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eddiejg
Senior tube assistant
Posts: 219
(2/10/04 12:21 am)
Stealing DC Voltage for Standby LED
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I built an amp & I added a standby indicator LED.
I'm trying to steal the voltage off the power rail with a dropping resistor (10K), which gives me the correct voltage on the LED (2V).
Howevere, it also drops the preamp voltage by 30-40 volts.
I guess I should have expected a voltage drop, but I wouldn;t have thought it would be that big.
What do I have to do to isolate the LED- use a capacitor?
Do I hook it up like a bias circuit?
Or will just the 1 cap & 1 resistor do it?
Thanks.
-Ed
ganzonimx
Hey get your own solder
Posts: 359
(2/10/04 10:22 am)
Re: Stealing DC Voltage for Standby LED
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I think the problem is the power consumption of the LED as you have to drop a lot of voltage.
Try with a neon indicator, they run at much higher voltages and lower power consumption. You find neons that run at 90 VDC @ 0.3 mA.
Cris
eddiejg
Senior tube assistant
Posts: 220
(2/10/04 10:33 am)
Re: Stealing DC Voltage for Standby LED
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Cris-
Thanks.
I might have to go that route.
The PT has sufficient HV current capacity, though.
Is it the big drop in voltage that is causing the problem?
If I take the voltage off the power tube cathode voltage, I would only have to drop ~10 volts.
Would that work better?
Thanks.
-Ed
wayneosdias
Senior tube assistant
Posts: 232
(2/10/04 10:43 am)
asdf
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Kevin Occonors TOT shows a 100k 2w off the b+ feeding a 6vdc LED to ground. Ive never hooked this up, but dont know why it wouldnt work. I plan on doing this on my next build, makes good sence, please let us know how yours turns out.
wayne
ganzonimx
Hey get your own solder
Posts: 361
(2/10/04 10:53 am)
Re: asdf
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Ed
I don't know, I just thought that you dropped something like 300V over the 10K resistor, that would be around 30mA for the LED, thats quit a lot of current (did the resistor get hot?) and affects the power supply.
If the dropping resistor would be 100K like Wayne says, it would be only 3 mA, - have a try, look what happens and tell us..
Cris
eddiejg
Senior tube assistant
Posts: 221
(2/10/04 1:05 pm)
2 Volt LED
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Alright, so I hooked it up with a 560 ohm resistor coming off the ~+12.5 volt power cathode resistor.
That gives me ~2 volts on the led & ~ 20mA which works.
It also dropped the cathode voltage to 11.5.
So, I'm going to drop the cathode cap from 180 to 150.
Anyone see any flaws or problems with this approach?
I did try the 340 B+ for the voltage, but the largest value 2 or 3 watt resistor I have here is 27K ohm & that dropped the voltage to 2 volts as required, but started smoking in about 30 seconds.
Thanks.
-Ed
Ritchie200
Senior tube assistant
Posts: 163
(2/10/04 2:10 pm)
Re: 2 Volt LED
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Hey guys,
How about taking it off your filament voltage? A full wave recto with about a 47 ohm resistor and a smoothing cap should handle it. Load should be negligible.
Please check my math, I'm at work and the controller ran off with my calculator! I'm surprised he could even function today. Damn accountant, probably off in a corner somewhere calling it "My precious!"
Jim
eddiejg
Senior tube assistant
Posts: 222
(2/10/04 2:39 pm)
Standby & Power Indicators
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Jim-
I'm using the filament for the power indicator, it was a 180 ohm resistor to drop the voltage to ~1.7 at 25mA.
Doug has this info in his catalog as he sells this led & holder.
What I didin't realize is that LED's can run on AC, so that one was pretty easy- just 1 resistor.
Pesonally, if I was only doing a power indicator, I would have used the Fender jewel lamp or an iluuminated power switch like I've used several other times.
But, the guy wanted a standby indicator too & I wanted to make the look uniform, so once I realized I could use the same LED for the power & standby, I was off to the races.
As far as my experimental solution, I'm guessing that no matter where I take the voltage from, 20mA is going to drag the overall voltage down somewhat across the board.
