• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

LED tube biasing, pros and cons

tapehead ted said:
I wonder if LED noise follows the same kind of curve as zener noise, where noise is proportional to the inverse square of current.
It might be unwise to draw conclusions about noise from forward biased diodes (LEDs) by looking at reverse biased diodes (zeners).

I seem to recall that SY (or was it someone else?) found that the major noise contribution from an LED was simply the thermal noise of the slope resistance. This means that it can be ignored in comparison with valve noise.
 
I did some calculations from Mr. Jones's measurements based on the assumption that the LED noise followed the same curve as zeners. At an input tube the noise at 1 ma (or less!) could be equivalent to that in quite a large resistor, one large enough to dwarf the valve noise.

Sometimes another few mA is nothing, other times it could be quite expensive- I'm sure it varies. Just noting a situation I ran into where the issue of potential capacitor ringing right into the cathode of the input tube seemed like something to be avoided- again, YMMV. Does anyone say YMMV any more?
 
I know I anticipate problems a lot- my mom calls it "borrowing trouble". Still, I'm expecting an unusual amount of vibration, so looking at a design to accommodate some. Doubtless I should ditch those potentially-microphonic tubes, but at that point love overcomes fear for me and I stumble serenely ahead...
It's not remotely working as a Zener (in either mode we call (Zener).

Go fishing. Walk the stream. The LED runs like water through a smooth constriction. The Zener "breaks" like a waterfall. Knowing one does not directly tell you much about the other.
Thank you. That helps.
 
Mechanical resonance of capacitors is unlikely to be a problem, assuming normal good commercial components (not hand-rolled audiophile items) of appropriate technology (not high-k ceramic). Anyway, I don't think you are yet at the stage in design where such fourth-order effects may matter; you need to concentrate on first and second-order effects such as good grounding.
 
To measure the LED forward drop properly usually requires a CC Source. I built mine in 1968, it has been used in support of a number of applications since. Eventually published in Glass Audio Mag, some here may have seen it. Two ranges, 40 & 400 mA. All in a small Hammond box.

Very useful.:)
 

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One of the more observant members of DIY has spotted some errors/omissions in the article I posted covering a variable constant current source used in testing. Thankyou Jon of Australia!

So I opened the Hammond box, the transformer I used is 20 volts, not 25 volts. But either will work. The 2N2907A will work OK in place of the 2N4143. Vcb 60V. The 2N3054 looks very expensive now. There are probably some good alternatives in a TO220. Zener diodes have a zero temp coefficient of voltage at 5.6 volts. Above that it is positive. And increases as the Zener voltage rises. Partial correction is possible by connecting a forward biased silicon diode in series. That is what the diode ‘D’ is doing in the corrected schematic.

There are better ccts out there now, but this was easy in 1969. And looks like writing the article in 2001, thirty-two years after the build required a little more care.

Still working & often used.:)
 

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  • 2N4143 Equivalents A.pdf
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Zener diodes have a zero temp coefficient of voltage at 5.6 volts. Above that it is positive.
<snip>
Partial correction is possible by connecting a forward biased silicon diode in series.

I've read that a single red LED used as a voltage reference with an NPN silicon transistor has quite a low tempco. Apparently the LED and the transistor's Vbe both have about the same change in voltage with temperature, and so cancel to a large degree.

The most recent batch of green LEDs I bought is more than bright enough for use as a power-on indicator at 80uA forward current, and is quite unpleasantly bright by the time you get to 500uA. With today's high-efficiency LEDs, it might be nice to extend your current source downwards to include 10 mA, 1 mA, and 100uA ranges.

That could be done with cheap small-signal transistors instead of the relatively beefy power transistor you used for your original design. Something like the BC550C / 560C would do the trick.

-Gnobuddy
 
LEDs are a good example of a Disruptive Technology. Read the attachment, my experience with LEDs at HP & other. I had wrote this for my sons a while back..



Packard always said he did not want to be in a business of commodities. So the HP OED (Opto Electronics Division) was eventually sold off.:)
 

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Packard always said he did not want to be in a business of commodities. So the HP OED (Opto Electronics Division) was eventually sold off.:)

And now that business is larger in market capitalization than Agilent, Keysight and all the other HP entrerprise businesses combined.

My first exposure to LED's was at the Bell Labs research facility in Murray Hill NJ in 1967. Our scout troop was supported by Ohio Bell and they arranged a bus tour.
 
And now that business is larger in market capitalization than Agilent, Keysight and all the other HP entrerprise businesses combined.
My perception as a (then) young person in a technical field was that Carly Fiorina gutted and destroyed everything that was truly special about HP during her tenure as CEO. She appeared to have not the faintest clue as to what had made HP one of the most astonishingly creative tech companies of its era, and so destroyed it all in the quest for more profits.

I think one of my favorite HP disruptive technologies was the inkjet printer. For the first time, much of the public had access to a printer that was more than a glorified typewriter; now you could have full-colour images as well as crisp text.

Those of you who grew up with home PCs and home printers can hardly imagine how dramatically they changed the world we had known before them.

-Gnobuddy
 
The 2N3054 looks very expensive now.

I don't see how that's possible. This is an old school power transistor that's been around since the '60s. Last time I checked, they were going for under a buck a pop. hfe~ 25; ft= 3.0MHz -- that's pretty awful by today's standards, and you wouldn't want them as audio power finals..

A lot of really decent audio solid state went by the boards (thank the gov't for that) but 2N3054's are not the solid state equivalent of 300B's.