Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Buck112 on January 01, 2019, 01:31:02 pm
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Hey all, Happy New Year! I recently finished building a single tube tremolo/boost pedal and besides working out the kinks in it, have decided to overbuild a bit and add two switchable caps to slow down the very speedy tremolo. I know how to do that just fine. What I don't know much about is adding an led to each switch as an indicator. I believe I have figured out how to calculate the resistor values I will need for current limiting, but have never designed my own circuit (for anything!) and have some questions about rectifying the 12v secondary to power the LEDs. The original circuit uses 2 120v to 12v pts back to back for the heaters, and then the B+ and each transformer is center tapped. For economy of space, as well as the low voltage demand of the LEDs, my research thus far leads me to believe that full wave center tapped rectification on the 12v side is the way to go.
What I need to know is: What type (part number) of diode should I be using (seems like 1N5400 should work?)
Do I need a smoothing cap to run the LEDs without any visual blinking/throbbing?
How is the DC voltage calculated after rectification? I have about 7 volts on each winding as is, plugged in with the tube. I would like to have a fairly accurate idea of the rectified voltage before I place the order so I can have all the components I need. Being new to this, the only spare parts I have are pretty much what is left over from this build.
In the schematics I see for CT rectifiers, there is always the resistor labeled "RL". Is this just showing where the load (LED) would be, or is this a separate component?
On a separate note, I am adding a volume pot at the end of the signal path, should I be thinking of adding a high pass filter to it for this type of application? If so, what are some good resistor/cap values to try? I am ordering 250k, 500k, and 1M pots to mess around with for this.
Here is the schematic of the box I built in case it comes in handy.
http://www.ricktone.com/schems.shtml
Sincere thanks to anyone willing to help out with this rudimentary sludge. I'm New! I don't know what to do! God bless, good luck in the new year!
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here's a start;
http://www.theledlight.com/LED101.html
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> 1N5400 should work?
GROSS over-kill.
1N4007 is also over-kill but now so cheap that you should just buy a 10-bag and use them in all 1N400x holes; also for fuzz clipper diodes and reverse-polarity protection.
You can not pre-predict the resistor because LEDs come in a wide range of brightness for the same current, and "improving" every year. Where we used to (1973) use 330r, now sometimes 10K is plenty bright.
You can run an LED on low-volt raw AC, no diode no capacitor. If you turn your head quickly, you see the "dotted streak" of the flicker. Stationary the flicker is hardly visible. (If it was your main stage light the flicker could be bad, but as a dot on your pedal/amp it is no big deal.)
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Thanks for the reply, shooter! That is definately useful information, I am more interested in this moment in the actual design and implementation of the rectifier circuit, i.e.; do I need the filtering cap, what voltage will I end up with, do I need a resistor in parallel with the load (RL)?...so I can use the information on the page you shared without building the rectifier first. Thanks again for the reply :)
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Thanks for the reply, PRR! I figured the 1N4007 was overkill but it was the lowest voltage rated on the site I was looking at. If I were to run the LEDs straight from AC, would the reverse voltage wear the bulb out faster, and would the dropping resistor just be placed on the anode of the led? Is it worth it to use a half wave rectifier, or will that add more flicker than just the AC itself? thanks again, I know there is other info online, but I have read so much without a fundamental understanding that my head eventually just starts spinning:) Figured its easier to just ask the pros and more experienced, so thanks. Again.