Looking for help with 'Vibe mod from Geofex site (Dano Chicken Salad)

[danielzink]

Okay....

I've successfully rehoused a Danelectro Chicken Salad:

http://pws.cablespeed.com/~danielzink/salad.htm




Got rid of the switching board, fabbed up a bias supply etc. etc.

While browsing the Geofex site - I came upon this nugget.

A Single Pot Speed Control for LFO's - Univibe, Trem Face, and Others
Here's a trick to get rid of those pesky dual section, odd taper pots for speed controls and a way to get wider range out of your swept pedals that use the phase shift oscillator style sweep.

Where it really shines is in the univibe. The 'vibe uses a dual section, reverse log taper pot that's just a bit easier to find than a dinosaur's tooth. If we notice that the pot is really wired as two variable resistors, we can use an LED/dual LDR module like the VT5C3/2, which costs about $5 from Newark and other electronics distributors. To give us good control, we use a transistor  to vary the LDR current. With a single 10K pot for speed, the transistor varies the current in the LED from nearly off to a voltage set by the V+ voltage minus the LED 1 voltage and the Vbe of the transistor. You take that voltage, divide  by the max current you want in your LED, and insert that resistance into Re. The 'vibe circuit has resistors to limit the min and max resistance the circuit sees. I did this in my tube univibe, and it works great. You can mess with the taper of the single speed resistor (See The Secret Life of Pots) to adjust the effective taper of the speed control pot as you like it.

I emailed RG about it - he replied once - and I haven't heard from him for a couple weeks now. I'm sure he's busy and I don't want to be a pest so - I was hoping the collected minds here could help me sort it out.

The one reply I did get was:

On the dual gang pots - of the 6 tabs on the pots - only 4 are used - and 2
>of them are tied together. That makes for 3 connections - I've got that
>(per your schematic ie: "to speed control pads")
>
>I guess I'm kind of lost from there. I'm not sure where the 2 leads from the

>vt5c go to. Let's think a minute, and I believe you'll figure it out.

A dual pot with only four terminals used is the same as two variable resistors,
right? And with two of those tied together, you have, in effect, a single
center-tapped, variable resistor.

That is very much like the LDR in the Vactec module. What you want is to
find where in the circuit the two shorted-together pot terminals go, and connect
the center tap of the LDR to that place. Then connect the remaining
two terminals where the non-shorted-together pot terminals went.

Okay...I got that...

> As well as how to incorporate the transistor into the circuit.
That's the easy part. You only have to find two existing signals in the circuit -
+9V and ground, like the diagram shows. The two diodes, pot, transistor,
resistor, and LED terminals only share +9V and ground with the rest of the
circuit. So make up the pot circuit including diodes, transistor, resistor, and
probably the Vactec on a small bit of perfboard, then run wires from the
perfboard to +9V, ground, the actual physical single speed pot, and the
three resistor terminals from the output of the Vactec.

.....I think I broke something in my head....I'm still lost here....I have no ideas on values for the resistor, diode and transistor...

I can source the LDR module no problem here: http://optoelectronics.perkinelmer.com/content/Datasheets/DTS_vtl5c2232.pdf

If it helps - here's the Chicken Salad schemo as well.....




Thanks, Dan

[R.G.]

Dan,

(1) find +9V somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a white wire to the pad. 6" should do it.
(2) find ground somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a black wire to the pad. 6" should do it.
(3) Make yourself up a small perf board with the added circuit on it.

LED1 is a red LED, any kind. It will not be seen outside the box and is used only for its voltage drop.
D1 is a silicon diode, any kind, both 1N4148 and 1n400x will work fine; it's used only for its voltage drop.
Transistor Q1 is an NPN transistor with a gain over 100. 2N4400, 2N3904 will work fine, as will most other NPN silicon transistors you can find. Be sure to check the pinout on the transistor you have before soldering it in.
Re is the scaling resistor that converts the voltage on the emitter of Q1 to a current through Q1. It should be sized so that  the 9V supply minus the approximately 1.5V LED voltage plus the approximately 0.6V D1 voltage gives a workable current through the opto isolator LED.

What's a "workable current"? It depends on the LED/LDR's LED. Over 20ma probably kills the led, so Re is going to be bigge than (9-1.5-0.6)/0.02A = 345 ohms. How much bigger? You need to put a pot in there and diddle it until the control range from min to max is what you like for your LED/LDR.

Some LED/LDRs are very sensitive, needing less than 1ma for full-on to essentially full-off on the LDR. That would be an Re value of 6900 ohms. So you're looking for a value between 345 and 6900 ohms.

(4) wire the new little perf board into your box.

(5) take two aspirins

(6) let us know how it works.

[danielzink]

Can do - thanks R.G. !


Dan

[danielzink]

The aspirin did help.........

How'd I do ?

Thanks, Dan

[R.G.]

Pretty well, except that it looks like the base of the transistor is going to the LED and the collector is going to the pot wiper.

And that it's an odd pinout transistor. If that was a US- style TO-92 like a 2N3904, the collector would be where you show the emitter, the base would be in the middle, and the emitter would be where you show the base. So a 2N3904 in that position, but turned 180 degrees so the collector contacts the VTL should work.

[danielzink]

Pretty well, except that it looks like the base of the transistor is going to the LED and the collector is going to the pot wiper.

And that it's an odd pinout transistor. If that was a US- style TO-92 like a 2N3904, the collector would be where you show the emitter, the base would be in the middle, and the emitter would be where you show the base. So a 2N3904 in that position, but turned 180 degrees so the collector contacts the VTL should work.

