Opening a chassis - Bassman content

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ruraltom, Mar 3, 2012.

  1. Ruraltom

    Ruraltom Member

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    Thought I'd ask ya'll here in the tech section...

    I was thinking about looking inside an amp chassis see if it looks 'like it should'. Is this inherently dangerous? Anything I should know or do before doing so? Should I just not?
     
    Last edited: Mar 3, 2012
  2. jaymeister

    jaymeister Member

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    Do you have your mojo capture device? If the mojo escapes, you can never put it back in.

    Seriously, if you want to take out the chassis, that is fine. It just sounds like you are new at it...so be careful and don't touch anything inside.
     
  3. wall_of_sleep

    wall_of_sleep Member

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    edit: what Jay said.

    Why do you want to look around the inside of your amplifier?
     
  4. VaughnC

    VaughnC Supporting Member

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    Well, even unplugged, an amp can store voltage...so, if you get your fingers in the wrong place, it could throw you, burn you, or even stop your heart if the current loop goes through your chest. However, if you're not tech savy enough to know this, looking inside the amp may not mean much unless its something very obvious like burned parts. But it many cases electronics can look normal but still not be working properly. So why even take the risk to just look? You know what they say about curiosity and the cat ;).
     
  5. zenas

    zenas Member

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    Learn how to do it safe before you do it.
    I've got some shocks over the years from rushing things. Thankfully just enough to remind me not to get in a rush.
     
  6. JTJ

    JTJ Supporting Member

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    Technically speaking you can drain the remaining voltage from the amp using small alligator clips through a resistor, but if you have played the amp in a few days your good to open it up and look around, just dont go feeling and grabbing errthing.lol
     
  7. Ruraltom

    Ruraltom Member

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    Hiyo Fellas

    Went fine, though I am texting from a gurney.. LOL (just kidding)

    Just picked up an AA165 Bassman head that I was told had some 20+ yr old mods. As I knew there were plenty of 'gut shots' around to compare, I thought I'd pop the chassis and see what we actually had... that is to say- the extent of the damage. I don't think it looks too extensive and should be relatively easy to reverse, by someone other than me! I may have overpaid, but it's nearly my duty to resuscitate the old beast if it's financially feasible. Photo below, any feedback would be great, I can take more pictures.

    [​IMG]
     
  8. hank57

    hank57 Silver Supporting Member

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    Well at least they didn't alter the board. Easy to fix with all the layouts at ampwares.com Even a person such as yourself!
     
  9. Ruraltom

    Ruraltom Member

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    So if I can solder, I can most likely accomplish the task? The only thing that worried me at all is the idea that the amp can somehow store high current... I've done some wiring -mainly household, but also all guitar electronics- just a little afraid that weeks after it was last plugged in I could get fried....
     
  10. Ruraltom

    Ruraltom Member

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    LOL, I don't think I can go from those schematics to getting it back to speed.

    The amp has obviously been modded for a guitar (though I bought it from an older dude who plays bass) and had a lot of gain added. All the pre-amp tubes are MESA 12AX7-As, that could be a BIG part of the tonal change!
     
  11. wall_of_sleep

    wall_of_sleep Member

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    If you can read the schematic, it's really a game of connect the dots to match the physical layout. As far as discharging goes, that is a very simple process. Learn which capacitors store said high voltage and learn a method or two to remove that to ground. 1 set of test leads is all you need.

    I'm not one of those guys who can tell you what's original or modded from a picture, but that is a nice clean layout. Good luck!
     
  12. Baxtercat

    Baxtercat Member

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    'back to speed', 'good luck', 'the extent of the damage'...?

    ...but how's it sound? Why reverse as a matter of course? Listen & see if the mods make the thing sound better. Stock Bassman amps usually have some dull/mud-sounding caps on the board. Unsolder one end or even swap a few and these sound so juicy. It's just a Bassman [they made many thousands] not gold bullion.
     
  13. john l

    john l Member

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    Careful man that kinda talk will light a fire around here lol. That said I absolutely agree! I have a totally cluster f---ked silver face AA371 that would make yours look like a work of art. I got it for a song almost a year ago with the intention of restoring it but after giving it half a chance Im going to fine tune the mod and call it done lol!

