Electrovibe

Gordo

Well-known member
And the money shot. I got the knobs in from SmallBear today (and if you've been noticing a delay in shipping lately due to staffing and Covid, it's pretty much gone and service is very fast). This thing is sick.
 

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Gordo

Well-known member
Thanks folks. This is a really special one for me in that it duplicates both the coolest parts, and the short comings of the original. I think it would be hard to do one without the other in terms of being legit.

I'm building the other board I got for a buddy of mine so I'll get a bit more detailed on this one. I need to bone up on the tech dissection from GEO to see if I'm missing anything.

Is this the be all end all Univibe? Maybe. But I'd bet no two vibes sound the same and with a half dozen under my belt none of them sound the same either. It's like building Fuzz Faces. This one doesn't cut corners or introduce much in terms of progress. It simply displays what the original was/is. I'll detail any differences on the second version. I'm interested to see how this pans out.
 

Gordo

Well-known member
Here's the revised (new LED's) first one and the buddy-build. Fancy light shield and shielded cable don't seem to make any difference at all, nor does making sure the transistors are all in range of each other. For the record I found it easier to dial in the sound if there was some distance between the LDR's and the lamp. You can drive the lamp a little bit brighter and have a bit more range on the Bias trimmer.
 

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cooder

Well-known member
Excellent and thanks for the details on lamp ldr placement and stuff.
As I have never had the chance to play an original, what do you mean exactly with 'coolest parts and shortcomings of original'; would love to hear your take on that a bit more in detail. You mentioned noise being an issue?
 

Big Monk

Well-known member
Anybody know the current draw for the Electrovibe? I don’t have any taps greater than 100 mA on my two power bricks anymore.
 

Big Monk

Well-known member
Got the little beastie together and waiting for paint to dry completely. My switch doesn't look as nice as the PedalPCB version (the painted part) but will touch it up tomorrow.

I'm noticing a slight volume drop at max volume (which, again, is true to the original). Any thoughts on getting more signal out of this puppy? Was hoping to hit unity at about 3/4 or so.

@Gordo did you figure out how to squeeze a little more volume out of it after all?

I was looking through the schematic after seeing these two posts and here's my take (could be way off here)...

Not knowing a ton about the univibe architecture, I assumed that the switch right before the Volume control is the SPST for Chorus/Vibrato. I ended up pulling R.G.'s "Technology of the Univibe" to confirm:

UV1.PNG

UV2.PNG

Again, I'm taking a shot at this but could be wrong. I'm assuming most people don't actually use Vibrato mode here but I'll crunch the numbers anyway.

In Chorus mode, you take the output off the 100k/100k voltage divider into the Volume control. I assumed the Volume control in parallel with R17 here and then calculated the output signal as a percentage of the signal hitting R16 and got 33%. This should be similar to those who have ever modified an Epiphone Valve Junior, as that circuit has a similar arrangement but with 1M.

So if we simulate an infinite impedance by using 5M for R17, we'd get 50% signal through instead of 33%. What I'm not sure of is how this affects the rest of the circuit. A less invasive measure for the actual circuit topology might be to swap the Volume control out for a higher value.

If we swap the 100kB for a 250kB and 500kB, respectively, the available amount of output signal goes up to 42% and 45%.

For Vibrato mode, the output is taken off the R20/R18 divider. There seems to be considerably more signal getting through to the Volume control here so you'd have to adjust the Volume control in this mode or tweak the resistive divider, which again, may have downstream impacts.

A simpler way might be to change R4 and get more signal into the circuit in the beginning:

UV3.PNG
Again, fans of the Epiphone Valve Junior will remember that one of the earliest mods was correcting the input voltage divider that was present on the V1 and V2 circuits. In this configuration, the 1M resistor originally found across the input jacks in the Tweed Champ circuits was placed AFTER the input resistor in the early Valve Juniors. This formed a resistive divider at the input.

Here, the R1/R4 divider can be tweaked to allow more signal to pass into the circuit. Stock, it lets 41% of the signal get through. So if you find that you are having volume issues, you can tweak R7 up. For example, 100k goes to 45%, 250k to 48%, 2M to 50%, etc.

Here, the R1/R4 divider can be tweaked to allow more signal to pass into the circuit. Stock, it lets 68% of the signal get through. So if you find that you are having volume issues, you can tweak R7 up. For example, 100k goes to 82%, 250k to 92%, 2M to 99%, etc.

(Edited because my simple voltage divider calcs were off because of a copy/paste error in Excel)

Not sure if I'm way off base here or not.
 
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Gordo

Well-known member
I might diddle with this a bit. My assumption is that the unity or even slight volume drop matches the original to my recollection. It doesn't bother me but it would be cool to have a bit more umph. Vibes round out the top end a bit so there is a perceived level difference as well.
 

Big Monk

Well-known member
I might diddle with this a bit. My assumption is that the unity or even slight volume drop matches the original to my recollection. It doesn't bother me but it would be cool to have a bit more umph. Vibes round out the top end a bit so there is a perceived level difference as well.

When it comes to reaching Unity Gain, there is no need to be traditional!
 

Big Monk

Well-known member
I’m getting my parts order together. The only thing I’m going to deviate on is increasing R4 for better control over unity gain and to use a bi-color LED for speed.

I’m going to have a matte dark gray enclosure drilled through Tayda.
 
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