Troubleshoot a modified AB165 circuit?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 60strat, Jan 29, 2015.

  1. 60strat

    60strat Member

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    Hi, New to the forum. I've acquired a 67 blackface AB165 Bassman. Tube chart shows AB165 with a QH stamp.Transformer codes are 606-638 & 606730. The amp does not work. The amp hummed when turned on and then a puff of smoke near the power tube and a lovely fragrance! The filter caps and bias board were changed. If I post pictures, can someone tell me what mods were done and what could have blown? Thanks
     
    Last edited: Jan 29, 2015
  2. Blue Strat

    Blue Strat Member

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    A few photos might help.

    Smoke COULD be because of a shorted power tube that burnt one of the 470 ohm resistors on one of the power tube sockets. Could also be a lot of other things.
     
  3. 60strat

    60strat Member

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    Those 2 large filter caps are 800uf 330v
     
  4. mark norwine

    mark norwine Member

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    Your amp has had minimal basic maintenance...incorrectly and incompletely performed.

    You need to take this to a tech and have it fixed.
     
    smolder and mscaggs like this.
  5. 60strat

    60strat Member

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    Ok, I kinda figured.
    What was the thinking with the 800uf caps? They should have been 70uf.
    Thanks!
     
    Last edited: Jan 29, 2015
  6. mark norwine

    mark norwine Member

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    ...and they should have been axial leads

    ...and the cathode bypass caps should have been changed

    ...and a grounded cord should have been added

    ...and the deathcap should have been removed
     
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  7. 60strat

    60strat Member

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    Is it worth going to the AA864 circuit or just clean up the mess?
     
  8. Blue Strat

    Blue Strat Member

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    Looks like some of the offboard feedback stuff from the 165 has been removed, moving more towards the 864.

    Before anything, you have to find out what caused the smoke and do a complete checkout/repair to get it back to stock. You don't modify amps with problems unless you want a mystery that will last until the end of time.

    Many a player loves his stock, or semi stock, 165. Some of the conversion has been done and converting the bias circuit should be a definite mod to have done.
     
  9. telenut62

    telenut62 Member

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    800 or 80uf?....330v is too low a rating anyway....2 x 80uf in series gives 40uf
     
  10. 60strat

    60strat Member

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    I'll have it checked. You're referring to the 220k resistors that were on the tag coming from the 6L6 right? Once the bias circuit mod is done, anything else to consider? Also, what does the 100k resistor going from the volume to treble pot do?
     
  11. 60strat

    60strat Member

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    I checked the caps and its 800uf 330v
     
  12. 60strat

    60strat Member

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    I checked the caps and it's 800uf 330v
     
  13. mark norwine

    mark norwine Member

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    never, ever, ever "mod a broken amp".

    Step one: Fix this amp....including a complete overhaul of all old (or incorrectly-installed new) electrolytics.

    Step two: make it safe.....grounded cord, no death cap, etc.

    Step three: consider mods.
     
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  14. schmidlin

    schmidlin Member

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    Listen to Mark. And you still don't know what the puff of smoke was. Find a competent tech to get it like it just rolled out of the factory.
    Edit: minus the ungrounded AC cord and death-cap of course.
     
    Last edited: Jan 30, 2015
  15. 60strat

    60strat Member

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    Thanks for all the input!
     

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