    As far as yours goes nothings jumping out at me as far on the gain mods front, mostly just some shoddy work and some parts swapping. Looks like whoever modded it didnt have the right parts on hand so did some serries stuff to get where he wanted to go. I also dont see a ground on the AC cord and that the death caps still hanging around in there. Untill these things get handled either check the outlets you plug it into or hold your strings down and give your mic a quick knuckle tap before you stick your mouth up to it. Getting bit on the lip sucks.
     
    Last edited: Mar 3, 2012
  14. Ruraltom

    Ruraltom Member

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    To make a short story long, it does sound pretty decent. When I went and checked it out, I played for a little bit at a reasonable level and it had a nice clear, clean Fender sparkle. It sounded quite a bit like my long lost '65. When I got it home and cranked it up... loads more gain that my old one and strangely much, much quieter (as in no louder than my RX Jr ((18W)) through the same cab).
    If I hadn't specifically bought the amp for it's native qualities, I'd probably be happy with it- no argument with that logic. However, I DID buy it to have a louder clean amp with more headroom... in fact, I actually bought it to be what it is (was)! I could try to put it back out on the market, but it really isn't a worth much as a sloppy, amateur-modded "Bassman" and I'll have to find another one at a half decent price... I'm prepared to spend a little more $$$ to restore it to how it 'should' sound.
     
    Last edited: Mar 3, 2012
  15. Ruraltom

    Ruraltom Member

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    Here are a couple more... I believe you are right, the AC cord's ground looks to be cut at the root... dunno what a death cap is... I think I'll avoid it.. I guess I'll be looking for someone in Pittsburgh to go over it.
    [​IMG]
    [​IMG]
    Most of the wiring looks original and the 'newer' plastic coated sticks out pretty clearly
     
  16. john l

    john l Member

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    The death cap is the blue cap that comes of the ground switch. The quick run down is that if the cap ever fails the chassis becomes live and since your plugged into it and touching metal strings you can become a path to ground in a variety of circumstances IE my microphone example. Its just the 110v wall voltage but still it can hurt and there are old wives tails of guys getting killed, hence the name death cap. Theres certain way you have to rewire it despite adding a proper 3 prong cord.

    Heres how
    http://www.google.com/url?sa=t&rct=...zNG0Bg&usg=AFQjCNHt4hC18aqtbfSaO7UGAnioHIppsw
     
  17. corn husk bag

    corn husk bag Silver Supporting Member

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    Well! Thanks for this link!

    Steve
     
  18. jbltwin1

    jbltwin1 Member

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    The only thing that bugs me is the cluster %&^& on the bias supply board. Post a better pic of that if you could. It looks like they were hacking on THAT>
     
  19. gldtp99

    gldtp99 Member

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    I've got a modded BF Bassman on my bench now----- it was modded long ago and the owner brought it to me because it "Buzzed".
    The PI tail resistor had become un-soldered (very poor workmanship on mods)---- and also someone had reversed the OT secondaries to try and cure the buzz.
    i got these issues in order and bench played the amp for @10 minutes to see how these mods sounded (not that great)---- and then smoke started to drift off the main board. i shut the amp down and put it aside until i could get back to it.
    Last night i gutted the board and disassembled enough to flip up the eyelet board to look at WTF was going on--- found huge solder blobs between eyelet board and insulator board that, along with the dirt/debris build-up, was providing new paths for current flow--- hence the smoke that i observed.
    This was an AB165 at one time--- i'll use AA864 specs for the Normal channel--- they've already converted to bias voltage adjustment (instead of bias balance)--- i'm talking to the owner about what type of tone he wants out of the Bass channel--- i've got two 12ax7 positions to work with there (4 gain stages available)--- not going to go all Gain-Wild on this one----- thinking of just running the 12ax7 halves in parallel (to get two Big gain stages) and altering tonestack ground resistor for more mids---- cathode Rk's as needed.
    i'll use all new components when assembling---- the Blue Molded caps were replaced long ago and not much was OEM on the board---at least the orig pots and knobs are still there.
    It's sad to see an old Bassman in such poor condition, i couldn't bring myself to take pics of the mess--- i'll fix it up......................gldtp99
     
  20. Krayon

    Krayon Senior Member

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    looks like someone did a version of what some call the "Hyperion mod"
    which is where the PI and Driver section is returned to AA864 Bassman specs. while the pre-amp sections are modded to either JTM45 or some other Marshall-esque pre-amp.

    a great sounding mod if done well.
    though I can tell they didnt' do the whole mod as the second pre-amp tube still has the first section unused... a complete waste on these amps.
     

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