47k ohms.

    The boards are available. 3 boards for under $6 shipped.

    Link to order boards: OSH Park ~ B-15N bias pot V1.2
    SherpaKahn, JimmyM and beans-on-toast like this.
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The fixed bias circuit board fits in there nicely.

    To provide some background, the bias circuit from the 70's B-15N is shown below. It consists of R36, D1, R34, C15, R42, and C19. The high voltage is tapped off the power transformer, the voltage is dropped, rectified, and filtered, converting it to a DC voltage. R34 sets the level. Normally, this resistor is swapped out to change the bias. Re-soldering is hard on the circuit board pads. Adding the adjustable pot makes life a little easier when adjusting the bias. It is also easier to fine tune the value with a pot than it is with a fixed resistor.

    The original bias circuit is shown below. The idea is to replace R34 with the adjustable bias circuit which consists of a multi-turn pot (R1) and one of two resistors (R2 and R3). The hard part is finding a way to mount the board on the Ampeg PCB so it is both stable and reversible. The board has a cutout to allow for air to circulate around R36, the big resistor in the image.

    Fixed Bias Circuit:
    B15 bias schematic.jpg

    Adjustable bias circuit that replaces R34:

    B15 bias pot circuit.JPG
    Last edited: Jul 26, 2015
    SherpaKahn and sjrash like this.
  3. Attn. Portaflex geeks! There is a 2x15" Vintage Portaflex cabinet on CL in Portland, Or. It's in rough condition but I thought I'd let everyone know seeing as how rare they are. I am in no way affiliated with the seller. Just a long time B-15 owner and player.
    sjrash and beans-on-toast like this.
  4. To be clear, on my cutout solution, I made it simply to allow the amp feet to clear the handle. I have never had an amp move around on the cab. I also loop my guitar cable under the amp from the input Jack and around the left hand corner of the back of the cab.
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Am I missing something Kirk?
    Last edited: Jul 30, 2015
  6. Bass 45

    Bass 45

    Jun 23, 2011
    Tempe, Arizona
    It looks like he thought he was replying in this thread:
    An approach to putting Amps on Cabs
    Discussion in 'Amps [BG]' started by bucephylus, Friday at 11:26 AM.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    An advantage of tube amps is that they tend to stay put. Micro amps don't take a lot of persuasion to take a dive off a cabinet.

    Vibration isolation is something else.
  8. I don't think you miss a thing Beans!
    JimmyM likes this.
  9. southpaw723


    Oct 20, 2013
    Finally got a matching cab!

    Attached Files:

  10. Holy crapola batman posting in multiple threads at once is ill advised late in the evening! Thanks bass45.............Thank God I wasn't drinking who knows what might have happened! Sorry guys ........slinks off quietly.......
    JimmyM and Bass 45 like this.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    How does it sound? Nothing like a wall of sound behind you. It really makes a difference.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Dang computers. You get a pass when you're over 50. :laugh:
    JimmyM likes this.
  13. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Just see that it never happens again.


    Sheesh, if I had a dollar for every time I did that, I could retire!
  14. LowEndOperative

    LowEndOperative ✓ᵛᵉʳᶦᶠᶦᵉᵈ Supporting Member

    Oct 8, 2013
    Yeah, but they don't look like matching drivers! (heh heh)
  15. southpaw723


    Oct 20, 2013
    I love it, real full sound. I think the pf500 sounds better with two 210's rather than one.
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Yes, more speakers are always better. But having the sound elevated so that you can hear it better makes a big difference as well.
  17. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Finally got (almost) everything settled in my new work space so I decided to finally get around to updating the PEC modules in my 65 B15. At one point shortly after I got this amp I had a friend checking it out, and his comment was that it was the darkest sounding B15 he had played. I didn't think much of it at the time, but having played a few of different eras in the intervening time I started to realize he might have been on to something. Having ordered a set of David's PCB replacements I figured I'd give it a shot. I had intended to start on channel 1, but when I opened the amp and actually looked at the pots I noticed this:


    I'm not sure how I didn't notice this before, I replaced the volume pot on channel 2 a while back. Obviously it's been like that for a while, the missing section was corrected in the circuit. As I disassembled the circuit a few of the connections broke free from the module with just some slight movement of the leads. Definitely time to go.

    Anyways, here we are at the outset:


    I also replaced the 120K resistor between the pots (it was reading about 127K in circuit) but the 270K read in proper range so I left it.

    Being limited on time for the night I opted to hold off on doing channel 1, so I zipped the amp back up and tested it out.

    Grabbed my bass VI, and fired it up. Rolled up the volume and nothing. Odd. Swapped to channel 1. Still nothing. Cab was connected, I could hear a slight sound as I flipped the standby so sound was passing.

    Pickups were off. Perfect.

    Definitely a bit more brightness now. The inputs are different, so it's not a perfect comparison, but channel 1 still seems a little dark/dull compared to the repaired channel 2. Definitely going to take care of that one tomorrow.

    Thanks again David designing the PCB replacement, works out great!
  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008

    Thanks Corey. Something took a bite out of your PEC module.

    The thing that I like about it is the low cost. Of course, there is the possibility of trying different components to see how it affects the sound. Maybe even changing the values of the components. I was just thinking that it would be neat to try making the module pluggable. Tone modules with different componemts could be plugged in to change the tone circuit. Might be interesting for recording.

    The 120k resistor the you changed sets the center frequency. Some people replace it with a pot to provide a mid shift. It isn't a dramatic shift but it does work a bit. The duncan tone simulator shows you what varying the resistor will do.

    If anyone is interested, here is an analysis of the James circuit and how to go about modifying it. You don't have to understand the equations to see how changing a component will affect it and shift the treble and bass break points.

    Analysis of the James Tone Stack
    Modifying the James Tone Stack
  19. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Ended up installing the second one last night. I did some hot glue around the back and front solder pads just to be sure the contacts were insulted, but I think I might grab some silicone to add in there and hold them in place a little better. As is they're basically held in place by the connecting wires. It's stable enough for now, but I want to figure out something more secure.

    I've seen the james stack calculators before but thanks for the link. I'm working on toying around with my traynor YVM-1 which uses a james stack but very different component values so something like that would be good for plotting out changes.

    Anyhow, back to the portaflexes...

  20. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008

    Looking at your boards, I was thinking, if you mounted the components on the other side, the board will sit parallel to the pots. You could add a dab of goop the back of the board to hold it against a pot. Sorry for the late advice.

    I use nylon hex standoffs to hold the board away from anything that it shouldn't touch. I hold down the nylon standoff with silicone goop.

    Nylon hex standoff: Keystone 1902B
    Nylon screw: Multicorp SPC13374 4-40
    Or just the screw and a nut to hold it in place: Multicorp SPC21811
    The nylon parts can be sanded down to the needed length.
    Last edited: Aug 1, 2015
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

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