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PostPosted: 05 Jun 2018, 04:32 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Oh, and that seemingly skinny wire is 600V rated teflon coated wire from Evatco. Nice to work with.

Danny, I would always leave headroom, but it depends on many factors as to whether it is ok or not, like ambient temperature, conservative ratings on the transformer.

Get back to basics. Try it as it is and add some LED's. Check the temperature and if all is well, add some more. Bear in mind that it is power you are fighting, not just current. At 10.9 VAC, that is around 12V DC, and that is roughly 6 LED's in series (2v per LED). So 6 LED's, running at say 10 mA you could get 22.5 LED's in there. How many do you need?


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PostPosted: 05 Jun 2018, 11:32 
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Joined: 29 Jun 2016, 15:59
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Trackhappy, hi.

Sorry maybe I am missing something but I do not understand how you get a total of 22.5 LEDs. I believe that you are using my assumption of having 225 mA remaining.
If you have 6 LEDs in series (2v per LED) running at 10 mA, then 10 mA would be used for each 6 LEDs track.
Then, wouldn't 225 mA support 135 LEDs ((225 mA/10 mA) * 6 LEDs)?

Again, I am just trying to find-out how much current I have left over. But, I guess I could measure it once I complete the project before installing the LEDs.

Thanks,

Danny


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PostPosted: 05 Jun 2018, 12:53 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, You need not worry about the current draw for LEDs. Compared to everything else it is so small as to be nearly non-existant. There is around 100s of times more available than needed. Think along the lines of 10ma or less at around 2 volts each. Typically I find the 1 to 5 ma range about right for most. Tap into the heater circuit (any of them, across the tube heater pins...watch for polarity though) and use a series resistor of around 1000 ohms with the LED. If it is too bright go up in resistance, too dim go down, but not below about 470R as you may damage the LED. On the high end you can go quite far. I have some that are really bright on 12 VDC with a 100K resistor in series.

Good listening
Bruce

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PostPosted: 05 Jun 2018, 12:55 
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Trackhappy, nice build. Clean layout. Easy to work with and if something needs fixing, easy to do.

Good listening
Bruce

EDIT: which power transformer is that one?

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PostPosted: 05 Jun 2018, 15:37 
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Team,
I see conflicting answers about this.
Do I still need a current limiting resistor if I have a 12 VDC supply and put four 3 V LEDs in series?
I would not think so. But some people say it is a must.


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PostPosted: 05 Jun 2018, 16:10 
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LED's have a fixed voltage across them when forward biased depending on colour and design. If you increase the voltage across them a very small amount over this voltage, the current goes up something like a square law or logarithmic law and the LED will quickly destroy itself. Thus you still need to limit the current so yes a series resistor is needed.

Sorry my maths might be off. If you had a string of LED's say 4 in series (with resistor) to run off 12 volts, that were using 10mA, then you could have up to 225/10 strings which equals 22.5 strings of 4 LED's.

As Bruce said, the current can be way lower and still get good brightness. Get some LED's of the type you want and measure is really the only way to be sure, Start with a large series resistor and lower it until you get a nice light, and measure the current.


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PostPosted: 05 Jun 2018, 16:11 
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gofar99 wrote:
Trackhappy, nice build. Clean layout. Easy to work with and if something needs fixing, easy to do.

Good listening
Bruce

EDIT: which power transformer is that one?


Thanks Bruce. XPWR117.

Just working out the ground in my head. read all your stuff on it last night.


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PostPosted: 06 Jun 2018, 14:22 
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Joined: 24 Oct 2010, 07:05
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Sorry to say, Trackhappy, but I haven't understood your intensions. Have you thought about some kind of indicator (one LED, for example) or something like modern computer cases with LED lines like on the christmass tree?
Your countings for the current left are not correct. 5751's heating current 0.175A + 2x88s in serial = 1.6A definitely gives the 1.775A, but tubes' tolerances are usually 10%, so it could be easily 1.95A. Then, don't you think the transformation of the 10.9V AC of your transformer secondary to the 12.6 DC to your tubes is for free? Even if we assume that there is no loss 12.6*1.775=22.365W which transforms in 22.365/10.9=2.052A for AC RMS secondary. As you can see there is no current left.


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PostPosted: 06 Jun 2018, 14:42 
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Hi, the math is correct, but a few milli amps for some LEDs would not matter. Plus (perhaps not good practice in general to assume this) Edcor trannies are seriously under rated. Based on my experience there is a reserve of between 25 and 50% on most low voltage windings. I would not generalize this to other manufacturers though.

I guess the question is sort of what Poty asked.....what exactly are you trying to do?

Good listening
Bruce

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PostPosted: 11 Jun 2018, 12:59 
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Joined: 29 Jun 2016, 15:59
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Just finished building my first OddBlock using 5751s & KT88s. February 22, 2012 schematics. For the power supply I used the May 6, 2015 Unified Schematic for Oddblock Amplifier. Here's some pictures.
For the cabinet I used Maple and Sapele (African Mahogany) wood. The amp includes a subwoofer output.

Some of the readings, which are a little bit low, but I guess still OK. Please, let me know otherwise.
B+ - 445V
SRPP - 293V
Heaters - 11.6V (5751) and 5.8V (KT88s).
DC reference heaters to GND voltage - ranges from 64V to 75.5 V, depending on where in the heaters it is read
Heaters to cathode voltage - ranges from 63V to 84V

It sounds great. With great bass and good clarity. My only complaint (minor) is that when I put my ear to the speaker, there is some white noise (like the sound of the ocean in the distance), which does not change as I increase the volume. Maybe because I am using high efficiency speakers that I built - Fostex FE206En Double Bass-Reflex Speakers at 96dB. Any ideas?
It really rocks!!!

I want to thank everyone for the support. Especially Bruce. Your help has been invaluable.
Thanks again.
Now to the Right (or is it the left?) channel.....

Danny


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