AB763 Bandmaster vs. AB165 Bassman...How similar are the circuits?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by KBN, Mar 20, 2006.

  1. KBN

    KBN Member

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    Some questions of curiosity here. My guitar amp is a 1965 "AB763" Bandmaster and my bass amp is a 1970 "AB165" silverface bassman. From what I understand the silverface bassman is still the blackface circuit. So last night at practice my Bandmaster just wasn't feeling or sounding "right". It was lacking attack and punch...I don't know, hard to explain. Weird. It sounded a little comb filtered and just wasn't reacting to my playing. It also seemed less loud. So I grabbed the Bassman and plugged that in. (I should mention that I play the Bandmaster on the Vibrato channel.) I plugged into the Normal channel of the Bassman and Holy cow, there I was. The bassman seemed more aggressive and much louder than I remember the Bandmaster ever being. I had the Bandmaster rebuilt about 5 years ago and the Bassman rebuilt about 2 years ago. Both by the same tech. So my questions are: How different are the Vibrato channel of the Bandmaster and the Normal channel of the Bassman apart from the vibrato (tremelo)? Would it seem that the Bandmaster needs to be retubed, or would it be a deeper problem than that? I thank you all in advance for your help. I know what I like to hear, but I am still in the early education of the technical side of things.

    Thanks,
    Keith
     
  2. jpervin

    jpervin Supporting Member

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    Supposedly the transformers in the Bassman are bigger (more powerful?) than the Bandmaster.
    I have a Bandmaster Reverb head hooked up to a 1x15 cabinet that I play bass through, using the normal channel. I have to turn it up to at least "5" on the volume but it seems to work out pretty good.
     
  3. Dave C

    Dave C Gold Supporting Member

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    The preamp circuits are pretty similar, the big difference is in the output section , just look at the output transformers , that's the biggest difference.
    Dave C
     
  4. Wakarusa

    Wakarusa Member

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    The AB165 Bassman uses both global and local negative feedback (several local NFB loops, in fact) that make a significant contribution to the punch and tighter tone.

    The AB763 Bandmaster, transformers excepted, would more closely resemble an AA864 Bassman.

    And the output transformers are very different.
     
  5. KBN

    KBN Member

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    Hey, thanks for all the info. So now I have a follow up question. How involved (i.e. "expensive" haha) would it be to modify the output section (done by a knowledgable tech of course) so that it would be a little more like the bassman. I like the tremelo in the Bandmaster and I like how it cleans up. Or should I just get a different amp and buy a good tremelo pedal?
     
  6. John Phillips

    John Phillips Member

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    What Todd says - the Normal channels of the AA864 Bassman and the AB763 Bandmaster are near-identical except that the channel mix is DC-coupled in the Bassman, which extends the low end and increases punch, and the output transformers are different (and the second tube stage shares its bias network with the Vibrato channel on the BM, but that probably doesn't significantly change the tone). Adding a Bassman OT to the Bandmaster is possible, and bypassing the coupling caps can be done... so then you'd have two AA864 Bassman Normal channels, one with tremolo. Personally I think that the AA864 Normal channel has the outright best tone Fender ever put in an amp :).

    The AB165 is different in that both channels share the intermediate gain stage with negative feedback - only the Bass channel uses it on the AA864.

    I have a thought... a friend of mine has a '64 Bandmaster with an already-changed non-Fender OT. I wonder if I could borrow it... ;)
     
  7. KBN

    KBN Member

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    Thanks, I may have that done. Sounds like what I am looking for.

    Keith
     
  8. saxophonist56

    saxophonist56 Supporting Member

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  9. John Phillips

    John Phillips Member

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    Different from either - the Normal channel is quite like the AA864, but the Bass channel doesn't have the intermediate/negative-feedback stage either; there's one less tube stage overall. It's cleaner and more open-sounding, but the Normal channel has a Mid control so you can give it a bit more attack. A nice amp for both guitar and bass IMO.
     
  10. opus

    opus Member

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    this may help on the bandmaster..., i've been plugging my guitar into input 1 on normal, then running an instrument cable from 2 on normal to 1 on vibrato. i found it fattened up my sound and gave me the ability to tailor it more..., the downside of course is that the vibrato has a less pronounced effect.
     
  11. saxophonist56

    saxophonist56 Supporting Member

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    yes i agree the normal channel's clean just blew me away.....then the guy played a 70's jmp and man did that sound bad...real trebly no matter what. and that was 4x the price of the bassman 100. i wonder if it could be dumblysized quite easily? i know the TR guy thinks his modified bassman's sound as good or better than his production stuff......
     
  12. John Phillips

    John Phillips Member

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    The 50W Bassmans are usually thought better candidates for mods, because they not only have the extra gain stage, they have a whole unused tube stage as well, which you can hook up - this basically gives you two complete 12AX7s to use on the Bass channel, which is plenty for all sorts of gain mods, and still allows you to leave the Normal channel untouched. You could add the extra tube socket to a 100, but it's obviously slightly more work.

    (FWIW, originally the extra gain stage was used in the 50s as a very heavy negative-feedback loop to widen and flatten the frequency response - to get enough bass and clean volume out of only a 50W power section, rather than increase distortion - because it was intended as a bass amp of course :). The 100 simply has enough power and headroom that it doesn't need it.)
     
  13. countandduke

    countandduke Member

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    I have had PLENTY of emails and talks with Trace from voodooamps.com not only is he SUPER cool but very knowledgeable about all kinds of amplifiers. He would also tell you that the bassman heads are much easier to work with too but would also quickly point out that your OT needs to be a nice high quality one so the amp sounds and performs at its maximum.

    Give Trace a call, he can help you with what you are looking for and answer your questions.

    Chris
     
  14. doctorx

    doctorx Member

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    I have a BF Bandmaster and a BF Bassman, as well as two silverfaced Bassmans.

    Why would you want them both to sound the same? They are both cool amps, but different as everyone has described. The Bandmaster is a little cleaner and a little less gainey but that is its charm.

    If you want a little more gain you can install a switch to disconnect the tremolo, it makes a noticible difference but keeps the character of the amp. It is also a whole lot less complicated than radically modifying a classic vintage amp :eek: by changing transformers, rewiring circuits, etc.

    Click here to see how to make the tremolo switchable.
     

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