AB763 Bandmaster Intrigue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TheAmpNerd, Jun 8, 2005.

  1. TheAmpNerd

    TheAmpNerd Member

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    Hi,

    I have a Vintage Bandmaster that is really strange.
    It is an AB763.

    Interestingly, it has a high pitched squeal when turned
    on.

    This is not the result of the out of phase Brn/Blu lead on output tube socket
    being on wrong socket (tried both ways, still squeal). This sounded like
    a pre amp squeal.

    I did find the following, and according to the owner
    he took it in for service about five years ago.

    I imagine a tube went south and took out the
    CC resistors. there is still some carbon scarring
    And some heater wire insulation was burnt and
    Peeled back and separated (no insulation for about
    And inch.

    Output tube socket resistors replaced with
    wire wounds and metal oxides (grid).

    Mains filters replaced--however they were mis-wired.
    I found them in with in series with a common ground,
    and amplifier wiring leads to the caps were:
    red to pos and black to the other positive. (on the second cap)

    Because of the age of the amp, and the mis-wire all the electrolytics
    have been replaced: mains, bypass, and bias.

    Result amp still squeals.

    TUBEs, have been installed with a known good test set.
    They make no difference the amp squeals.

    Stand by voltage = 450V.
    Operating voltage = 413V.
    Bias voltage to the tubes = -48.2
    The other voltages are all close to spec.

    The amp is stable and no squeal with out a PI tube.
    With a PI tube installed it squeals.
    With a PI tube and the .001 cap removed from the circuit
    There is not squeal.

    However, when my Fluke is measures the voltages
    On the PI resistorsÂ….1M, 470, 1M.
    On any of those it squeals again.

    Every CC Resistor in the amp has drifted out of Spec.

    That is it in a nut shell.

    Any Ideas? I have the tools, and will post results and
    Voltages accordingly.
     
  2. John Phillips

    John Phillips Member

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    Check that the OT output is not out of phase - not on the wrong socket, actually both wires on the wrong connections. I've worked on a mid-70s Bassman 100 that was wired up like that, absolutely stock. Strange but true. A real Friday-afternoon job... they'd "cured" it by grounding out the feedback loop at the PI end.

    You can find out easily - temporarily disconnect the feedback loop, which is the 'other' wire that comes from the speaker jacks and goes back to the board. If disconnecting that wire stops the squeal, it may well be that.
     
  3. TheAmpNerd

    TheAmpNerd Member

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    Hi John,

    Been busy and haven't had the time
    to check this amp again.

    Besides new tubes (for testing)

    spliced in:

    New Output tranny
    New Choke
    New Power Tranny

    Still the same problem.

    So there must be something else going on.

    AND

    Lifting the feedback loop?

    Appears to solve the problem...None of the squeel.

    However

    It is possible the amp is just out of phase with itself?
     
  4. John Phillips

    John Phillips Member

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    Yes, that's the problem. Or technically, the problem is that it's in phase with itself :). Someone must have hooked it up wrong at the factory.

    I didn't believe it at first either when I came across the same fault on that Bassman... but it was so. OT secondary leads reversed, definitely stock.

    That should be all you have to do to fix it - just swap the OT wires on the speaker jacks.
     
  5. TheAmpNerd

    TheAmpNerd Member

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    John,

    But this doesn't explain when I use a different output transformer
    that it also squeals.

    Thinking, if the amp is out (in) of phase, I also tested the feedback
    wire...when i place it on the other OT wire, it doesn't squeal.

    Trying to think; which is the BEST solution?

    Also, at some point your Bassman, and my Bandmaster started
    this...I can't belive it came this way from the factory.

    Strange, indeed.

