1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Ampeg B18 speaker ohm question

Discussion in 'Amps and Cabs [BG]' started by harryh, Jan 2, 2014.

  1. harryh


    Aug 20, 2011
    Hi guys, I'm fixing up a B18 for my best friend. I noticed that the speaker was re-coned. I checked the ohm rating and it showed that it was an 8 ohm speaker.

    I kinda thought it should be an 16 ohm speaker. Hmm, what do you guys think?

  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It should be 8 ohms. If you plug in two cabs, they are connected in series and switched to the 16 Ω tap on the output transformer. Maybe that's where the 16 Ω confusion is coming from.

    There are some models that show a 16 ohms speaker. In that case it should be 16 ohms. When you plug in an external 16 Ω cab, the two speakers are connected in parallel and connected internally to the 8 Ω tap. Again, maybe that's where the 16 ohm confusion is coming from. :p

    So there are two different ways that these amps are wired. Take an ohm meter and measure the resistance of the speaker in the main cabinet. The speaker needs to be disconnected from the amp when you do this. This will allow you to verify whether the speaker is an 8 or 16 ohm.

    Then check which wiring there is in the amp. It is different depending on which type of speaker originally came with the amp. If it came with a 16 ohm speaker and was wired for one and someone installed an 8 ohm speaker without changing anything, things are going to get confusing.

    If your B-18 sounds strange, check the speaker and the wiring.
  3. Vintage-Blue

    Vintage-Blue Gold Supporting Member Commercial User

    Feb 13, 2008
    Cincinnati, OH
    Owner, Vintage Blue (repro cabinets)
    David - I've always found this to be a point of confusion when it comes to the B18N. I always believed the speaker output of the B18N was wired just like the B15N: 8 ohm internal speaker setup so that when an 8 ohm extension speaker is plugged in the speakers are put in series and switched to the 16 ohm OT tap (as you mentioned). Then I had some emails a couple of years ago from a customer that discovered some inconsistencies with a couple of B18N heads he was having serviced. I think he even mentioned the output being labeled differently on one of the heads. I'm trying to find the email to see if I'm mis-remembering.

    On this schematic dated 1-66 they have actually labeled the speaker as 16 ohm.


    On this 10-66 schematic the speaker impedance has been removed.


    I don't know what any of this means; I just remember it raising questions in my mind.
  4. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Mark, on the schematic where it shows the 16 ohm speaker it still lists 8 ohm as the main tap on the OT, 16 ohms as the tap connecting to the extension speaker. I wonder if the 16 ohm on the speaker could have been a misprint? The later schematic still has the transformer impedances but no speaker impedance.
  5. Vintage-Blue

    Vintage-Blue Gold Supporting Member Commercial User

    Feb 13, 2008
    Cincinnati, OH
    Owner, Vintage Blue (repro cabinets)
    Corey, did you notice how the external speaker jack is drawn on the two schematics I posted? I've never been good at interpreting switching jacks on a schematic but they are drawn differently than they were on the B15N schematics. Here is an early B18N schematic (SSR) that has the jack drawn like the B15N, and has the speaker labeled as 8 ohm. The PT also appears to be the one used in the B15N, PT-108.


    Here is part of an email I received while my customer was having his B18s serviced:

    "Did you notice that the schematic has the output to the speaker (and Ext. speaker) listed as 8-ohms ? The factory-original Fane is 16-ohms, and the schematic shows both the 8 & 16-ohm taps of the OT. Some other early B18's got Goodman speakers, also English-made and 16-ohms. The later 18" Cleveland speakers are 8-ohms.

    Also the PT is listed as a PT 108, and I thought that model was for a B15 ?? The B18 PT & OT are potted in taller cans!

    I'll send you some pics of the chart in my other '64 B18N as soon as I can get to it. Jess wrote on that one after reconing the original Fane speaker to 8-ohms."

    As I said previously, I don't really know what any of this means but I figure the Portaflex owners on this forum will come up with some useful info!
  6. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    I did notice but the diagram wasn't really clear to me either. They are different than the B15N as you said, and if I'm tracing it right it could be a parallel connection. I could be reading it wrong, the diagram is odd. The only B18 schematic I have around is the same one that is posted in the wiki and shows the standard series connections.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Another Ampeg mystery!

