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AA764 Tremolo Depth

Discussion in 'Shock Brother's DIY Amps' started by Linkjr, Sep 14, 2020.

  1. Ten Over

    Ten Over Tele-Holic

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    No. It's the one in the picture that is above it and that connects to the orange capacitor. You have the stock 100K instead of the 220K in the Mojo schematic. However, the Mojo layout shows 100K in that position.
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I agree, Link, it's good to practice reading schematics -- in a UK analogy, they're sorta like the London Underground map, showing how electrons get to work, while the layout is like a London street map that just shows where the underground stations are located.

    Still, they ought to agree!!! I look at Mojo diagrams but I don't build from them. Mojo is a fine outfit, but they're a small shop and their diagrams may contain errors, inconsistencies, and outmoded or idiosyncratic choices. When I was starting out I'd often mark up schematics and layouts. Whose? Fender's. Their component values are often the 'gold standard' (you may nowadays use a 2 or 3W first dropping resistor, but you probably still want 1K value there). I happened to label this when I was restoring a VC. Note Fender actually numbers the pins here (helpful going back to the layout) -- the only other thing you need to know is they put dots where wires connect, and no dots where they just cross each other. So it becomes obvious where the plate resistors are on V1; they connect to pins 1 & 6.

    VC scheme V1 detail.jpg
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Link, a brief study of the history of vacuum tubes might help one to understand a schematic..

    lhttps://en.wikipedia.org/wiki/Vacuum_tube


    Along with that, Tube Data Sheet Locator will lead you info on a particular tube....including the pinout which identifies those elements that are discussed in that history and development article on Wikipedia to which I linked.
    http://tdsl.duncanamps.com/

    oh yeah, speaking from experience——some of it very recent——I will note that it helps one knows what schematic one should be looking at!!! Lol...of course, this is a reminder to myself as much as anything!
     
    Last edited: Oct 2, 2020
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  4. Linkjr

    Linkjr Tele-Meister

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    Thanks for those [email protected] @King Fan that is a nicely labelled up schematic! That information is a revelation, Why doesn't someone come up with one of these for all the old fender schematics ... had plenty of time lol

    Anyway i swapped in a 220k on v1b - thanks for explaining @Ten Over
    The trem is the same as before but ..... the amp seems to have more bass, also it definitely sounds warmer to my ears. As far as these amps go im a traditionalist and since it hasn't helped my Trem and its not on the fender schematic i think i will swap it back for the 100k.
     
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  5. Ten Over

    Ten Over Tele-Holic

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    You can get a general idea of the tremolo strength at the cathode follower by placing an LED between the 68K resistor and the wiper of the Intensity pot with the tremolo running. This LED should light up pretty good with the Intensity at "1" and decrease in brightness as the Intensity is increased towards "10". The brightest flashing should be considerably brighter than the faint flashing when the LED was used for the cathode circuit of V2a.

    If you decide to try this, the cathode end of the LED goes to the wiper. Heat sink the LED while soldering.

    If you turn the tremolo off with the footswitch and then turn the Intensity up, V1b will become cold biased and the volume will decrease. The sound quality will degrade at higher settings. Some people like this.

    The cathode bypass capacitor on V1b is shown as 25uF on the Mojo schematic. 5uF would be better because 25uF starts shunting too much of the higher tremolo frequencies to ground. Fender used 10uF, which is better than 25uF but not as good as 5uF.

    The 50K-RA pot is not optimal for this tremolo circuit. If the cathode follower is found to have a strong signal, you might try a pair of 20K resistors on the Intensity pot.

    Vibro Champ Intensity Alt 4.png
     
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  6. Linkjr

    Linkjr Tele-Meister

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    @Ten Over thank you for that fantastic schematic, and sorry for lack of updates, i have ordered in components and have put in a single 20k on the intensity pot to ground i noticed the span of the pot seemed better im not sure if that was the intention. i also subbed in a 4.7 uf cap in v1b
    I have not received another LED yet to check the strength of the trem but i have noticed an improvement with the 4u7 cap. it sounds on par with a 70s bronco amp i saw a video that displayed its trem settings in the video.

    one question do these mods to get more depth out of the trem change the tonal characteristics of the vibrochamp eg the “shunting higher tremolo frequencies to ground“ or are my ears deceiving me? there seems to be more than 1 way to skin a cat and i would like to keep my vibrochamp as close to sounding like a 60s blackface vibrochamp as possible.
     
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