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An vintage BFDR problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Mar 4, 2006.

  1. mbratch

    mbratch Member

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    I just bought a vintage 1965 Deluxe Reverb. Unfortunately, both channels sound buzzy/fuzzy/fizzy especially low frequencies. Even at low volumes (as low as 2 on the dial). I tried changing power tubes, rectifier tube, and speaker. I also changed out the 12AX7s (I don't have spare AT7s). Still the same issue. I made sure the baffle is tight as well.

    What sort of component failure causes this? Should I just check voltages inside the amp as cited on the schematic (I know how to do it safely). If you need sound clips, I can try to make some.
     
  2. BozoTone

    BozoTone Member

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    I guess you have to start at about the simplest place, the input jack..check it for good contact with your guitar cord, check wire connections from the jack to the rest of the circuit and for rust (not being a smarta$$...if my a$$ could do tricks I'd be with the Wringling Bros.). Might even look at, and, reseat the fuses. From that point look at speaker connections and start considering voltages through out the circuit...man I hate the basics and will be the first person to look straight past them.....
    BZT
     
  3. Blue Strat

    Blue Strat Member

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    Definitely start by checking the power supply and plate voltages. Also, check the bias of the power tubes (probably first). Doing these 2 things can tell you a lot about an amp and whether or not it's safe to keep the power on.

    If it hasn't had a cap job in the last 15 years DO IT NOW!
     
  4. billdurham

    billdurham Member

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    Sounds like Electrolytics to me.

    BD
     
  5. mbratch

    mbratch Member

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    Thanks guys. I'll check all that out. I talked to the guy who sold it to me and he said it had just been in for service. I'll open it up and see if anything weird happened in shipment.
     
  6. Blue Strat

    Blue Strat Member

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    One tech's "service" and another techs "dusting". ;)
     
  7. Blue Strat

    Blue Strat Member

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    Another thought, in amps that distort at low volumes, one power tube or one side of the phase inverter could be out. I know you've tried new power tubes, but one of the sockets or associated components could be preventing one power tube from working correctly (no signal, no bias, etc). A quick bias and voltage check will lead you to something.
     
  8. mbratch

    mbratch Member

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    Thanks again guys. I just went through and measured all the voltages. Everything looks good (at least within 20%) except one, and that's the heater voltage. The schematic says it should be 6.3V(AC) but I'm reading 3.3V(AC).
     
  9. fr8_trane

    fr8_trane Member

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    Did you check the filter caps? If they are original they may need replacing.
     
  10. mbratch

    mbratch Member

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    I didn't check the filter caps. Is the 3.3V heater voltage OK?
     
  11. Geetarpicker

    Geetarpicker Member

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    Check that heater voltage again!

    If its that low, there's your problem. Remember to check it across the two tube socket heater terminals, not from one pin to ground. If you did that you probably would get 3v as the ground point is half way between the two for hum cancelling reasons.

    Also even though 12ax7s have 12 volt heater windings they also have a center tap and can be wired (and are usually) to run @ 6v.

    If it's still off you might have something wrong in the power transformer like a partial short.

    I would normally think the tubes and pilot light would look mighty dim @ 3v, but it probably would play.
     
  12. mbratch

    mbratch Member

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    Duh. That explains it. I was measuring from ground. I'm reading 6.6V across the heater terminals, which is within spec.

    So I guess I'm back to potential issues with caps.
     
  13. Blue Strat

    Blue Strat Member

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    If the amp isn't humming and your plate voltages are correct, the caps aren't your primary problem (though they should be changed).

    Have you checked the bias on the power tubes? This is you next step for sure. I'll bet one of them isn't operating or is biased at meltdown. Either that or your PI tube or circuit isn't functioning properly.

    As a quick test, plug your V3 tube into the V6 socket. Both are supposed to be 12AT7s. In fact, remove all preamp tubes except for V2, V4 and V6 (which are all that are required for the reverb channel to operate) and see what happens. Also, clean all tube pins and sockets as per the FAQ on my site, under Resources.
     
  14. mbratch

    mbratch Member

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    Thanks Mike. I'll give that a try.
     
  15. mbratch

    mbratch Member

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    Here are pics of the inside of the amp circuit (BFDR). It looks like a bunch of caps were already replaced (the organ Sprague electrolytics). The large caps in the can underneath also look new (not shown in the photos).

    [​IMG]

    [​IMG]

    [​IMG]
     
  16. mbratch

    mbratch Member

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    I played with this a lot more this weekend. I think there are two issues. One is the Jensen C12N speaker in the amp does contribute a little. After A/B-ing with another cab, there's more buzz going on with the internal Jensen. I need to now take the speaker from the cab and put it right in the BFDR to see if the BFDR's cab or baffle is contributing.

    The second thing could just be my misperception of the amp, since this is my first DR amp. How much clean head room should I expect in a DR? At what I'll call a loud living room volume (probably the lowest gigging volume), it's not totally clean. It has some decent hair on the chords when played.
     
  17. John Phillips

    John Phillips Member

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    Check the screen resistors. They are both replacements and look under-rated for the job. The one on the right-hand (furthest from the rectifier) tube socket looks fried in the pic - and if it is, that tube is not operating... which would definitely give you a thin, harsh, rough distorted sound at anything above very low volume (and maybe even then). Neither of them look in good shape. I'd replace them both with 470-ohm 2.5W or 3W wirewounds before doing anything else.
     
  18. mbratch

    mbratch Member

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    Thanks John, you got a good eye. Those particular resistors do look suspicious. I'll give that a shot.
     
  19. mbratch

    mbratch Member

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    John, forgot to ask... do they need to be wirewounds?
     
  20. Blue Strat

    Blue Strat Member

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    Metal film 2 watters is what I use. I think the originals were 1 watts.

    BTW, checking the bias would alert you to a problem with these resistors and lots of other issues associated with the power tubes.
     

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