SF/BF Twin Reverb- a couple of questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by postalblowfish, Sep 22, 2008.

  1. postalblowfish

    postalblowfish Member

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    I have an AC568 Twin that has been blackfaced. Is it possible to convert the Bright Switch into a Bright Knob to control the amount of brightness? If so how would it be done? Also: the reverb on my Twin has developed a low level buzz/hiss (120hz?) that increases with the reverb knob but not the volume knob. I've switched the 12AT7's, V4 12AX7, and send and return cables with known good, and cleaned the inputs on the amp, tank, and footswitch. Still there. What else could be the culprit? Reverb tranny? Tank? The reverb still sounds great except for the little buzz, which is noticable with the volume on 2-2 1/2 and the reverb on 4 or so.
     
  2. GearHeadFred

    GearHeadFred Member

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    The bright switch inserts a capacitor - not a resistor - into the path, so it's not as easy as adding a pot (variable resistor).

    Regarding the buzz... you've done the right things.. Disconnect the footswitch and make sure the noise is not coming in on that path.
     
  3. postalblowfish

    postalblowfish Member

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    I appreciate your reply, Fred. I've disconnected the footswitch and still *zzzzzzzzz* Any other options? To be clear it's not terrible, just loud enough to be annoying. Also, I've checked the outlet ground and plugged into an A/V power filter. Still there.

    About the Bright Switch: I kind of figured it would be involved, I just haven't heard of it being done before and wondered if it were possible and how much work it would take.
     
  4. SatelliteAmps

    SatelliteAmps Member

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    You could add a rotary switch to switch in different value caps. It wouldn't be as smooth as a pot, but it would add more variations to the bright value.
     
  5. postalblowfish

    postalblowfish Member

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    Kind of like an ES345? Neat idea. When I get some time I'll experiment with it. Thanks!

    Any other ideas on the reverb?
     
  6. SatelliteAmps

    SatelliteAmps Member

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    Orange used a cascading capacitor rotary switch that would probably be easiest to copy.

    Noise can come from a failing cap, or a bad resistor. That is where I would look next.
     
  7. Swarty

    Swarty Member

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    Try a 1m pot wired as a variable resistor in series with the bright cap. Regarding the reverb noise, did you change the orientation of the tank, if so, that can do exactly as you describe.
     
  8. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Has the amp had the electrolytic capacitors replaced?

    I find Twins to be quite bright...what speakers are in your Twin?
     
  9. postalblowfish

    postalblowfish Member

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    That's an interesting idea, too, Tim. And, yes I played around w/orientation. The noise seems lessen w/ the tank out of the cab and away from the PT and increase as I put my hand near the little transformer on the output side of the tank. If I get a chance I'm going to switch tanks w/ a SFVR this weekend to hear the effect. I'll have to make an appointment w/ you at some point if I can't hunt it down. I like my 'verb and this is annoying.

    Thanks. The amp sounds good w/ humbuckers & bright switch on. Just a tad shrill w SC's and bright switch on and a bit dark w/ switch off. I was thinking about a way to tweak a happy medium on the amp as I generally prefer the tone knob on my guitars wide open.

    Yes, serviced about 3-4 years ago, played 3-4 hours a week since.

    Jensen Vibrantos. I've come to prefer running my Twin 'Nels Cline' style: Treble=2, Mid=9-10, Bass= 4-6, depending on circumstances.
    http://www.nelscline.com/tech.html.
    Tames some shrillness and sounds meatier at 3-4 volume, which is about the max w/ this amp in my world. I like the bright switch on for a little extra sparkle but not too much extra.

    Much appreciation for the input, everyone.
     

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