The set-up procedure described in the following you will get the Photon Vibe working over its whole speed range and produce a Univibe tone as true as it gets.

Obviously, in order to cram the originally clumsy unit into a modern stomp box format some things had to be changed and features deemed unnecessary had to be dropped. The kit, as it comes, appears very refined and does not need any major modifications. The only improvement I would recommend is a very simple tweak of the front panel LED´s circuit in order to make the visual feedback of the low frequent oscillator (hereafter: LFO) speed more pronounced. This change is quickly done, cheap and non-invasive.

To make it clear, units like that are very old technology that reflect the technical restrictions of the time being. Many modern adaptations have some of those flaws ironed out, but they will inevitably sound - modern.

The methods applied here can be applied to any similar unit of course.
I hope the following will be of advantage.

Before you make any changes, it helps to understand what the corner points are. PedalPCB´s Photon Vibe is a clone of an original Univibe to a high degree.

The  LFO is, exactly like the original, comprised of discrete transistors, but the phase shifting stages are implemented by operational amplifiers (hereafter: OPAs). There is no improvement to be gained by staying discrete what so ever.

Original supply voltages have been approximated by using a charge pump. The LFO is thus really identical. It is an optical unit using a filament lamp and light dependent resistors (hereafter: LDR´s) exactly like the original. Beyond that, they have even spent extra trim pots to hone in for perfect tone unlike the vintage forefathers (a curse as some may come to think).

"The Vibronaut is like an awesome argentinian grilled piece of meat. But the salt and pepper are under the table." - TGP, Lovepedal Vibronaut Photocell Vibe (post #15), user Haunting Mids, https://www.thegearpage.net/board/index.php?threads/lovepedal-vibronaut-photocell-vibe.1779271/

For achieving a Hendrix-style tone in a bedroom environment, the  generally agreed upon opinion is to place the unit before any high gain units (see for example discussion at above link). Remember we don´t have a true Hendrix-like signal chain. The downside of this is, although the unit does not appear overly noisy per se, that residual hiss gets amplified alongside. This is particularly obnoxious since it is the first effect in the queue, but tone indeed is most Jimi-like this way.

This phenomenon is not limited to the Photon Vibe, it happens with other effects too. I tried R.O.G.'s TriVibe, which ends up not any quieter overall.

It uses TL072 OPAs, but similar contemporary units^[1][2] use NE5532 probably for the very reason of keeping noise down. I do not recommend NE5532 in this position, since they draw a lot of supply current which might push the charge pump to its limits. I have tried LM833 (drawing a comparable supply current with with one third of the noise figure), but it was not feeling too well there despite adding ceramic rail bypass caps. So the TL072 OPAs have to stay.

The Photon Vibe has, unlike the original, no vibrato mode selectable, but this may be added easily by basically cutting the dry signal. That said, the vibrato mode appears pretty "special" to a point of being useless, which is probably why it has been dropped. (You can try it out during the setup procedure).

The LFO, as it does in the original Univibe, has as some compensation built into to counteract the inability of the bulb to follow higher frequencies, by increasing the lamp drive with frequency. In other words, the effect depth increases potentially with higher speed settings. Due to this peculiarity the effect depth will appear exaggerated on higher speed settings.

Use the depth control to compensate for this. Although I have never played through a vintage unit, I have reasons to believe that this is a property innate to all filament lamp units. (LED driven units will not suffer from this, but will create problems of their own).

On the contrary, if you optimize tone for higher speed ranges (or don´t care when adjusting), low LFO speeds most likely won´t work, as many people have complained consequently.

The original unit had no full bypass mode (as none of those units of yesteryear due to the non-availability of decent 3p2t footswitches), but the clone has of course.

I noticed that upon powering up, the bypass switch seems to pop a lot, but this effect disappears after a short time.

Note that the filament lamp is permanently on, despite the front panel LED suggesting being off (during bypass mode).
This could be remedied by tying the emitter side of Q3 via R27 to the pin on the footswitch that activates the LED. You thus may want to remove power during prolonged periods of non-usage.

