| dsavitsk |
I have a circuit that uses an LED to set a tube's bias. The amp, with a basic schematic, can be seen at http://www.ecp.cc/semha.html
The current tube is I am using is a 7788, and I have it biased with a single LED at about 2.35V. With a B+ of 160V, this is leading to a current draw ~30mA ... maybe a tad more. I would like to reduce this slightly by upping the bias to closer to 3V. So, the question is, should I pick up some 3V LEDs, or will a Schottky diode in series with the currently used LED work just as well? Anyone with any reason one solution is better than the other? |
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| EC8010 |
| A pair of red LEDs in series will give about 3.2V and a nice low AC resistance. A yellow LED in series with an infra-red LED will give about 2.9V but not such a low AC resistance. |
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| Sheldon |
You can do as 8010 suggests, but no reason not to try a Schottky or other diode and see how it sounds to you. The dynamic impedance of the Schottky will not be as low as the LED and will cause some distortion, but don't know how audible it will be. Another option, if you can give up a little gain, to try just a small series resistor, say 100r or so. A little degeneration will reduce distortion, might sound better, might not.
Sheldon |
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| kevinkr |
| quote: | Originally posted by dsavitsk
I have a circuit that uses an LED to set a tube's bias. The amp, with a basic schematic, can be seen at http://www.ecp.cc/semha.html
The current tube is I am using is a 7788, and I have it biased with a single LED at about 2.35V. With a B+ of 160V, this is leading to a current draw ~30mA ... maybe a tad more. I would like to reduce this slightly by upping the bias to closer to 3V. So, the question is, should I pick up some 3V LEDs, or will a Schottky diode in series with the currently used LED work just as well? Anyone with any reason one solution is better than the other? |
Hi Doug,
I've found certain red leds with a 1N4148 in series to be a reasonable compromise. (Larger diodes tend to sound distorted.) |
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| Jeb-D. |
| quote: | | A pair of red LEDs in series will give about 3.2V and a nice low AC resistance. A yellow LED in series with an infra-red LED will give about 2.9V but not such a low AC resistance. |
How do you know the AC resistance of the different LED's? |
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| SY |
There have been quite a few measurements published. Sometimes it's even a specification. In any case, it's quite easy to measure and reasonably consistent.
There's a nice table of typical AC impedances for different diodes in Morgan Jones's "Valve Amplifiers" 3rd edition. |
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| mach1 |
| quote: | | How do you know the AC resistance of the different LED's? |
Measure the slope of the line at your chosen operating current on the datasheet forward current / forward voltage graph (if available).
BTW, very few of the commonly available red diodes produce a 1.6.- 1.7 forward voltage - most are now in the region of 2.1V. If you want a 1.7V drop red diode with a guaranteed low dynamic impedance pay a bit extra and get HLMP6000. |
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| arnoldc |
Might be of interest ;)
National Semiconductor
Online Seminar
LED Application and Driving Techniques
September 14, 2007 (Friday)
1130 - 1300 (India)
1400 - 1530 (Singapore/Malaysia/Philippines/Hong Kong)
http://www.eeplace.com/eeplace/even...IL&eventid=1447 |
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| kevinkr |
| quote: | Originally posted by mach1
Measure the slope of the line at your chosen operating current on the datasheet forward current / forward voltage graph (if available).
BTW, very few of the commonly available red diodes produce a 1.6.- 1.7 forward voltage - most are now in the region of 2.1V. If you want a 1.7V drop red diode with a guaranteed low dynamic impedance pay a bit extra and get HLMP6000. |
I can confirm this, all of the red leds I have are hovering around 1.95V - 2.1V at 10mA. (I even have a bunch of jumbo red leds from about 1982 which fall into this range.)
You can also measure two or more points on the curve directly with your chosen LED type. Simply measure the difference in mA between two "operating" points and simultaneously measure the voltage across the LED at each of those operating points, then just take the difference in voltage and R just becomes E/I. (More correctly delta of E over delta of I)
I would recommend doing the measurements using the expected range of current in the amplifier stage.
