Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: skeprico on February 20, 2022, 02:58:44 pm
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Hello all. I'm new to the forum. I have some basic questions about using vactrols for switching. I'm building a Rick Campbell Trem-O-Drive. It seemed this would be a good opportunity to experiment with switching. I'm taking the LED power supply from the 12.6v transformer output. I'm using a rectifier bridge to convert to DC, then installing a resistor to get the voltage down to ~3.5V. Amperage is ~25mA. The problem I have is that the cell resistance is still about 2.5K. If I'm reading the spec sheet correctly (for a VTL5C1) the resistance with the LED on at that amperage should be between 200 and 600 ohms. I didn't check the resistance of the cell till I'd gotten the voltage and current right. I may have hit the LED pretty hard at some point so I'm wondering, is it possible that I've damaged the led with over amperage while experimenting? Maybe it's not emitting enough light to bring down the resistance? Since they're so expensive I'd like to find out if I'm doing something stupid before spending the money on replacements.
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I don't see what DC smoothing cap value is? What DC voltage you actually get? And math-errors are popular: what resistor value do you use? Do you have other higher values to make sure the photocell resistance really is changing?
IMHO the most likely burn-out would be total darkness inside and nearly infinite (>>1Meg) photocell resistance.
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The LED is probably shot. I've seen damaged LEDs that just glow at about half brightness when damaged. Others don't glow at all. It only takes 3V connected wrong polarity to kill that LED. If nothing is connected to the LED does the LDR measure extremely high, like 50MΩ?
Here's a safe circuit to experiment with LEDs or a VTL5C1 without killing anybody...
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Hey guys, thanks for responding.
PRR- I did use smoothing caps in the power supply. 3 250mF in parallel. And I tried your suggestion of different values of resistor and the resistance is changing. It's just very high.
Sluckey- Thanks for the schematic. I didn't think about using a 9V battery. That worked nicely to verify that it's the same as with my power supply. I get 3.2K ohms from the cell at 13mA to the LED using that setup. Looks like the LED's are roasted. Wish I'd have thought that out better (researched) before beginning to experiment. Live and learn.
Once again, thanks to both of you for helping. It's much appreciated.