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Before you start, I recommend improving the LED indicator on the front panel. We can put this to good use in the setup process lateron. The LED is meant to provide a visual clue as to what the LFO is doing. The original unit did not have such a visual user feedback, and it was a self-evident improvement to add this feature.

Unfortunately, the implementation chosen does not work well (linking the LED directly to the bulb), since a LED does not vary brightness (linearly) with current the way a light bulb does. The result is that you will barely see the LED fluctuate during operation. A larger resistor does not help, it just shifts overall LED brightness into darker ranges.

We will need to operate the LED close to its knee of turn-on (or, if you wish, to cut-off), and this can be achieved with a Zener diode in series. I used a 10V 1/4W specimen successfully with a blue LED, but a 9V2 gave a smoother turn on characteristic much like the lamp itself. However on low speed settings (where swing is reduced) the display was considered subjectively superior using the 10V rating.

Just insert the Zener diode where the wire between PCB board terminal "SW" goes and the footswitch contact, cathode pointing towards the PCB.

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Adjustment Procedure

Most people (like me when I started off) make the mistake of adjusting the trimpots with both depth and speed pots set wrong. I believe that most people just twiddle the pots until it sounds better somehow. The method described hereafter optimizes for the weakest point, which is low LFO speed, by listening. All other settings fall in place naturally.

Make all adjustments in a dark environment, unless you use a shroud over the opto section. Remember the case is open when you adjust and lamp and LDR´s are exposed.

Trying to adjust the LFO by optical clues alone is not feasible, because the LDR´s change at low illumination conditions is more pronounced than you may think.

Adjusting effect depth and waveform symmetry

TR2 (illumination center value) and TR3 (lobe balance resp. signal symmetry) are both somewhat interactive. Follow the procedure exactly and always just turn one pot. There are several workable settings, but only one will give the deepest signal at low LFO settings.

• before you start, set all trimpots to center.
• turn off depth. Adjust the lamp to a dark orange glow with TR2, somewhere near center.
• turn depth fully on again. set speed to 10:00 (this will yield a LFO frequency between 1 and 2 Hz, which makes it easy to adjust by listening)
• pluck strings on the guitar and listen to the effect.
• adjust TR3 for deepest effect and balance for treble and bass. You will hear if the swing abides more time on the bassy side with the treble lobes hardly "peaking out" or vice versa. All of a sudden the effect becomes "deep", when both swings are balanced. This happens on a very tiny adjustment range on the TR3 pot.
• note the settings!
• turn speed to min and observe how deep the effect is. You will have to turn depth fully up.  Look at the LED as a second clue to get a visual indication (have you fixed the LED display already?)
  
• optionally revisit TR2 and then TR3 (as above) to hear if a small adjustment to one of the sides will make the pulse in low speed modes more pronounced.
  
• note the settings again.
• you can try another run towards the other side of TR2 and see if this is more to your liking.
• decide upon the setting that produced the deepest sweep on low speed settings.
  

Adjusting the mixer

Now we attend to the mixer. TR1 mixes the pitch-shifted signal with the original signal. Near center is optimum, CCW is the original signal, CW the pitch shifted version. This is done by listening. RG.Keen says^[3][4] the impedances of both signal paths be matched perfectly for the most deep effect, but I cannot find such a critical pot setting around the center range.

• set speed to 10:00 or 11:00
• pluck strings on the guitar.
• rotate TR1 fully clockwise and listen to the pitch shifting. This should sound like a warped LP record.
  
• rotate TR1 gradually towards center. Get the hang of what to listen for by going back and fro a couple of times. The warped LP tone is gradually becoming smaller towards the center. Maybe turn back a smidge.

In this PCB, there is no space for a "dustcap" over the optical arrangement. However, no external light can intrude into an aluminium case, and the lid is close and reflective enough for the light. That said, I installed a highly reflective piece of tape onto the lid to be on the safe side.

I have mounted the LDRs flat onto the PCB, as it is shown on the component location print,  and not facing the lamp as some considered. This works flawless.

I have also covered the LED´s interior side (on the other side of the PCB) with black insulation tape to eliminate all possible leakage from the LED flashing on.

This procedure was inspired by R.G. Keen^[4]. Thank you R.G.

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Reference