Help with 1967 Blackface Bassman Head

Discussion in 'The Workbench' started by surfguy13, Feb 3, 2012.

  1. surfguy13

    surfguy13 Member

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    I am picking up a 1967 blackface bassman head next week that will be used with a stepdown transformer as I'm in the UK.
    I intend to use a non-original cab with it and was wondering what ohms options I have?
    I believe the 2x12 bassman piggyback cabs ran at 4 ohms - can anyone confirm this?
    If this isn't the case, or there is a means of adjusting ohmage on the amp, I'd be really grateful for any advice.
    Thanks!! :)
     
  2. clel miller

    clel miller Member

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    If the OT is original, it will be 4 Ohms.
    These amps did not have an Impedance selector.
    Not to say that A Lot of guys did not run the amps at 8 Ohms "back in the day"......but that is up to you.
    So you are either looking at new speakers or a new OT.
    Best
     
  3. mickeydg5

    mickeydg5 Well-Known Member

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    Yes, as mentioned, those amplifiers were 4 ohms out at 50 watts.
    The original output transformer is a model 125A13A.
     
  4. surfguy13

    surfguy13 Member

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    Thanks guys, really appreciate the help. Yup, it's the original OT so I gotta live with 4 ohms.
    As matter of interest, what actually gets damaged if you run a head through a cab with a different impedance?
     
  5. mickeydg5

    mickeydg5 Well-Known Member

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  6. clel miller

    clel miller Member

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    That is an ON GOING debate.....with MANY variables.
    Tubes are VERY forgiving creatures. They operate over a range of spec/impedance. If you read a receiving manual, look at the specs.......you will be shocked to see that Fender and Marshall (all of them really) would frequently run their plate voltages WAY past spec.
    Anyway......the OT itself will have a lot to play in this. How well it is made, material, construction and how "overbuilt' it may be.
    Run the OT into a bigger load, and you risk the damaging "Fly-back" voltage. It can ruin OT's and Power Tubes.
    Run the OT into a smaller load and you risk over current that can do the same. But wait, tubes are current limiting by nature......RIGHT.?
    It is "best" if you can run the "correct" load. If you read the Internet enough you will see there is a general wisdom that you can run a 100% mismatch in either direction. For you that would mean 2 Ohms or 8 Ohms.
    There is a reason that output jacks default is to GROUND and not to OPEN.......again,in theory.
    Good Luck
     
  7. MajorNut1967

    MajorNut1967 New Member

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    Is it a AB165 model?

    If it is 4ohms is the speaker impedance.
     
  8. gldtp99

    gldtp99 Active Member

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    Fender Output Transformer, Super Reverb, Bassman, Concert, 4/8/16 with M-6 Upgrade

    This is an excellent sounding BF Bassman replacement OT with 4/8/16 ohm output taps------ i've used many of these OT's in different builds/repairs--- one resides in my '66 BF Bassman that came to me in a trade-- it was pretty well butchered (poorly modded) and had a bad non-orig OT.
    The '66 Bassman is used in the left panned guitar track here: ejtap by gldtp99 on SoundCloud - Create, record and share your sounds for free
    This is the Bass channel that i've modded to a Classic Rock tone--- the Normal channel has been modded for a cleaner than stock tone.
    With the MC 4/8/16 ohm OT the amp can be used with almost any cab without an impedance mismatch---- so the amp can sound its best and there's no worry of overcurrent or flyback problems from a mismatch--- I think this is the best way to go about it but this amp had a bad OT when it came to me, so my decision was an easy one.
    On the other hand a friend has been running his SF Bassman 50 (4 ohm output) into a 16 ohm 4x10 cab for years without any problems and it sounds good----- but he's a musician and not an amp tech---:cool:.......................................gldtp99
     
  9. clel miller

    clel miller Member

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    Magnetic Components really seam to be a decent company.....
    I have used that OT in two SF Bassman that I pieced together, and just a month ago, installed one in a Drip-Edge that I rebuilt for a friend.
    I also did some "mods" to the first channel for a bit more mids and gain.
    Your comment about what you did to the Normal Channel intrigues me.......mind if I ask what you did.?
    Thank You
     
  10. gldtp99

    gldtp99 Active Member

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    I just looked inside the Bassman--- I "modded" the Normal channel back to AA864 Normal channel specs----The amp was pretty jacked up when i got it and i couldn't remember exactly what i did to it--- it was many projects ago--- still has the Blue Molded caps in the Normal channel----PI is back to AA864 specs except output coupling caps are .047uF Mojo Dijon (TAD).
    So the Normal channel isn't modded at all but has been restored to stock AA864 specs--- my apologies---- it has a wonderful BF Fender clean tone---go figure.........................gldtp99

    AA864 schematic: http://www.el34world.com/charts/Schematics/files/fender/BASSMAN_AA864.pdf


    Amp Pics: http://s46.photobucket.com/albums/f112/gldtp99/BF%20Bassman%201966/

    This amp had an FX loop installed in the back panel when i got it----i used one hole (had to enlarge somewhat) to mount the Impedance switch and plugged the other hole---- amp's not a beauty but it sounds very good.
     
  11. clel miller

    clel miller Member

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    Thanks.
    ......if you remeber.....did you run both channels through V2 or just the "bass channel".?
    Thanks
     
  12. gldtp99

    gldtp99 Active Member

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    Half of V2 is used for the modded Bass channel----three gain stages in this channel
     
  13. clel miller

    clel miller Member

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    10-4
    Thank You
     

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