Install a new Power on LED in an Ampeg B100r

[pbman1953]

I bought a a B100R that needs a new power on light. The original light is marked at 28 v. If I read the schematic right  there's 40 watts that goes into a 220 ohm resistor to tame the voltage down to match to the stock light. I bought a light but all they had was a 24 volt light. The salesperson gave me a 220 ohm resistor , so I need to know if that's the right value. Plus, I have to see if the factory 220 ohm resistor is actually on the board or it could be built in to the housing of the stock light. The stock light is over an inch long so there is room for the resistor.


Thanks for any advice.

[DummyLoad]

that's a 40 VOLT supply -


the original lamp is marked 28V - you have a 40V supply feeding a 28V part via a series resistor - apply KVL: (40V-28V = Vr220Ω) so that leaves 12V across the resistor - 12V/220Ω gives us 55mA of current. compare to the specs of your new lamp. 

based on the schematic, it would seem that the 220R is located on the board.


--pete

[pbman1953]

Thanks, but now I have a 24 volt part, would I have to add another resistor to get to 24?

[DummyLoad]

perhaps a larger resistor is needed - i'd run with what's there - 4V isn't going smoke the lamp, however, it may not last as long. if you're really fretting over the 4V then we'd need the specs on the lamp you have to determine what the dropping resistor value is. otherwise, buying the exact replacement part is your best option.


--pete

[sluckey]

28V lamps are very common. Show us a pic of the original and tell us what all is written on the lamp.

[pbman1953]

Will have to send later. But all it shows is the makers name and 28V.


Can I simply add another 75 ohms or so ? to bring it down?

[pbman1953]

I had a chance to measure the spade tabs off the board and the measurement was 37.5 volts. The salesman at the store told me to use a 220 ohm resistor to bring it down o the 24 volts. The resistor really didn't do much at all. The package on the light said to use a rule of 50 ohms per volt @ 2 watts. I suppose this means I need 650 ohm resistor at 2 watts.


I'm assuming that the 220 on the board took it from 40 volts to 37.5 and there's a resistor built in to the housing of the stock light. Tonight I can try and push the tabs on the light to see if it's there

[sluckey]

This is a common failure on that amp. The original uses a 2187 bulb, not a LED.

This should be the correct bulb. If it were my amp I'd just replace the bulb with a real LED and change the on board resistor to about 2.2K to 4.7K. Higher resistance gives dimmer bulb. Never have to replace another light.

     https://www.amazon.com/Eiko-2187-10-2187-Terminal-Light/dp/B076RRQ9K9?th=1

[pbman1953]

Thanks!