    I edited my post above to try to clarify things. Mark, in your first schematic with the 16 Ω speaker, the speaker cognacs to the 16 Ω tap on the output transformer. If you plug in an extension cabinet it should also have a 16 Ω speaker installed for the impedance to be matched. In this amp, the extension cabinet is connected IN PARALLEL to the 16 Ω speaker in the main cabinet. The two speakers are connected to the 8 Ω tap on the amp.

    So if the amp came with a 16 Ω speaker, you would use it with a 16 Ω extension cabinet and the amp would be wired to work with one or two 16 Ω cabinets.

    The earliest B-18N amps came with an 8Ω 18" speaker. The amp was wired like a B-15N where the external speaker jack was wired to connect two 8 Ω speakers in series and switched to the 16 Ω tap on the output transformer. Very different from the wiring of the amp that came with a 16 Ω speaker.

    Both versions used the same output transformer. It was just wired differently.

    Why Ampeg did this I don't know. At one time they had an agreement with a British company. Ampeg sold Burns instruments in North America, on the other side of the pond, their partner sold Ampeg products. Part of that deal might have been to use a 16 Ω British speaker. The speakers were probably installed over there and they gained a tax advantage. They did this in Canada with Jensen speakers in 1963.

    All the B-18N's that I've seen had an 8 Ω speaker installed. They also had 8 Ω ext speaker silk screened on the back of the chassis. I've never seen one with 16 ohms silk screened on the back of a B-18N chassis. Doesn't mean that this wasn't done.

    So if your B-18N does not sound right check the speaker and the amp's speaker out wiring. If you have an 8 Ω speaker connected to the 16 Ω tap there would be an impedance mismatch. If you connected an 8 Ω extension cab, the two speakers would be connected in parallel, 4 ohms, to the 8 Ω tap. Again a mismatch.

    Kind of a confusing mess, isn't it?
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    They took the B-15NA schematic and modified it to do the B-18N schematic. That amp used a PT-109, later they used a PT-12XA. The PT-108 must be an error on the schematic.
  9. MR PC

    MR PC

    Dec 1, 2007
    My old B 18 came stock with a 16 ohm speaker. That amp won't sound right with anything else.
  10. My 1966 B-18, still with the original Cleveland speaker, is clearly marked as 8 ohm.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Wasn't the B-18N only offered between 1963 and 1967? Makes me wonder how many were 16 ohms and how many were 8 ohms. The Cleveland seems to be very popular from what I've seen.
  12. harryh


    Aug 20, 2011
    Wowsers, lots of info! Thanks so much. I think we'll switch over to the 8 ohm tap and move the feedback connection, too.

    The speaker is a big Cast Frame, but I don't know the brand. Did Cleveland make Cast Frame speakers? The speaker cone says WHF and I think that comes from the Waldom Company, yes? I'll post a pic in the next few days.

    Mil Gracias, harryh
  13. xoir


    Oct 25, 2007
    This becomes even more confusing for me, over the years I've seen portaflex users talking about heat in their amps, I've faced the same issue with pretty much all the amps I've had without been serviced properly, except for my B18n, I've used hard and loud for hours some times, and I've never experienced nothing inusual, I mean, no heat, not distortion, no popping, etc. And I think this is the only amp I have with original caps, which makes me wonder is my amp working without been stressed? (underworking) is miss pair with the speaker?, or is just working good? About a month ago had an issue with the primary speaker output that was fixed with only cleaning, besides that, the amps works good, but it works dang good when I use both outputs I hear a lot improvement in the cleveland.

    My confusion now is... I'm looking in to the original diagram that came with the amp, and more strange is.. WTH is this resistor doing in my cabinet hehehe

    I'll go buy some cheap multimeter tomorrow and figure it out this mystery!!



    The cab came with the green/back cable and the resistor but I change it at some point.

