What are Vactrols? (VTL-5C3 VTL-5C1)

[rockon1]

I noticed in the schematic of my Palomino V32 theres a bunch of these devices in the circuit. What is thier purpose? Some sort of resistor? Why use them? theres a few VTL-5C3's and VTL5C1's . Thanks Bob

[Robert M. Martinelli]

They're optically-coupled variable resistors; the ones you mentioned are a modern version of the "bulb ( or neon ) lamp/photoresistor assemblies" often found in old amps ( mainly in vib/trem circuits ).

In these new types, the lamps have been dropped in favor of LEDs, which have a shorter response time and a longer life ( provided they're operated within their safe limits ).

The way they work, however, is pretty much the same: the emitting device varies the light intensity together with the input signal and the receiving device changes its resistance accordingly. As I said, they' re usually found in vib/trem circuits, but they can, of course, be used for other purposes as well ( e.g. optical volume pedals/wahs and the like ).

Hope this helps

Best regards

Bob

[rockon1]

Thanks any info helps! Why is my V32 riddled with them? No tremolo or anything? Thanks Bob

http://www.schematicheaven.com/newam...2_palomino.pdf



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[Jerry]

They're used for channel switching.

[rockon1]

They're used for channel switching.

Ive got a bunch of channel switchers. Never noticed them in the schematics. More than one way to skin a cat I guess. Im learning but what I know could fit in a thimble. lol! Bob

[Don Moose]

On schematics, they are often just called LDRs* - Mesa uses them by the truckload, and refers to them as such.

Hope this helps!

* Light-Dependent Resistor, properly the name of just the resistive element.

[Enzo]

And if you just google "vactrol" the first hit on there is the manufacturer.

It is a resistor that can be controlled. It is a photocell with a light source in one package. Turning the light off and on turns the resistance up and down. You can use it for anything you want.

As a trem it is obvious - wire it across the signal path and the light can control the volume by varying the resistance. Put it in series with a signal and you can turn it on and off, either blocking the signal or letting it through. Likewise, if wired across the signal path to ground, then you can turn it off and on to either allow the signal past or to ground it off. Those two methods have obvious applications in channel switching. Or turning reverb off an on or whatever. How about in series with a cathode bypass cap - turns the cap off and on to control gain, like a boost switch. Many uses. It is another way to do a similar job to a relay or a JFET.

[RWood]

Would the following symptom indicate a failing LDR........or does it 'come with the territory' with this method of channel-switching?

The amp's owner reports that when engaging his channel-switch, there is a slight delay (he described it as 1/10 sec) before the channel switch actually occurs. There's a brief drop-out. Does that seem normal or is something faulty?

The amp is a Crate Palomino V-32

Schem: http://www.schematicheaven.com/newam...2_palomino.pdf


RWood

[sean k]

Just as an aside, those things can get pretty pricey and often you can find LDR's (Light dependant resistors) by themselves and add an LED and file off the top of the LED and glue it to the LDR with super glue and cover it in heat shrink. This can be good because you can somewhat tailor the response you want to have by choosing an LDR with the right dark and bright response and an LED tuned to the specific wavelength of the LDR or just outside it.

Oh, and with channel switching theres often 2 vactrols with one coming on and the other turning off and then theres the switching circuit which controls the vactrols and that can be logic stuff or something else thats cheap to implement so theres quite a signal chain to follow to find out where something is wrong.

With your Palamino it looks like the switching is acheived with Q4 and Q3 on page two and the most likely cuprits are the electrlytic caps C50 and C51 given the switching lives of all the other parts in the circuit the caps are the most likely to fail. and given the fault has a charge up time involved, even though it's only a 100ms, it could quite possibly be C50.

I'm not up on flip flops at all but it's likely the circuit of Q4 and Q5 is one so that when ones on the other off and changes in current to the diodes at the front end flips it the other way each time. So you can either do a comprehensive reading of voltages around the circuit and isolate the failing part or just start with the most likely to fail... and I think that'd be the 50c electros.

[RWood]

Thanks Sean, I appreciate your pointing me in the right direction. I'll post back here with the results.

RWood