Do Vibrato circuits suck down output?

[EETStudent]

Im having an issue with the vibrato circuit you see in the schematic. I sorta accidentally stumbled across this problem. First off, this amp is rated at about 35watts, but I wasnt getting nearly that much. I suspected some leaking components or something mechanical. but tonight, after I redirected the vibrato circuit to channel 2 instead of 1 I got full volume, but I couldnt engage the vibrato. So I poked around a bit, and I got the vibrato to come on but the output went straight back down to where it was. Do I have a circuit problem, or is the vibrato affecting the output?

[chipaudette]

Wow, that's guitar amp cuircuit certainly isn't your standard Fender-type layout. What kind of amp is this from?

As for your vibrato question, the first place to check is usually the optocoupler (your LDR). Oftentimes, the circuit is slightly out of spec thereby making the the optocoupler light is never really "off"...ie, the light is always a little on, which makes the resistance of the circuit lower than it should be, which makes current leak down to ground, thereby sucking volume from your circuit. Measure the voltages across the light side of the LDR. I don't know what the voltage should be (zero?) but hopefully you do. Check it out. If you've got a problem, it looks like you've already got a pot in your circuit (R26) that you can adjust to control the current going through that tube (and, therefore, through the LDR light).

My second thought has to do with the resistance of the signal side of the LDR. Even if the LDR light is fully off and the LDR resistance is maximum, what is the resistance value? I'm not sure what the value is supposed to be for your LDR, but it might be only 50K or something. Unhook the signal side of your LDR and measure its resistance.

If it is this low, it will indeed drop your audio volume. Look at where the connects into the audio path of your circuit -- it connects in right after your "loudness" pot. If your loudness pot is set to half way (or on "7" if an audio taper), then the top half of the pot is like a 125K resistor and the bottom half is another 125K resistor. Your signal must pass through the top 125K on the way to the rest of the circuit. But, immediately following it is the LDR to ground. If the LDR only 50K to ground, you've got a sweet voltage divider going on that'll lower your signal volume by 125/50= 2.5 in voltage or nearly 20*log10(2.5) = 8 dB in SPL. That's a lot!

So, (1) check to make sure the LDR is fully off. (2) measure the signal-side resistance of the LDR and confirm that it is correct for the part called out in the parts list.

Chip

[EETStudent]

Yeh I suspected it was something with the LDR, because when I was poking around all it took to get full volume was to wiggle it around a bit. I went to check it today like you recommended and one of the legs fell out. So i gotta get a new one anyway I suppose, but maybe that was the problem needed fixing.

[EETStudent]

Just received a brand new LDR from Weber today, supposed to be a direct replacement. Unfortunately, I cannot get the vibrato circuit to function. Ive got the legs connected correctly according to the schematic, yet nothing. I dont know much about these, but maybe these LDRs arent applicable in all situations?

[Bruce / Mission Amps]

Just received a brand new LDR from Weber today, supposed to be a direct replacement. Unfortunately, I cannot get the vibrato circuit to function. Ive got the legs connected correctly according to the schematic, yet nothing. I dont know much about these, but maybe these LDRs arent applicable in all situations?

I was the one who suggested that LDR to Weber and did the prototype/beta testing for them.
They work fine in every Fender Vibrato amp using an LDR.
You probably just have something else going on there.
Did you recreate the Fender circuit for this amp?
If it is connected correctly, does the neon bulb come on and blink when the RCA jack's center pin is shorted to ground?