Ampeg AFP2 help please!

[xI Jonathan Ix]

I’m trying to have an amp switch made that exactly matches the Ampeg AFP2 switch’s wiring. The thing we can’t figure out, even after looking at the schematic, is whether there are resistors in the switch or not for the LEDs.

If you have an AFP2, a picture of the inside would definitely help. Below you’ll find the schematic provided from Ampeg. They don’t show the resistors on the parts list or on the schematic. The switch builder says they could be built into the amp’s footswitch circuit (SVT-7 Pro) but, I wouldn’t know what I’m looking for even if I could get the amp’s schematic. HALP PLZ.

 

[agedhorse]

There are no resistors used in the foot switch, why not build it according to the documentation rather than trying to reinvent the wheel?

 

[xI Jonathan Ix]

There are no resistors used in the foot switch, why not build it according to the documentation rather than trying to reinvent the wheel?

That’s the plan but, I was asked by the builder to clarify that there are no resistors used within the switch. They told me that is different from most switches of the same type.

 

[agedhorse]

That’s the plan but, I was asked by the builder to clarify that there are no resistors used within the switch. They told me that is different from most switches of the same type.

Not my experience, many foot switches on amps do not use resistors.

 

[xI Jonathan Ix]

Not my experience, many foot switches on amps do not use resistors.

Perfect, that’s what I needed to know.

Just as a quest for knowledge; say the builder made the switch with the resistors in line with the LEDs anyway, would it (lights and switch) still function properly if they were in both the amp & switch? Doubled-up resistors?

 

[agedhorse]

Maybe not. Without knowing the schematic of the amp, adding resistors would be nothing but a wild ass guess. Second guessing the original designer’s intent is rarely successful IME.

 

[xI Jonathan Ix]

Maybe not. Without knowing the schematic of the amp, adding resistors would be nothing but a wild ass guess. Second guessing the original designer’s intent is rarely successful IME.

Just wondering. I have no intention of changing the amp or switch’s schematic. Thanks for your help.

 

[BassmanPaul]

Why not just buy the proper foot switch? So much simpler that way and no infringing on IP rights.

 

[agedhorse]

Why not just buy the proper foot switch? So much simpler that way and no infringing on IP rights.

I'm assuming that it's NLA?

 

[xI Jonathan Ix]

Why not just buy the proper foot switch? So much simpler that way and no infringing on IP rights.

The proprietary switch is more than twice the price, and almost thrice the precious pedalboard real estate. I wouldn’t think I’m infringing on any rights if their switch schematic is readily available on their own website?

 

[Greg East Yorks]

An LED can only cope with low voltage, normally about 1.9 volts. Anything more than that you insert a resistor to drop the voltage for the LED. If you have one, measure at the LED with a voltmeter and if over 1.9V then, yes, insert resistor. If not then no need. Worst case scenario though is a blown LED, just a few pennies in the UK so not worth loosing sleep over!!

 

[Wasnex]

An LED can only cope with low voltage, normally about 1.9 volts. Anything more than that you insert a resistor to drop the voltage for the LED. If you have one, measure at the LED with a voltmeter and if over 1.9V then, yes, insert resistor. If not then no need. Worst case scenario though is a blown LED, just a few pennies in the UK so not worth loosing sleep over!!

The footswitches ground out the bases of couple of NPN transitors to turn them off. The emitter of the transistors are tied to ground. Between each transistor's base and each switch, there is a 22.1K resistor. At this junction there is also a 5.1V zener tied down to ground and a 1K resistor tied up to the positive supply. I think the supply is 15V.

My guess is the 1K resistor will drop enough voltage to protect the LED. I believe the max current one of these LEDs can handle is 20mA. If 20mA were flowing through the 1K resistor it would drop 20V. 0.020x1000=20V So this suggest the LED will be fine since the supply is only 15V. AFAIK, the LED will reach an equilibrium point below 20mA where it is happy with the voltage.


This is a simplified version of the mute circuit.

The signal passes on the red line. My read is when the switch is open, the red LED is lit and the transistor conducts shunting the signal to ground. Close the switch, and ground is applied across the LED, and also to the base of the transistor. This turns the LED and transistor off, so the signal can pass.

The FX ON/OFF circuit is similar, except the collector of the transistor drives a relay that is used to open and close a physical connection that bypasses the Effects Loop.

 

[agedhorse]

An external resistor is not necessary.

Also, an LED is a current controlled device, the voltage across an LED is the byproduct of the junction’s band gap potential.

 

[Frank_Mtl]

This may come lately.
I had the same question about No Resistor. I've checked on a BA-112 and found out Resistors are'nt needed. The Amp limits current to 5mA. The tip of the footswith presents an 14 Volts.
So any efficient LED will do if properly polarized. The BA series manual answered that the BA-112 required only 1 SW though AFP1. Bigger models could have more SW.
Enjoy your Ampeg