Sluckey Trem-O-Nator optocoupler drive

[W5FH]

Hello-
   I am wanting to get the Sluckey Trem-O-Nator to drive the X-Vibe VTL5-C1 optocoupler to a very low resistance, in order to get a very deep tremolo. I am getting a pretty deep tremolo now but not as deep as a pedal that this Trem-O-Nator is replacing. I am using a 250K intensity pot that is NOT grounded; there is no draining of audio signal to ground when not in use. The intensity pot is simply a variable resistor in series with the VTL5-C1 resistor cell going which is going to ground. It is connected to signal path right upstream of coupling capacitor going into phase inverter (off of Master volume wiper). So, in theory I should be able to get a very deep tremolo with intensity pot at zero resistance. I assume the greater the DC drive to the VTL5-C1 LED cell the lower the cell resistance. Can I experiment with cathode resistor values in order to increase the DC drive to VTL5-C1? My circuit is an exact copy of the one shown by Sluckey. Thanks.

[sluckey]

Remove the 10K that's connected across the LED. Any better?

[W5FH]

OK Sluckey-
  I believe you are speaking of the 10K resistor from pin#3 cathode to ground (There is only 1 of 10K resistor in circuit, yes it is in parallel with the VTL5-C1 LED). Just making sure. I will try that this evening when I get amp out of cabinet. Just curious right now - by removing this 10K resistor, will that affect osc operation in any manner?
Thank you!

[sluckey]

It "might" increase the brightness of the LED. This is what you are wanting. Best way to tell would be to temporarily replace the VTL with a 5mm red LED. Notice the brightness with and without the 10K. I would connect the 10K with gator clip leads so you can quickly touch the resistor to the circuit. The whole purpose of that resistor is to keep the LFO running for quick start up. Increasing the size of that resistor up to about 47K may be better than totally removing the resistor.

Experiment.

[W5FH]

Yessir-
   Just the guidance I was seeking, I will experiment with that resistor. Thanks!

[TenderTendon]

I spent a lot of time with this and honestly think the information I gathered will help some here. I also noticed that the tremolo using a 5C1 Vactrols does not go as deep as I had hoped. I have found that there's 2 reasons for this. The first is that there is not enough voltage on the cathode to drive the Vactrol to a low enough resistance state. More on that later. The second reason is that I have found through my testing that the XVive Vactrols are pretty much cheap junk, that don't perform as good as the spec sheet says they should. I bought 5 of the XVive vactrols and tested all of them. Actually I have been testing these for quite a while, as I was going to use them in another build for channel switching.

When I started testing the tremolo circuit, I noticed the tremolo wasn't going quite as deep as I knew it could. I put a scope on the cathode and noticed the peak voltage feeding the vactrol LED was quite a bit lower than needed to get it to a low resistance state. Playing around with load resistors and cathode resisitors/caps, the most peak voltage I could get was 1.56v. Next, I took all 5 of the vactrols to a very good power supply and put 1.56v on them. On average, the vactrols passed slightly over 1 mA. At this current, i checked the resistance of all of them and they ranged from 3200 to 4500 ohms. This explains the tremolo effect not being as deep as it could. Next, I drove all of them to 40mA current to check resistance there. The resistance of the 5 ranged from 210 ohms to 400 ohms. None of the met the specification sheet stating 200 ohms@40mA. Even so, 400 ohms is much lower than the 4500 I'm getting now. Getting the vactrols to 40mA required approximately 1.86v and that was consistent amongst the 5 I tested. If we can get the cathode to source 1.85v or so peaks, I think it will make a dramatic difference in the depth of the tremolo pulses, but getting there is beyond my pay grade and I wouldn't even know where to start. For now, I have left it as is, as I like a mild tremolo effect. The next person to use this amp next might find it lacking though.

[TenderTendon]

I forgot to mention that due to the reasons listed above, I decided not to use them for channel switching in my SLO-100 build. I found some vactrols that have fantastic input current to output resistance characteristics. You can get them binned/sorted that are guaranteed to have no more that 124 ohms resistance with only 1mA LED current. Some of the ones I received are less than 100 ohms @ 1mA and less than 30 ohms @ 10mA. The only problem with these is that they have a slow recovery time when the LED is turned off. The ones that I have take approximately 1 second to go from 30 ohms to 1M ohms. This is fine for channel switching, but obviously too slow for tremolo applications.

[sluckey]

TT, the voltage across the vactrol LED is determined by the forward voltage drop of the LED, not the current flowing through the tube. Once enough current flows through the LED to forward bias it, the voltage remains constant. The amount of current flowing through the LED determines how bright it is and that brightness controls the resistance of the LDR.

I think your LED is not getting enough current through it to reach hard forward bias, causing the led to not glow as bright as it should. Possibly changing the 10K to 47K (or just totally removing it) as I suggested to W5FH would help? Didn't I suggest this to you also?

[TenderTendon]

Yes, I tried everything you suggested and more. I could never get more than 1.56v peaks on the cathode and 1.56v correlates to slightly over 1 mA passing through the vactrol LED.