Which of the caps in the picture need replacing? (64 Bassman Content)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MarkWorth, Jan 2, 2008.


  1. MarkWorth

    MarkWorth Member

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    I know the filter caps do on the underneath of the chassis, but what about out of these guys...it looks like most of them are original, so i think i've got to replace a lot of them.

    [​IMG]


    My guess is the two black caps that are on the middle right of the board, the two black caps that are on the middle left of the board, also the red/brown one that's next to the two black caps on the left side of the board, and also the one all by itself in the bottom right hand of the picture.

    Is that correct?

    *Also, is there anything else you think i should do? I've got new tubes and a new 3 prong. Everything else is probably ok then?
     
  2. AL30

    AL30 Member

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    The black caps you are referring to are the electrolytics and are the ones that usually need replacing. Those aren't the originals though so they may be OK. But I don't know why someone would use radial caps in there. I would replace them just "because" but that's me.

    Also, the big Mallory (tan colored) cap on the bias board in the bottom right of the picture should be replaced. Make sure you check the polarity.

    I also replace the resistors on the tubes (1.5K and 470ohm). There's a lot of heat there. It may not be necessary. And I replace the resistor on the bias board as well. Again, it may not be necessary.

    AL
     
  3. John Phillips

    John Phillips Member

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    +1.

    Remember that a failure in any of the bias supply chain (the resistor and cap on the board next to the pilot light, and the 1.5K grid stopper resistors on the power tube sockets are the most common) will fry one or both power tubes.

    Replacing the 470-ohm screen resistors will improve reliability, although nothing bad usually happens if one fails.

    These components don't affect the tone, so use the best and most robust modern types - carbon-film or metal-film for the small ones, metal-oxide or wirewound (2 or 3W) for the screens.
     
  4. MarkWorth

    MarkWorth Member

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    ok great (thank you!), and do i need to replace that brownish/redish cap on the left as well?
     
  5. John Phillips

    John Phillips Member

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    No - not unless it's leaking DC.
     
  6. MarkWorth

    MarkWorth Member

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    ok, awesome..

    oh! and one more thing! shouldn't i replace the four on the board with 25uf/25vdc? One group is 35uf, and the other group is mismatched with one 25uf and one 50uf:confused:

    *it's an AA864 if that matters at all
     
  7. MarkWorth

    MarkWorth Member

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    bump for this last question!
     
  8. AL30

    AL30 Member

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    Those are the black caps correct? They've already been replaced although there's no way of knowing when. I would replace them.

    I don't know why they are different values. They should be 25uf/25V. I usually replace with 25uf/50V.

    Here's your layout

    http://www.ampwares.com/ffg/schem/bassman_aa864_layout.gif

    Just use the layout. DO NOT ORDER FROM HERE. There's a thread going on right now about this (and they'll a similar thread in about 3 months).

    You can order from Mouser, Digi-Key, Hoffman etc.

    AL
     
  9. MarkWorth

    MarkWorth Member

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    yeah, the black caps that've already been replaced. I don't trust 'em. Hah

    I thought they were supposed to be 25/25, but i figured i'd ask before i ordered. And geesh, not gonna lie, that layout's so blurry, i wouldn't be able to tell whether it says 25 or 35 or whatever... : /
     
  10. Flameout12

    Flameout12 Member

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    I can confirm the warning about ordering from Ampwares. :mad:
     
  11. devbro

    devbro Member

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    It's hard to see in the photo, but it looks like it needs a three prong cord upgrade.:eek: I don't see the death cap though. Also, allot of guys replace the bias cap with a slightly larger one - I did. Also, I don't think the brown turd cap was original. If restoration is important to you, you may consider finding a replacement blue molded cap. A few guys on this board have some available. Make sure they're not leaking.
     
  12. HKGuns

    HKGuns Member

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    Just DON'T touch those blue ones. PLEASE!!!
     
  13. MarkWorth

    MarkWorth Member

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    ...?

    theyre THAT good?
     
  14. brad347

    brad347 Member

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    they're THAT original. ;)

    The relative merits of the blue mallory caps is up for debate. If you like the way blackface amplifiers sound, I'd leave 'em in. If they're leaking DC or suspect in any way I'd replace the faulty ones. But there are as many opinions on that as there are opinions on anything.

    I like 'em just fine and have 'em in my blackface stuff. If it ain't broke...

    Also, you've got some funny values for the cathode bypass caps (the black ones that you will be changing). If you really like the tone of that amp as-is, you may want to make a note of what values are where. Changing values of cathode bypass caps changes the tone a lot. I think you should hear it with stock values, but if you absolutely hate it you'll want to know what was where.
     

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