  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Best to post a pic of the speaker. If you have an ohm meter, disconnect the speaker and measure the resistance. It is important to know if it is 16 or 8 ohms.

    The feedback resistor is normally connected to the highest impedance tap on the output transformer. In this case, the 16 ohm tap. If the speaker is connected to the 8 ohm tap, you didn't need to move the feedback resistor to that tap.
  15. xoir


    Oct 25, 2007

    I took measures in all my house outlets and all of them are at 121.2 volts, by the way, when the house was build, I requested two separate lines threads from the electric company in order to separate devices like fridge, dryer, microwave, etc from the rest of the house, so I would say that my voltage is stable, the neighborhood it's new and I know that two person from the electric company live here, so spikes and blackouts are pretty rare in the neighborhood, in mexico we use 110-120.

    I took a reading of cleveland speaker and guess what, 4.1ohms so now I'm trying to remember and figuring what my tech told me once and what is been discussed here with the ext.speaker...

    If the 16ohm tap is activated when two cabinets is in used and I have my main speaker at 4ohms an the external speaker is 8ohms, I'm at 12ohms, but may someone think that using two cabinets at 4ohms in series will still work in the 8ohms OT tap? is that why someone added a resistor in the speaker?
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Sounds like you have a very good voltage level and wiring in your home. Having neighbors that work for the electric company doesn't hurt either to ensure that things are done right. If not, they would have had it looked into.

    Was the resistor connected across the speaker terminals when you took your reading and got 4.1Ω ? If it was it would be better to disconnect it first. In your pic there is a screw attaching the resistor. If you can, undo it and take a reading of just the speaker. Make sure that you do not touch the ends of the probes as you will add a parallel resistance. Also, read the value of the resistor. Is there a value printed on the resistor?

    A speaker typically has a DC resistance that reads 70% on average. So an 8Ω speaker would read 5.6Ω on average. It could be higher or lower. 4.1Ω is on the low side for an 8Ω speaker.

    On the schematic that you posted that came with your amp, you have a head that is wired to be used with a 16Ω speaker. BUT you don't know if your amp is wired for 16Ω or if it wired for 8Ω.

    You have to look to see how it is wired to be sure:

    The color code of the output taps is green-16Ω, yellow-8Ω, black 0Ω. Find these three wires and see where they are connected. I can see clearly enough how it is done in the pic that you posted.

    It looks like you have isolation washers on the ext amp jack. Please check this and let me know.

    If your amp is wired for 8Ω, the yellow wire should be connected to the shunt (switch) on the jack as shown below. Just like on a B-15N.


    If the amp is wired for 16 ohms and the jack is like the schematic, there should be two switches one normally open, one normally closed, like the ext speaker jack in your SVT. It is a more complicated looking jack than the one used for the 8Ω wiring.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Adding the resistor sounds like they are trying to compensate for something. It isn't the right way to do it.

    We need to know how your amp is wired and what type of jack is in there and what wires are connected to it. The jack for 8Ω is not the same as the one for 16Ω.

    Is the added resistor in parallel with the speaker -- across both terminals, or is it only connected to one terminal -- indicating that it is in series with the speaker.
  18. 66Atlas


    May 19, 2012
    Interesting that this topic recently came up considering my latest project. I recently tracked down an original 60s b18 extension cabinet in kinda rough shape. Unfortunately the label inside is missing so I cant confirm an actual year. The speaker appears to be a 60s 15 Ohm (yes 15) Fane 60w speaker. Obviously I cant be certain that the speaker is original to the cabinet but it adds to the mystery I guess :bag:
  19. joplin


    Aug 31, 2008
    I have a B-18 cabinet with the same speaker - it looks like cast aluminum.

    Does NOT have a resistor across the terminals and doesn't look like it ever had one. I used a multimeter and it reads around 10 ohms.

    The cover over the magnet has an Ampeg sticker on it - I have no idea what the speaker type is.

    Are the cast aluminum speakers the Fane type?
    MR PC likes this.
  20. spellcaster


    Mar 20, 2007
    Having the same curiosity with my 66 B18 head. Is there a way I can check what ohm tap the amp is wired for?