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I need help wiring a LED for amp channel switching

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roadtrip

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Hi. I don't know much about electronics so I have a question. I am making an amp foot switch for a guitar amplifier. The current switch I have has a 5mm red LED and a resistor. The voltage coming from the amp on the tip of a tip/sleeve 1/4 inch cable is 9.62 volts. I have purchased the appropriate resistor for a Super Ultra-Bright LED. I am making a custom pedal that also switches between two FX pedal loops at the same time as the amp is switched between channels.

So the problem is this. I don't want to take apart the old foot switch circuit if I don't have to, but using a DMM across the resistor gives me zero ohms for some reason (?), and I don't know why. The foot switch still works. So if I put a LED and a resistor in series to run them at 9.6 volts (I used an online resistor calculator), doesn't the amp need some voltage, to say hold a relay open? I could make the connection using the 9.6 volts without the LED and have the loop LEDs display the channel I'm on, but I am worried that if it is , say a 5 volt relay with the current LED setup, that I may burn out the relay.

My only options if I can't figure this out (with your help of course) is to use the LED from the old foot switch.

So my question is, without wiring it up to a LED with a resistor, is there any way of figuring out if the relay or whatever needs current., or will a zero volt continuity circuit switch the channels, after the voltage has dropped through the resistor and LED. Much thanks in advance, Roadtrip
 

alec_t

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It's hard to make out what's going on there. Can you post schematics of your pedal, old foot-switch and amp? What does the LED indicate?
 

roadtrip

Member
Amp Foot Switch.jpg Thank you for your reply alec_t. I looked at the old footswitch again, and the reason I get zero ohms is that there isn't a resistor under the clear plastic sleeve that is covering it. It was hard to see at first. The sleeve covers the join between the LED and the wire going to the switch. sorry about that. Here is the diagram of the old foot switch. (My apologies if I have drawn the LED the wrong way around). So since it doesn't have a resistor, I guess I'm going to have to find the voltage drop across the LED. I don't know how to do that yet. Can I just hook it up to the amp and put a DMM over the LEDs leads?
 

roadtrip

Member
Seymour Duncan Schematic 1c.jpg Here is the schematic of the amp. You can see the relay in the upper left corner that switches between the preamps. The amp switch and jack for the footswitch are in the lower right corner. Its hard to read the values so let me know if any of the values will help in figuring out the voltage needed for the LED in the footswitch. Thanks in advance!
 

roadtrip

Member
OK so I plugged in the foot switch with the bottom removed, and tested the voltage drop over the LED while it was lit. The voltage drop is 2.252 volts. Does this sound about right? Is it possible that the relay circuit is 7.37 volts?

I still don't know the milliamps of the LED though.
 

alec_t

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Useful info.
From the bottom of the amp schematic it is clear the relay coil K1 is in series with red LED 1 and also with the LED in the footswitch, so that's why you don't see or need the usual resistor in series with the footswitch LED. Operating the pedal grounds the relay coil bottom end via both LEDs.
Your new LED will be fine in your new pedal without a resistor.
 

roadtrip

Member
Thanks for that alec_t. The LED that I have purchased (with resistor for 9.6 volts lol!) is a Super Ultra-Bright LED rated at 2.0 vdc typical, and 2.4 vdc max. Average 50 mA and peak 200 mA. So it looks like it will work like you say.

Interesting to note: the mA across the tip/sleeve of the foot switch cable is 66mA (and it changes to the red channel), but when I toggle the foot switch to open so I can read the mA across the LED as well, I get 49.4 mA. Do you have any idea why it is less and not more? (remember I'm an amateur, and that includes working with a DMM !). Thanks
 

alec_t

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Most Helpful Member
It occurs to me that perhaps you were hoping to power other items in your new pedal from the 9.6V? Unfortunately, the pedal switch merely provides the path to ground for the relay and LEDs, so the voltage in the pedal is 9.6V without the pedal pressed but will drop to ~2V (the LED forward voltage) when the pedal is pressed.
Regarding the mA values, can you confirm the voltage between points G and H (bottom of the amp schematic), and/or the values of the two resistors connected to the LM317 voltage regulator VR1?
 

roadtrip

Member
Thanks again alec_t. No I wasn't going to try to power anything else from the 9.6 volts. There will be a couple of LEDs in the pedal (to indicate loop A or B active, and a blinking LED to indicate the loops being bypassed) that will require a separate power supply, but for now the pedal will be passive and will be able to run without power if necessary.

I don't want to open up the amp at this point so I don't have the values for G and H. The resistors for VR1 are 1.5k (1/2 watt) and 200 ohms (1/2 watt). The 1.5k (R69) is the vertical resistor and the 200 ohm (R70) is the horizontal resistor.
 

roadtrip

Member
Help my amp is making a terrible crackling noise when I turn it off! The amp was made in 1989 and it worked fine until I put it in storage in 2002. It's been in storage until a few months ago, and was sounding fine except for a crackling sound that I thought might be just a dirty pot or something. I have heard of things like having old amps re capped and the pots "golded" ( I don't know what that is, but it said these amps should have it). It has removable preamp modules and it said something about doing something to the contacts on the modules or the slot connections, Although I haven't removed the modules many times, only to change preamp tubes. The amp crackles even with the preamp levels turned all the way off. I should probably also mention that it was repaired twice in the 90's, once for a fire inside and I quickly turned it off. It worked fine after being repaired though. The tubes are the same as when it went into storage 15 years ago, it has two preamp tubes, a center driver tube, and two power tubes. The amp was just cracking a bit but now it's really loud for a few seconds when I turn it off, so I'm afraid to turn it on again in case I'm making it worse. Any advice is greatly appreciated. Thanks!
 

schmitt trigger

Well-Known Member
Tubes can have shorts (or leaks) between the cathode and the heater....many, many moons ago, I saw something similar to that.

But most likely one of the electrolytics is going defective.

Another one is that the insulation inside some of the output transformers is breaking down.
 

roadtrip

Member
Thanks for the informative reply shmitt trigger. I hope it's not the transformer. I forgot to mention that when I turn up the reverb it hums badly. I guess I should take it in for repair to save myself wasting money on tubes if its going to die soon. It's weird because it still has a warm fluffy tube sound. I hope it's just old tubes or a capacitor like you say. Although I wonder why the reverb hums so bad.
 

alec_t

Well-Known Member
Most Helpful Member
If the hum is worse than it was, I'd suspect a capacitor somewhere has become defective.
 

roadtrip

Member
Alec_t thanks. The noise is like a crackling like as if I was turning a dirty pot. But the last couple of times when I turn it off there is a loud crackling that goes on for a few second before the amp goes completely quiet.
 
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