vactec VTL5C1 opto switch

[blues man]

After building a few amps I want to move into learning channel switching. I want to learn how to use the vactec VTL5C1 opto switch. Can anyone suggest a fender amp sch for examples. I of course don't know how to power them up like votage supply to turn them on and off. The fender 75 amp looks like it  has opto switches am I correct on this. I recently converted a 75 into a supper reverb but all that stuff was already removed from the amp before I got it. Just looking for some direction. Thanks for your time.

[sluckey]

The Fender 75 uses a couple of optoisolators (opto switches).

The VTL5C1 uses a LED for the light source and a LDR (light dependent resistor) for the switching element. When the LED is off/dark the LDR will be very high resistance. When the LED is on/illuminated the LDR will be low resistance. Look up the data sheet to see the exact LDR resistance readings.

The LED is powered by a low voltage DC supply. 5V to 24V is typical. If I were building a supply inside a guitar amp I'd use something like Hoffman's 5V Relay Power Supply Board that can be connected to the 6.3VAC filament string. Plenty of info about that supply right here. The Fender 75 uses the bias supply connected to a simple resistor/6.2V zener to power the opto switches. (I dislike using bias voltage for anything except bias.)

As with any LED, you must use a current limiting resistor (CLR) in series with the diode. Size of the resistor depends on the dc supply voltage, ie, higher voltages require larger resistor values. Typical CLR value when using a 5 volt supply would be 270Ω to 1KΩ. Lower values would result in a brighter LED, but you'll never actually see it.

To use the VTL5C1 you would either connect the LDR side in series with a signal path or in parallel (shunt to ground) to a signal path. Shunt is more common in guitar amps. When using shunt switching you would need to turn the LED on to kill the signal or turn it off to enable the signal. I would connect the LED side like this... positive lead thru a CLR to the positive power supply and negative lead thru a switch to the power supply negative. NOTE: The negative side of the low voltage supply does not have to be connected to chassis ground. It can be floating.

The above example describes how to mute a single signal path. But you may want to mute one path and simultaneously enable another signal path. This requires two VTL5C1s. Just turn one on and the other off with a single SPDT switch or use a SPST switch combined with a transistor switch circuit.

This should give you a good basic understanding of optoisolators. I'm sure Google can find a ton of info.

[blues man]

Thanks a lot Sluckey you have indeed helped me.

[2deaf]

If you're new to LDR's, some bits of knowledge may jumpstart your designs.  The more current you give the LED the lower the resistance of the LDR with the lowest resistance coming right before you burn out the LED.  The things have a few hundred ohms with the LED on so that you can't completely kill the signal by shunting it to ground through the LDR.  What people typically do to really knock the signal down is to use two LDR's as in the attachment below to switch between two different extreme voltage dividers.  These devices make really quiet switches, but the network you use to control them isn't necessarily quiet.

[maggit13]

I have a fender 75 and it sounds like some reverb signal is leaking through even when it is switched off via the footswitch. would the opto switches have anything to do with this if they might be going bad or not working properly? I've checked many other places in the amp for possible causes but can't find anything that gets rid of the reverb signal totally, besides pulling the tank plugs from the back of the amp 

[2deaf]

I have a fender 75 and it sounds like some reverb signal is leaking through even when it is switched off via the footswitch. would the opto switches have anything to do with this if they might be going bad or not working properly?

No.  The LDR's are not involved with the reverb switching.

The footswitch mechanically grounds the wiper of the Reverb Control in order to turn the reverb off.  The cord and/or the footswitch may be defective.  You could try shorting the ring to ground on the plug at the amplifier end and see if that completely kills the reverb.   

[maggit13]

Thanks for the suggestion. I tried doing that and I still here some reverb. I've been working on this for over a month now, I built a brand new footswitch, replaced the filter caps and bypass caps, check this thing up and down, got a new reverb tank, it needed one anyway, and none of this worked. I can't get rid of this leaking reverb signal and it's driving me nuts. Is this typical on fender amps to bleed a little reverb or is this a genuine problem? something must be leaking somewhere because when I pull one of the reverb tank plugs from the amp it goes away, so some signal must be going through the tank and not being grounded totally. any help is greatly appreciated.

