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pitch-shifting tube vibrato unit

Discussion in 'Shock Brother's DIY Amps' started by tubeswell, Dec 17, 2011.

  1. tubeswell

    tubeswell Friend of Leo's

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    Saw Steve Luckey's (over at the Hoffman forum) one a few months back and liked the sounds of that so I decided to try it for myself.

    The unit uses a saturable reactor from a hammond organ, which is a kind of triple transformer array with one side of each of the three transformers wired in series with the other two and fed by an oscillator. The other sides of the transformers are utilised in feedback loops from the 12AU7 triodes. There are 4 sigma (phase shift/pitch/harmonic) controls (one for each 12AU7 triode), a feedback control, an output/bypass mixing control and the LFO controls. The unit is driven by 2 x 12AU7, 2 x 12AX7 and a 12DW7. There is a CF bypass so you can run the unit un-effected by turning down the output level control, which is wired with a dual gang pot that also serves and the 'dry' mix control when turned down. Schematic here:

    http://ampgarage.com/forum/download.php?id=20207

    I was a bit concerned about how many parts I was cramming in there, but it didn't affect the sound ;-)

    Only delved into the tip of sounds so far but got several variations ranging from chorus to phase to vibrato to tremolo by tweaking the 4 sigma controls, and many variations in between. There are a few settings that sound quite magnatone-ish. I need to get a few .mp3s done.
     

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  2. celeste

    celeste Friend of Leo's

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    Yes you do. :)

    As I recall, the reactor is not so much a transformer as 3 chokes on common core, does that sound right? Any idea about the relative turns ratio between the windings? Or inductance?
     
  3. tubeswell

    tubeswell Friend of Leo's

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    Hi celeste. No they are definitely transformer-ish. There are three sets of iron, each with primary and secondary windings. I have a spare one sitting around and so it is easy for me to take some measurements of the Pr:Sec ratios. They pop up on e-bay from time to time.
     
  4. celeste

    celeste Friend of Leo's

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    I guess this means I am going to have to join amp garage to take a peek. Somewhere I thought I saw patent application for the hammond circuit, but I could easily be mixing circuits up.

    As far as ebay goes, it is my slow season, so I have more time then money, but I do have a coil winder
     
  5. megafiddle

    megafiddle Former Member

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    The second winding was used to control the saturation of the reactor.
    A DC current is run through it to allow variation of the saturation threshold.
    I believe.
     
  6. celeste

    celeste Friend of Leo's

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    Aww yes, that sounds familiar. That is why I remember them as chokes.
     
  7. Bolide

    Bolide Friend of Leo's

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    Mag-Amps? Sweet!

    Magnetic Amplifierss work by the fact that a ferrous core will, if saturated, have a lower effective inductance than it does before it reaches saturation.

    The cores are always in pairs, they are wound so that they are an extremly high inductance to the a.c. applied to the power winding, acting almost like an infinite resistance. In addition to the power winding is a control winding, one with many more turns than the power winding so that a small current can drive the core to the saturation point. Since this then would induce a high a.c. voltage from the power winding to the control winding this is the reason the cores are always in pairs with the control windings arranged to cancel this induced voltage out.

    When there is no current sent to the control winding very little current can pass through the power winding, and thus to the load (which is normally in series with the power windings). When current is applied to the control winding it causes the core to saturate for part of the power winding cycle, allowing large current flow through the power winding while the core is saturated. The higher the current through the control winding, the longer the period of saturation and thus the more power supplied to the load.

    In addition to the power and control windings there may be bias windngs to raise or lower the point of action, and negative feedback windings to adjust the slope of the action.

    In this I would guess that two of the cores are the reactors, and the third is a coupling transformer that acts as the load.

    This looks neat, and I am subscribed in hopes that I can learn something about what sounds like an interesting device.
     
  8. Bolide

    Bolide Friend of Leo's

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    Guess, one of the wild variety

    I am going to guess that this works somehwat like a quadrature amp, with the control oscilator shifting the phase of the audio, doing phase modulation to effect a limited frequency modulation.

    Am I even close?
     
  9. tubeswell

    tubeswell Friend of Leo's

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    Did some very rough sounds earlier this avo into garbageband not properly mic'd (mac built-in mic) and didn't get time to run through all the sounds, will try and find time later in the week. Guitar is a strat. Other amp is my PR clone. Please excuse the shoddy playing

    Links follow:

    http://music-electronics-forum.com/attachments/16501d1324188712-vibe-buggy.mp3

    http://music-electronics-forum.com/attachments/16502d1324188749-viberator-sky.mp3

    http://music-electronics-forum.com/attachments/16503d1324188799-black-sand-vibe.mp3

    http://music-electronics-forum.com/attachments/16504d1324188888-stray-vibe.mp3
     
  10. tubeswell

    tubeswell Friend of Leo's

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  11. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    Hey Tubes, sounds great man!
     
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