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Repurposing Vintage Electronics

Discussion in 'Glowing Bottle Tube Amp Forum' started by sllsll, Feb 25, 2021.

  1. sllsll

    sllsll TDPRI Member

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    Here's a little 5 Watt tube rehearsal/ studio guitar amp that I converted from a 1960s tape deck for a few friends. It's been evolving over the past year based on their feedback.





    This amp was designed and constructed using the basic foundation of a 1960s (Akai M8) Roberts 770X tube reel to reel tape recorder. The amplifier circuitry has been optimized to specifically serve as a guitar amp rather than as hi-fi equipment. Where practical the original vintage components, including the tubes, have been kept.

    There are 2 instruments inputs which share the common volume and tone controls. The 2 inputs however employ separate, and different, input stage tubes and therefore sound very different. The first input uses an EF86 (6267) tube. These tubes were used in 1960s vintage VOX amps, and are still used in some Dr. Z, Matchless, and other boutique amplifiers. The 2nd input uses a 12AX7 (12AD7) tube. 12AX7s are the most common preamp tubes used in guitar amplifiers such as Fender, Mesa Boogie, and Marshall. All of the tubes are the original vintage 1960s tubes. This amp is very quiet (background noise and hum) and breaks up nicely at reasonable volume levels.

    The tone circuit uses a modified passive Baxandall/James circuit (Bass and Treble controls) similar to the type of tone stacks used in the original Ampeg, Orange, and some Dr. Z amps. Additionally, a boost switch increases the gain and frequency response of the amp providing additional tonal flexibility.

    The output stage is a single-ended class A power amp. This amp uses an EL84 (6BQ5). Single-ended class A amps were used in many lower power 1950s-60s Fender Tweed and Blackface amps and in Vox's AC-4, which also used the EL84 tube. The amp uses a 6X4 tube rectifier. This amp works great for rehearsal, small venue use, and especially studio recording.
     

    Attached Files:

  2. kbold

    kbold Friend of Leo's

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    Nice.

    The tubes used in those old reel-to-reels were excellent, with low noise floor.
    It's unlikely you'll ever hear a quieter (background) preamp tube than a 12AD7. They also break up nicely when pushed.
     
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  3. sllsll

    sllsll TDPRI Member

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    Yeah, even the EF86 (6267) is quiet as the input tube for the 2nd channel and has a different tonal and overdrive profile.
     
  4. printer2

    printer2 Poster Extraordinaire

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    Cool. Nice demo also, nice variety.
     
  5. Bill Moore

    Bill Moore Tele-Afflicted

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    Sounds great.
    I have an old Akai reel that a friend brought that has some 7 pin output tubes. I think they are only like 3.5 watts though!
     
  6. sllsll

    sllsll TDPRI Member

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    3.5 - 5 watts, basically up to component variances. Probably the same amps.
     
  7. Willie Johnson

    Willie Johnson Tele-Holic

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    I have a Bell 2122 hifi amp that I got from a secondhand store with black plate/grey glass 6v6 power tubes and a 5y3 rectifier for $30. It's got to be useful for something--I've given it and my SCX2 some sideways looks with the thought of cobbling them together into some kind of tweed Deluxe. The power transformer is gigantic.
     
  8. jvin248

    jvin248 Doctor of Teleocity

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    .

    The builds/conversions of old gear into guitar amps is a fun thing to follow on a couple of youtube channels:

    Uncle Doug
    D-Lab

    Old cathedral radios converted, police scanners, and more.

    .
     
  9. archetype

    archetype Fiend of Leo's

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    Most human ears and brains can't discern a difference between 3.5 and 5 Watts. I'd make a project out of it.
     
  10. don71

    don71 Tele-Afflicted

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    I like to see these kind of projects. Nice job.
     
  11. sllsll

    sllsll TDPRI Member

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  12. Bill Moore

    Bill Moore Tele-Afflicted

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    I guess I'll try to see if anything works, can't make out the tube numbers, maybe when I finish the projects I have going now.
    (Don't know what tubes would cost if they are dead!)
     
  13. sllsll

    sllsll TDPRI Member

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    My Akai tape recorder manual actually specifies the output as 6 Watts per channel into 8 Ohms.
     
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