You can take several sets of points (Just 3 points is actually enough) to make sure that R is not too dynamic over the current range you intend to operate. (R varying with current is a distortion generating mechanism.)
Using this technique should allow you to choose diodes with the lowest dynamic resistance variation of the lot(s) you have in addition to determining the VF. |
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| richwalters |
| quote: | Originally posted by dsavitsk
I have a circuit that uses an LED to set a tube's bias. |
I've never found this an method ideal to set up an output stage, estimating LED brightness variations versus actual current is not a sensitive method for the human eye. And yet alot of manufacturers do it. I suppose it's just bog simple, but it isn't if one is slightly 3 colour blind.
richj |
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| dsavitsk |
| quote: | Originally posted by richwalters
I've never found this an method ideal to set up an output stage, estimating LED brightness variations versus actual current is not a sensitive method for the human eye. And yet alot of manufacturers do it. I suppose it's just bog simple, but it isn't if one is slightly 3 colour blind.
richj |
I don't know what this response means at all? I determined the current and voltage by measuring them. |
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| kevinkr |
Hi Doug,
I would hazard a guess that Rich thought you were talking about using a led as a bias setting indicator, not as the actual source of bias.
OT:
ARC of course used comparators that would light the led when the bias was correct, hyper critical to set. The comparators failed regularly when inappropriately chosen output tubes arced over.:smash: |
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| rcavictim |
This is related to cathode bias but from a different angle. Instead of using a cute glowing LED (or several in series) some have used the plate- cathode junction of a small indirectly heated thermionic diode such as a 6AL5 or 6H6 in small signal stage applications, and larger current capability diodes such as 6AX4 or their cousins which are single diodes designed for use as damper tube diodes good for over 100 mA.
Anyone here have any experience with the tube diode technique, or better yet with both techniques and comments? I have experimented with neither yet.
edit to fix typo. |
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| SY |
| Pretty high AC impedance, at least according to the table in Valve Amplifiers. |
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| richwalters |
| Yup Kevin, youre right.. I was a bit hasty...I should have really mentioned to an earlier post that using a 4148 in series ruins all the impedance advantages as diode introduces a wild temp cooef going the wrong way. |
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| rcavictim |
| quote: | Originally posted by SY
Pretty high AC impedance, at least according to the table in Valve Amplifiers. |
So are you suggesting it would act much the same as a cheaper and simpler to implement non- capacitor bypassed cathode resistor, just prettier to look at and ups the tube count (if that is important to your psyche)? |
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| SY |
| Probably a little less linear, but basically, yes. |
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| kevinkr |
| quote: | Originally posted by richwalters
Yup Kevin, youre right.. I was a bit hasty...I should have really mentioned to an earlier post that using a 4148 in series ruins all the impedance advantages as diode introduces a wild temp cooef going the wrong way. |
Um, LOL, that post would have been mine. :D And yes you are right, although the impedance is still far lower than could typically be achieved with a very good electrolytic at LF.. The variation in bias voltage 10mVdegreeC over the temperature range this thing operates has not proven to be problematic so far. I needed about 2.7V for bias with a high transconductance triode. The variation works out to be < 3% typically over the entire temperature range this amplifier is operating at. Less in fact than some of the ever popular CC resistors some people still seem to espouse.. LOL |
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| Sheldon |
| quote: | Originally posted by rcavictim
This is related to cathode bias but from a different angle. Instead of using a cute glowing LED (or several in series) some have used the plate- cathode junction of a small indirectly heated thermionic diode such as a 6AL5 or 6H6 in small signal stage applications, and larger current capability diodes such as 6AX4 or their cousins which are single diodes designed for use as damper tube diodes good for over 100 mA.
Anyone here have any experience with the tube diode technique, or better yet with both techniques and comments? I have experimented with neither yet.
edit to fix typo. |
http://members.aol.com/sbench/dio_bias.html |
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| dsavitsk |
| quote: | Originally posted by kevinkr
Hi Doug,
I would hazard a guess that Rich thought you were talking about using a led as a bias setting indicator, not as the actual source of bias. |
:) That would explain why it didn't make any sense.