If that is the case, dropping the 12 volts from the cathode probably is a viable solution.
Any comments or feedback on this is welcome.
Thanks.
-Ed
Lucid Alice
Senior tube assistant
Posts: 155
(2/10/04 9:02 pm)
Re: Standby & Power Indicators
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You want to create a voltage divider. Try a 100K off the power rail and select a resistor to place across the LED that will set the proper voltage for the LED. I did this and it worked fine. but I don't recall what value I used across the LED. You may want to tinker with the value of both resistors until you get the voltage and current you want.
eddiejg
Senior tube assistant
Posts: 223
(2/11/04 10:24 am)
Re: Standby & Power Indicators
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Alice-
Yeah, my 1st though was to wire it up like a bias circuit (without the diode).
But, I don't think the voltage divider is necessary, just the resistor.
Maybe it is though for the big voltage drop.
Anyway, the circuit works the way I have it set up & unless anyone has a reason not to leave it like it is, I'm done.
Funny coincidence, my other stupid problem with this amp was wiring the single/parallel jacks like the 18 watter.
Last time you helped me was to get the single/parallel to work with 1 jack & a switch.
My 1st attempt with the 2 jacks didn't work because I tried to work off the layouts on the 18 watt site & they all have the marshall jacks, which confused me (not hard to do).
So, I had to look at the actual schematic to figure it out.
But, it works, so I don't have to bug you about that again :<).
Thanks.
-Ed
GroundhogKen
Forum Moderator
Posts: 2257
(2/11/04 12:34 pm)
Re: Standby & Power Indicators
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Just to set me straight here:
Is the idea to have one LED that indicates that the power is on and another LED to indicate that the B+ is active with the amp ready to play? Or do you want to have an LED that is lit when the amp is actually in standby?
Or do you want two LEDs: one indicating power-on/standby and the other indicating power-on/play?
Ken
eddiejg
Senior tube assistant
Posts: 224
(2/11/04 1:30 pm)
Re: Standby & Power Indicators
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Ken-
1 LED indicates power is on.
1 LED indicates standby switch is in on position- B+ is active & amp is ready to play.
-Ed
Jackson Ampworks
Junior tube assistant
Posts: 1
(2/11/04 2:36 pm)
5V Regulated Supply
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I have already designed and built what you need. Here is a picture of it.
www.jacksonampworks.com/c...ge/155.jpgThe cost is $50.00
Simply connect the AC IN of the circuit board to the 6.3V filament winding on the power transformer and you will instantly have a clean and regulated 5V supply for LED's, and any other device that requires a 5V DC supply.
Incorporated into the circuit board are two DC OUTS. One of the DC OUTS is a dedicated 5V supply and the remaining DC OUT incorporates a dropping resistor to reduce voltage for devices such as LED's and other components requiring less than 5V DC.
Brad Jackson
Jackson Ampworks
wayneosdias
Senior tube assistant
Posts: 237
(2/11/04 5:30 pm)
asdf
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Ya, thats how the London power supply is set up and makes good sence to me. Ill forget and leave the amp on but not the standby, doesnt that hurt the tubes, just running heater current thru them? With this setup theres a visual cue to help remind you.
wayne
GroundhogKen
Forum Moderator
Posts: 2259
(2/11/04 10:30 pm)
Re: 5V Regulated Supply
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Why not just use a two pole switch?
One side is the actually B+ standby switch and the other side connects the standby LED and a resistor to the AC filament winding just like the power LED. Simple and cheap.
Ken
eddiejg
Senior tube assistant
Posts: 225
(2/11/04 10:49 pm)
Too Simple, Too Cheap
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Ken-
That was one of my original ideas, but the guy wanted the power & standby switches too match each other & the power & standby switches I used on the other amp I sold him.
Give the people what they want, right?
My convoluted solution is working so hopefully I'm done.
I got to try a few things I hadn't done before, like use the original Matchless OT & choke, which sound great.
And I learned a lesson about letting the customer micro manage.
Thanks for the advice, I will use the dpst switch next time.
-Ed
wayneosdias
Senior tube assistant
Posts: 238
(2/12/04 12:49 am)
asdf
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