[Willabe]

I can't get rid of this leaking reverb signal and it's driving me nuts. Is this typical on fender amps to bleed a little reverb or is this a genuine problem?

Depends on what you mean by 'problem'.

Will it hurt anything? No.

I've never turned off the reverb on an amp that has it. I always have some on.

[maggit13]

Understood. I know most people have some reverb on all the time it's just bugging me that I can't have a clean signal if I want one for whatever reason. And i never noticed it before until i had my amp serviced recently so i don't know if this is new or i just never noticed it before, but i think i would have considering i noticed it right away when i got it back. I just can't believe that an amp that bleeds as much reverb as i am hearing is the way it's supposed to work. unless i'm totally wrong and this is just how it is

[2deaf]

You could try moving the wire from the Reverb Control wiper to the grid of the reverb recovery stage (V2A on the schematic I have).  It will probably pick up some noise from the LDR switching line if I were to guess, but that could be solved some way if you're killing the reverb like you want.

[maggit13]

ok thanks, that's not a bad idea but would that kill the reverb signal totally without being able to turn it back on?

[2deaf]

ok thanks, that's not a bad idea but would that kill the reverb signal totally without being able to turn it back on?

Huh?  Step on the footswitch or pull the footswitch cord and it will come back on. 

[maggit13]

Yeah sorry, I misunderstood. So take the wiper lead from the resistor it attaches to and move it directly to the grid? or move it from the wiper lug to the grid? sorry for the stupidity.

[2deaf]

Stupidity and a lack of knowledge are two different things (although they are frequently found together).  The lack of knowledge about a totally new subject could hardly be considered stupidity.

The footswitch jack should be right next to the Reverb Output jack (NOT the red one).  The ring is the center contact on a stereo plug so the plug goes tip -- ring -- sleeve -- housing -- cable.  The terminal on the footswitch jack that contacts the ring has a wire that probably goes to the board.  Remove this wire and leave everything else intact.  Save that piece of wire in case you want to restore the original wiring.  Connect the ring terminal from the footswitch jack to the center terminal of the Reverb Output phono jack using a new piece of wire.

BTW, does your amp have a 4A or a 9A tank?

[maggit13]

thank for the explanation. being a newbie at this is a bit frustrating but i am learning quickly. I will try this and see what happens.

I have a 9AB2C1B tank

[maggit13]

wow, I thing that may have done it. just a few quirks. it is a bit noisier like you said. The hum is much louder but comes and goes as I move around and sometimes goes away, but then comes back full force. Also, the reverb will come in and out too. I can turn it up and down with the knob even though it is switched off, but then if I switch it on then off, it goes away completely, then may come back. Is this just bad soldering on my part?

[jojokeo]

Rather than use this type of optoisolator (that's also expensive and more used for compression circuits) why not use a separate optoisolator along with a triac working in tandem? Or is this better for switching voltages?

[maggit13]

Alright, that did it. I fixed the solder joint on the reverb output phono jack and all is working well. the reverb is off unless I switch it on and it sounds great. it does pick up some noise as I turn the reverb knob up so and suggestions for that would be great but I can live with it since I don't turn it up that much. Thanks again for the help!!

[maggit13]

one thing, now the lead channel has really gone to crap. not that it was that great before but as the string slows down, the drive starts to crackle and break up then stops before the dry signal stops as if there were no drive on. would moving this one wire have caused that or do I have a new problem now?

Also, would moving this wire affect the voltage or current in a bad way at all?

[2deaf]

Got moved, eh?  Maybe you should start a new thread under Tweaks and Repairs where people who might know about these amps are a lot more likely to see it.  Or maybe they just don't want this one there.

The other wire on the footswitch jack in question controls the lead potentiometer, so it may have been disturbed during the process and is affecting the lead channel.  That schematic is a little frustrating - - almost like they went out of their way to make it difficult to read.  I like that half plate voltage thing with the switching bias.

[maggit13]

ok I'll check. I just want to make sure that moving that wire to the grid of the tube isn't hurting anything in the amp, meaning did I change the current or voltage in the circuit or add extra to the tube that will cause damage. If that's the case i'll just move it back and live with it.