    Thanks
     
  6. John Phillips

    John Phillips Member

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    Try swapping the leads on the new OT too. My guess is that the problem will go away. It may not be the output wires that are reversed, it could be the primary wires, either of the feeds to the output tubes, or the plate wires to the PI tube. If removing the feedback loop stops the oscillation, it's a phasing problem somewhere in the PI or output stage. Connecting the feedback wire to the other OT output wire will certainly stop it - that's a ground connection.

    That Bassman left the factory like that, I'm quite certain. The OT output wires were the wrong way round (by color code), and the soldering looked absolutely original. Not only that, the problem had been 'cured' by bypassing the 100-ohm resistor at the PI ground (which kills the negative feedback loop), which also looked like a factory soldering job.

    I also once worked on a Vibrolux where a wire had not been soldered into the eyelet - just poked in there and left. No solder had been applied to the wire end, ever. Totally original - which I know because the guy who owned it bought it new and it had never been worked on. The funny thing is that it had worked perfectly well for about ten years before it started to cause trouble.

    Fender's QC was maybe not as high as you think, and mistakes like this definitely did occur. They were cranking those amps out by the tens of thousands.

    They even made mistakes at the design stage... they built Champs with totally the wrong bias for about fifteen years, and no-one noticed.
     
  7. TheAmpNerd

    TheAmpNerd Member

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    John,

    Yeah, I came across a Deluxe Reverb that ahd the same thing.

    Someone got in a hurry, took a break, or for what ever reason
    left and eyelet unsoldered. I was saying to myself "holy ****,
    this thing still works after all these years w/o solder!"

    Who said "Solder isn't glue" ?

    Anyway, thanks for the heads up on the feedback wire.

    I'll double check the PI wiring,

    I'll also post anything else I find.

    Oh, swapping the primary leads on the OT was the first
    thing I tried same squeal. It kept sounding like a
    pre amp grounding squeal that you can get sometimes.

    Ciao for now
     
  8. TheAmpNerd

    TheAmpNerd Member

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    UPDATE:

    After changing the OT Seconary leads,
    the amp works fine w/o squeal.

    Wiring the green to ground and black to tip
    did the trick. Keeping the feedback wire where
    it was.

    I've also taken care of all the other problems
    with the amp. All new filters,
    New final coupling caps,
    Fixed the Standard CC resistor blow up,
    New heater wire between outputs,
    New resistors on outputs
    added gound sense (measure bias).

    She works like a champ now,
    er ah, I guess that should be...

    Works like a Bandmaster now. : )

    I can post some pics if any one is insterested.
     
  9. Boogie92801

    Boogie92801 Member

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    I have a bandmaster reverb. SInce you are on the topic. I was wonder what you would sugest to tame the mids and tighten the bass. I recently had it tuned up but the voicing is not what I wanted. It has all new caps and tubes. I am running into a custom 2x12 closed back cab with two Texas Heats.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    I should think with a 2 x 12 closed back your bass should
    be plenty tight. However, I haven't tried the speakers
    in question and don't know if they accentuate the mids.

    If you are working with a local tech, then work through
    him to see what he's done. With all new caps,
    is that electrolytics (mains), bypass (inside), coupling?
    He should also be able to assist with voicing
    the amp to your liking.

    Also, if Bandmaster Reverb, with the 100K slope
    resistor, plenty of mids should be scooped. You
    can bump that up to scoop them even more.

    I also assume you are using the tone controls
    and turning the Mids down?

    You can also try a pair of .022uf coupling
    caps instead of the .1uf .047uf combination.

    That's a start any way.

    Get back and let us know whats been done specifically.
     
  11. darkbluemurder

    darkbluemurder Member

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    If I remember things correctly only the later Bandmasters have mid controls and only so on the vibrato channel. If you do not have a mid control you can try to turn up the bass and treble although I tend to keep the bass rather low to avoid mushiness and achieve better sound projection when playing with the band.
     
  12. TheAmpNerd

    TheAmpNerd Member

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    You are quite correct; that later amp is
    the Bandmaster Reverb thinking AA1068 or 69?.
     

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