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1968 Bassman Amp, can anyone tell what's been done?

Discussion in 'Amp Central Station' started by halfsmith, Apr 16, 2021.

  1. halfsmith

    halfsmith TDPRI Member

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    Hello. I got this incredible sounding amp from a guy...
    (I've had and gigged with a 60's Bassman 100 and always regretted parting with it, so when this guy pulled it out of his garage saying it didn't work, I couldn't walk away and we made a very good deal)

    Anyway, it worked right away after replacing the fuse, lucky!

    So far I can tell it is an early '68 and parts inside confirm that, but I've crosschecked multiple schematics and my best guess is that someone has made some changes. I would love any expert eyes or opinions.

    Thanks
    Bassman-guts-4.17.21.jpg
     
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  2. halfsmith

    halfsmith TDPRI Member

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    here are two areas I found that were kind of a hybrid AA165/AA864
    bassman-mods-1.PNG bassman-mods-2.PNG

    what can I do to help you help me?
    Thanks!
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Iirc early '68 Bassman had Blue molded Ajax caps and some brown turd (blob) caps. It looks like those are gone. There may be some additional high pass caps added in parallel with a resistor or two.
    The Orange drops should have date codes on them. Under the doghouse the filter caps should have date codes as well. So it might give an idea how long the new stuff has been in there.
     
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  4. tele_savales

    tele_savales Tele-Holic Platinum Supporter

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    Considering the number of components that were swapped out I would go ahead and change the plate load resistors to metal film. The amp will be a lot less noisy.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It looks like it has been changed to match the AA864 schematic.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Welcome, halfsmith. Fwiw, the amp you have now is quite different from the Bassman 100. The Bassman 100 has only two gain stages in each channel whereas the 2 x 6L6 amp you have has three gain stages per channel. This amp you have has been modded back to a true bias voltage adjustment circuit that was last seen in the AA864 amp. What else has been done would demand a closer look...best done in person.
     
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  7. Fixcinater

    Fixcinater TDPRI Member

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    Looks like the important stuff has been updated: bias board (first photo upper left), 3-prong AC cord, caps driving the 6L6s so you aren't getting DC on them (big Orange Drops on left side of board as pictured). Hopefully this means the filter caps have been replaced as well.

    I'd confirm (or have a tech do it if you're not comfortable working with high voltage stuff): filter caps have been changed and haven't aged out of their useful lifecycle;
    the "death cap" is out of circuit, and you could re-use the "Ground" switch for NFB switching or something else. Or just leave it disconnected and forget about it.

    With the components that have been already been changed, you could alter component values/brands to your taste, if you want a bit more mids or ______, since it won't be all original ever again. My '66 Bandmaster is like this, lots of board components changed so no harm changing values to suit.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    The AC wiring has not been done in the best and safest manner. This can be seen because both PT primaries still go to the area of the fuse, power switch, and stand by switch. Also, we cannot see whether or not the death cap has been removed.
     
  9. halfsmith

    halfsmith TDPRI Member

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    thank you all for the help!

    Hopefully this pic shows the absence of the "death cap"?

    Do y'all recommend I bypass the extra power plug and ground switch?
    Would following Brad The Guitologist's work on a similar Bassman be advised?

    bassman-mods-3deathcap.PNG
     
    Last edited: Apr 18, 2021
  10. halfsmith

    halfsmith TDPRI Member

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    Here's a look inside the doghouse, new filters and some resistors, how can I date the Caps?
    bassman-mods-4doghouse.PNG
     
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  11. halfsmith

    halfsmith TDPRI Member

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    Thanks Wally!
    Are you able to guess what the amp was built as? Isn't AA864 a newer layout than the AA165/AB165?

    What is the benefit or audible difference in the "true bias voltage adjustment circuit"? Is that just to make dialing in the Tubes better?
     
  12. Wally

    Wally Telefied Ad Free Member

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    Halfsmith, those circuit designations have a date code. AA864 was drawn in August—-8, 1964. this circuit has a true bias voltage adjustment circuit, 3 gain stages in the Bass channel, and two gain stages in the Normal channel. That Normal channel is the same as the Normal channel preamp of the Two channel guitar amps. The AA165 was drawn in January, 1965. This circuit has the same preamp structure as the AA864 Bassman but has the bias balance circuit. The AB165 is the first revision...AB versus AA...of that January, 1965 schematic. This amp has three gain stages in both channels. This makes the Normal channel the hottest guitar channel Fender built at that time since this amp has no reverb or tremolo to suck gain from those three gain stages that are also in the .vibrato channel of the Reverb amps. AC568 is the third revision of the first SF circuit...drawn in May, 1968, which is 13 months after the first SF cosmetics were introduced in April. These preamps were much the same in the following 2 x 6L6 Bassman amps except for the Bassman 10 amp which follows the preamp structure of the 4x6L6 Bassman amps...two gain stages in each channel...clean machines.
    The tweed schematics tell a person what model of amp and what year the schematic was drawn....5E3 = Tweed Deluxe—-5=1950s,
    3=the third model of Fender amp, and it was drawn in 1955—-E. The 5F6A Bassman was drawn in 1956 and was the first revision. 1950s, Bassman was Fender’s 6th model, F=6[1956], A = First revision.
     
  13. halfsmith

    halfsmith TDPRI Member

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    Incredible post Wally, I appreciate it!
     
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  14. Dacious

    Dacious Poster Extraordinaire

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    Looks like the hard yards on this one have been done. It's probably loud and in your face with lots of mids. Very direct sounding. These through a 210 are an ideal gig amp, they can punch even at lowish volume.
     
  15. Wally

    Wally Telefied Ad Free Member

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    Mids...in a BF preamp? I doubt it. One cannot tell the values of the tone stack in the normal channel, but the Bass channel has not been significantly modified as far as I can tell.
     
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  16. Dacious

    Dacious Poster Extraordinaire

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    The one I remember definitely had some mid quality. Wasn't dripedge but pretty early SF. I don't know the circuit or mods.

    I remember thinking it had some 'clean Marshall' without the woolly kachang overdrive but a ton of headroom. It was giving a 212 box a hard time, dunno what the speakers were, probably original.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    There is some gain grit going on, but ime/imho there is not a heavy mid thing going on....except for the ones that I have modded to make the Bass channel that is hotter than the Normal and having more mid content. I am sure others do something similar...
     
  18. halfsmith

    halfsmith TDPRI Member

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    This is the only deviation I can find from the AA864 schematic.

    Can anyone explain what this mod does?
    bassman_aa864_mod.jpg
     
  19. Wally

    Wally Telefied Ad Free Member

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    Starting with the AA165, they moved the cathode circuit for V1P3 over to the right end of the circuit board in these 2x6L6 Bassman amps. It appears to me that you have a circuit based on the AB165 and later Bassman. Trace out the junction of the two channels...you will see this. What are the dates on the transformers and pots? What is the serial number. The cloth covered wiring for the circuit board indicates an earlier 1968 amp, iirc...it may well be an AB165 BF circuit...with some mods to the bias and perhaps the power rail.
     
  20. halfsmith

    halfsmith TDPRI Member

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    I'm working out the schematic as best I can (very new to this), and like I've pictured (post #18 above) what's in there now is nearly identical to AA864, but I can tell that's not necessarily how it left the factory.

    I'm having trouble locating the cathode circuit for V1P3 on the board, can you point it out?

    Transformer date is 14th week of '68, there's only one pot I have been able to date, 40th week of '68, so late '68 seems in line with the A40000's serial - places it right in the depths of the transition right?
     
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