The Univibe Angle

His guitar slung across his back, his dusty boots is his cadillac.

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Eb7+9
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Re: The Univibe Angle

Post by Eb7+9 » Fri Dec 11, 2015 3:23 pm

Tek465b wrote:any tip and trick on setting the bias offset and bulb driver gain using an oscilloscope?
I know a years or 2 ago i used my scope and signal generator to do it, but i really can't remember how i did and it was probably not the best way.
sorry, I'm not sure how a scope would help here

the reason I say this is because scopes measure voltage
and the Univibe's bulb driver circuit drives a "current" into the bulb

scoping the voltage on the downside of the bulb will reveal little practical information IMO

---

this is how I do it ...

once the basic signal path and everything are working distortion free, etc etc ...
I sit the unit on the bench all opened up and keep a single desk-light on //
the rest of the room should be pitch black when said desk-light is switched off
the desk light is used to help adjust trimmers if need be, that's all ...

you'll notice right away how the light "kills" the effect when turned on ...
so, you need to turn off the desk light to ascertain Vibe performance ...

duh, yeah //ok ...

I have a test guitar is sitting in its stand nearby so I can graze the strings and let it ring
the pedal is connected to gtr and amp ...

the idea is to correlate behavior of bulb against the effect quality
you really need a visual on the bulb // so, now light shield ..

in particular you need to see how close to completely shutting off you want
when the bulb goes completely out you might get a bit of gating

the re"vibe will wound lame when the bulb doesn't come close enough to going completely dark during its cycle
you then make use of the offset control to incrementally approach the sweet spot

it takes many small iterations of both controls to get "there" ...

now, what makes it difficult is the fact that the oscillato does not put out an even LFO signal amplitudes over its frequency range ... so, I tend to focus at the MINIMUM speed setting and get that to work really well (or as well as possible) ... then I go to the mid-speed area where the LFO signal blows up,and verify that the DEPTH control gives me what I want

of course, it might take some experience to "know" how best to set things up here
but a little patience and you WILL nail it eventually

the key is setting the "edge of darkness" point at min speed
(if that makes sense)

once you feel you are done, stick the light shield back on and then the cover ... done !!

if you feel like tweaking it again, having gone thru the process visually like this
you don't have to remove the light shield, etc ...

you should then be able to do it by ear confidently

only last tip ... when the light is glowing onto the cells
make sure that each cell is being washed fully by light

ie., make sure there are no shadows // either from adjacent cell edges, or from the
posts inside the bulb hodling the filament

having done this, and crude-matched the cells, and adjusted the bias correctly ...
should get you the best/deepest Vibing possible

hope this helps
~jcm
modern VT circuit analysis and modeling: https://viva-analog.com/product/ifmta-book-pdf/

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Xplorer
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Re: The Univibe Angle

Post by Xplorer » Sun Jan 03, 2016 5:14 pm

all this reminds me a few things ;) i just didn't practice it yet but i'll soon have all the parts.

just wondering, does anyone knows perhaps a little company who could cut an aluminium sheet, anodised in black and printed with white paint ?
it would potentialy be for a Univibe front pannel replica. i'm expetcting some answers from a company who may do that but nothing sure.

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