Uni-Vibe cancel feature

Discussion in 'Effects, Pedals, Strings & Things' started by shakti, May 22, 2011.

  1. shakti

    shakti Member

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    I've been messing around with a KR Megavibe, trying to figure out how to make it do a "cancel" feature as well as the stock true bypass switching.

    From looking at the schematic here, it looks like the cancel feature is simply bypassing a 47k resistor to ground:
    http://www.geofex.com/article_folders/univibe/univtech.htm

    I've tried to bypass all the several 47k resistors inside this thing, but can't make it cancel out as it should. Has anyone tried this successfully?

    I want to do this in order to have the Uni-Vibe preamp running in the chain when I want it, but without the effect. I believe that preamp sound to be a significant factor in Hendrix' BOG tone (yeah, I know...another one chasing it...).

    One further question: when you turn down the intensity, the effect eventually "freezes" and the light to the bulb appears to be turned off. However, at this setting, the sound appears quite severely altered from stock bypass. Is this setting the "same" as with the vibe in "cancel" mode on an original? I also notice that the tone alteration is much less severe in vibrato mode than in chorus mode.

    Lastly, according to Stuart Castledine (www.wah-wah.co.uk), there were several small differences to the early Uni-Vibes. I've also heard many people say that different vintage Uni-Vibes affected the sound in cancel mode to a varying degree. Could this be one of the reasons that the BOG sound is rather unique in the Hendrix catalogue?
     
  2. catalinbread

    catalinbread Member

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    I wasn't able to glean from your post if you've shorted the base side of the 47K to ground or if you are lifting it off of the ground? Does your LFO shut off?

    Cancel setting sound depends on where the bulb bias trimmer is parked... Off the line the trimmers probably weren't set exactly the same unit to unit which is no doubt a big part of the variation in sounds on records... I doubt back in the day many players were messing around with that trimmer as a control surface. You'd probably get a lot of milage out of the bulb bias trimmer for your BOG parking spot sounds. Unfortunately, messing with this trimmer will change the offset of the LFO sweep, so if you find a good cancel spot, you may not like the throb range when it is reengaged. Put an offset knob on top! :)

    I suspect the intensity knob is parking the bulb at a different brightness than the cancel setting does. If so, the sounds will be different for sure. We've had a vibe on the breadboard for quite awhile now, you have me curious enough to check out its behavior tomorrow... Edit: Just looked at the schem, forgot about that inline 47K resistor, the intensity off will be different from the cancel sound. I am still curious to know by how much!
    :cool:
     
    Last edited: May 22, 2011
  3. shakti

    shakti Member

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    Thanks a lot for your reply, very informative!

    From what I could gather from the schematic, there's a 47k to ground. The cancel feature basically shunts in a bypass across that resistor, which was what I was trying to do (clipping in an alligator clip). I had one clip attached to ground and tried to insert the other clip at either side of every 47k I could come across inside there, as I don't know which one is the right one (can't accurately trace the circuit from the top side of the circuit board). I couldn't make the bulb turn off at any of these spots, but truth be told I didn't have it hooked up to an amp, so I don't know if maybe the LFO was canceled? I thought the cancel was supposed to shut off the bulb?

    edit: the bulb kept flashing, so the LFO must have kept running...
     
  4. Truxton Spangler

    Truxton Spangler Member

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    I can send you a picture of the bottom of my large KR Megavibe circuit board if you want

    The traces are identical to an original univibe though

    Best regards -T
     
  5. shakti

    shakti Member

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    Thanks for the offer! If you could upload it here, maybe someone will able to identify the correct spot? However, I have a small Megavibe, so it won't be identical.

    If I can get this to work, I have thought about rehousing the Megavibe and maybe even go all out and modding my speed pedal to have a heel down cancel switch...
     
  6. catalinbread

    catalinbread Member

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    If you can find the base of the bulb driver transistor you can shunt that to ground. You can also do a continuity check to the 47Ks from the base if you need a larger place to grab onto with an alligator clip.

    Cancel may or may not turn the bulb off all the way, it will stop the LFO from modulating the bulb though.
     
  7. shakti

    shakti Member

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    Success! Well, sort of...

    After taking out the board and tracing it according to schematic, I found the right spot; the junction between a 4k7, 47k and 100k. Shunting that point to ground turns the bulb off and, as far as I can understand it, acts just like the heel down cancel feature on the originals.

    The bad news is that it didn't sound very good at all. In fact, it sounded like a very weak ring modulator, which makes me think that the LFO somehow affects the signal even with the bulb turned off? This bad "cancel" sound was not affected by any settings or even the internal trimmer - same cheesy ever-so-slightly ring modulated sound. I can't imagine this is how it's supposed to sound, even though it doesn't have a reputation for being a transparent bypassed sound.

    Any suggestions? If I found the right spot, it would be very easy to mod it to accept a cancel footswitch, or even a more complicated heel-activated momentary switch. However, not the way it sound in these preliminary tests. I love the vibe sound of this thing, but I've been more and more set on having that heel activated cancel...
     
  8. catalinbread

    catalinbread Member

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    The cancel does not have a reputation for being transparent, yr right... Afterall your signal is still going through the input gain stage and all of the phase shift stages, they are just "parked". As far as a ring mod sound... The LFO doesn't go fast enough to be causing that, seems to me. I wonder if there is something going on with the biasing of the bulb driver in cancel mode that is putting the bulb in a sort of inbetween state and fast flickering as a result? I wouldn't expect a bulb to respond fast enough to do this though. Have you messed around with the bias trimmer?
     
  9. shakti

    shakti Member

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    I tried to adjust the only trimmer inside there, but it didn't seem to affect the cancel sound, at least not the ring mod aspect of it. I assume it's a bias trimmer - adjusts the brightness of the bulb and the "center" of the sweep - all the way from otuside audible range to quite deep and throbby.
     
  10. Morgan24

    Morgan24 Member

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  11. teddy boy

    teddy boy Member

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    Getting my castledine supra-vibe next week! Can't wait to try it out with the cancel-mode. The demo's I've heard sound fantastic and the cancel-mode was one of the main features why I wanted one! Yep, I'm all about rolling of some highs of possible...
     
  12. shakti

    shakti Member

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    I'm looking into the Castledine as well, for precisely that reason. Having it heel-activated would put it over the top for me, but giving up on the Megavibe sound would be veeery hard.

    Let us know how the Supra-Vibe works out for you. Clips would be great if you can do them - with the cancel compared to the bypass.
     
  13. shakti

    shakti Member

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    Thanks for the tip! I've just registered, will check it out. Maybe these changes affect how the cancel sound will be?
     

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