No.....that's an odd person not paying attention to diagrams.....(revised schemo posted).
From what I'm reading online a 2N2222 and the 3904 have similar gains ? (I have a 2222 on hand so it saves me a trip if it works - plus I already have the diode on hand - all I need is the vactec)

Thanks Again R.G.

[danielzink]

We're doing this to replace a dual gang reverse audio taper pot. Would it be wise to say the 10k pot in this circuit is also an audio taper ?
I see the notes that say we can adjust the effective taper of the pot with the resistor.....


Dan

[R.G.]

From what I'm reading online a 2N2222 and the 3904 have similar gains ? (I have a 2222 on hand so it saves me a trip if it works - plus I already have the diode on hand - all I need is the vactec)

No, the 2222 is typically 75-100, the 3904 is about twice that, 200 or so. But a 2222 ought to work OK. It's not a demanding app for a transistor.

The transistor is now correctish, but is 180 degrees reversed. The emitter goes to ground, the collector to the Vactec.

We're doing this to replace a dual gang reverse audio taper pot. Would it be wise to say the 10k pot in this circuit is also an audio taper ?
I see the notes that say we can adjust the effective taper of the pot with the resistor.....

That's a clever and insightful thing to worry about next, but no. You also have to worry about the curve of LED current to resistance. That's way nonlinear, and in fact just about does the unwinding you want to use a linear pot, at least on the few of these I've built. Put in a linear and replace it if you don't like the results.

[$uperpuma]

Dan... if you need some 3904s, I'll send some with your trotsky...

[selectortone]

Hi, I'm in the middle of re-housing a Chicken salad pedal - what exactly are the benefits of replacing the dual pot with this circuit? - Thanks.

[danielzink]

Hi, I'm in the middle of re-housing a Chicken salad pedal - what exactly are the benefits of replacing the dual pot with this circuit? - Thanks.

I'll go back to the original quote from the Geofex site:

A Single Pot Speed Control for LFO's - Univibe, Trem Face, and Others
Here's a trick to get rid of those pesky dual section, odd taper pots for speed controls and a way to get wider range out of your swept pedals that use the phase shift oscillator style sweep.

Where it really shines is in the univibe. The 'vibe uses a dual section, reverse log taper pot that's just a bit easier to find than a dinosaur's tooth. If we notice that the

Dan

[selectortone]

Dan, thanks. I'll be interested to hear how you like the mod when it's done.

[mike_a]

Hi there Dan,

Really nice work with the rehouse.
I wanted to know if you have a schematic with all the part values.
especially those of the caps in the opamps feedback loop.

Thanks,
Mike.

[danielzink]

Mike - the only CS schemo I've got is the one posted above (my first post - starting this thread - last picture).

If I find anything else I'll post it up.

Dan

[mike_a]

Thanks again Dan.

If it's not too much of a bother, could you tell me the values of the feedback caps of the phase-shifting opamps?

[danielzink]

I'm trying to etch only one board for the bias circuit and the LFO control.



It's funny.
The original thread over at HC was all about how to eilminate the switching board and rehouse the pedal.
Now we're back to two boards again...ah well...we're having fun..

If it's not too much of a bother, could you tell me the values of the feedback caps of the phase-shifting opamps?

PM me in engrish prease.....


Dan

[danielzink]

I finally have a few things behind me and I can get back it this.

need to put a pot in there and diddle it until the control range from min to max is what you like for your LED/LDR.

Some LED/LDRs are very sensitive, needing less than 1ma for full-on to essentially full-off on the LDR. That would be an Re value of 6900 ohms. So you're looking for a value between 345 and 6900 ohms.

RG - what's your thought on mounting a 10k small square trim pot on the board - and leaving it there - or - could I run a 10k 16mm pot to the outside of the enclosure ? will the value set (between 345 and 6900 ohms) be a "set-it and forget-it" ? or will it be any kind of a useful tool to vary the sound.

Thanks, Dan

[R.G.]

For me, it would be set and forget. It sets the range and feel of another control, and by getting it the way you like it best, you're already tuning the control. But there is a view that everything ought to be tunable on stage.

[beatstrat]

Good works, Dan.  The Spirit of DIY!

[Gunnar]

Felt like reviving this old thread since it contains the info needed to do the mods on the Chicken Salad.

well here's an aditional mod:
to mee the max speed of the unit was  too slow, so I decided to do a mod to make it go faster when the rate knob was maxed out.
It's a very simple mod although it's a bit difficult to fit in the resistors due to lack of space! But using a bit of wire it can be done quite easily.

Here's what you have to do:

Jumper the two 4k7 ohm resistors between the dual pot and the diodes, with resistors of the same value  (4k7 ohm)

That'll halve the value of the resistance between the rate pot and the diodes when the pot is turned to max speed (which actually means that it's at zero resistance)

(hint: do not try to jumper the actual resistors! jumper from legs of the pot to the leg of the nearest diode and to the cap between the pot and the other diode!..Do not jumper the cap!)

If you want even faster max speed go for 3k3 ohm resistors..that'll put the max speed way over the hill  

The change in effect is that you get a lot faster maximum speed, but the slowest speed will be the same (or maybe 1-2% less)

With this mod the pedal sounds like a great tube-like vibro effect when at maximum speeds, but without loosing any of the univibe swirl at lower speeds  

Enjoy