Anyhow, thanks for all the suggestions -- I am going to try a few of them and see what seems to work. Additionally, a back of the napkin calculation suggests that the pair of 10uF caps might be big enough to use as ultrapath caps (PS is a cLCRCRC followed by a regulator, so ripple should not be problem) though that might be wishful thinking, so I may give that a shot too. |
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| rdf |
| Hi Bob. I experimented extensively with a couple forgotten triode-diode-diode tubes and though it works never got them to measure or sound as good as simple LED biasing. |
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| dsavitsk |
| quote: | Originally posted by kevinkr
measure the voltage across the LED at each of those operating points, then just take the difference in voltage and R just becomes E/I. (More correctly delta of E over delta of I)
You can take several sets of points (Just 3 points is actually enough) to make sure that R is not too dynamic over the current range you intend to operate. (R varying with current is a distortion generating mechanism.) |
I thought this was interesting, and somewhat surprising as I have heard people recommend blue LEDs -- maybe some others are better:
Red
compared to Blue
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| kevinkr |
| quote: | Originally posted by dsavitsk
I thought this was interesting, and somewhat surprising as I have heard people recommend blue LEDs -- maybe some others are better:
Red
compared to Blue
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Hi Doug,
Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales?? |
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| Sheldon |
| quote: | Originally posted by kevinkr
Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales?? |
Looks pretty much like a wash to me. Red = 0.5V/.035A = 14.2Ohm. Blue = 0.3V/0.02A = 15Ohm.
Sheldon
Compare to these: http://www.avagotech.com/products/p...50,C6889,P91120 |
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| dsavitsk |
| quote: | Originally posted by kevinkr
Hi Doug,
Looking at those curves it would seem that the blue leds have a much larger dynamic resistance component in their characteristics - is that what you read out of this too? Or is it just the difference in scales?? |
I took it to show that the red led's impedance was constant while the blue led's was not. That is, dV/dI for red is constant while blue is a function of V. |
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| SY |
| That's higher for the red LED's impedance than I've typically seen with surplus LEDs. In the dozen or so different samples I've tested, the AC impedance was something between 4 and 10 ohms. Surplus rules. |
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| EC8010 |
| quote: | Originally posted by dsavitsk
I took it to show that the red LED's impedance was constant while the blue LED's was not. That is, dV/dI for red is constant while blue is a function of V. |
All diodes change their dynamic resistance with current, and LEDs are no exception. Having said that, once you're up to about 10mA, things tend to flatten out because although the LED's resistance continues to fall, the resistance of those tiny internal wires connecting to it becomes significant. |
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| rdf |
| quote: | Originally posted by EC8010
....the resistance of those tiny internal wires connecting to it becomes significant. |
Thx, I never considered that because my construction is always groundplane and the LEDs are tightly cut from pin to plane (and bypassed with a small cap for HF.)
It would be interesting to compare notes on the effect of LED impedance. From my experience the non-linearities aren't usually a factor from the perspective of measured performance. Quite often distortion goes down for me with high impedance diodes due to the small local feedback. Bypassing them with a large cap cause distortion to rise. As always, I also find the upper and more noxious harmonics drop to a much greater extent than the 2nd.
I'm currently laying with a Scott LT-110B tube tuner, replacing the common cathode, loop feedback 12AT7 output stage with a no feedback 5678 biased by two series generic green LEDs. At 4 volts p-p out the 2nd harmonic is below -60 dB and the third well below -100 dB with no higher H visible above -120 dB. If 'bad' LEDs cause measurable problems I'm not seeing them. |
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| kevinkr |
Hi Sheldon,
To make a valid comparison you would need to compare two points at the same (delta in) operating current, so in a sense you have confirmed my suspicions..
;) |
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| Sheldon |
I just picked a delta that was easy to read from the graph. My point was that the slope is comparable. The blue version does require higher current to be in the linear region, but without a specific application, can't know if that's an issue. In any case, there are LED's available with much lower dynamic impedance, and linear at much lower currents, so we don't have to split hairs on these.
Sheldon |
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