Hi CLS. Sorry, didn't check your bio first. I'ld expect an engineer to spec or measure first. 
I've found, just as the spec sheets reflect, the longer the LED's emitted wavelength the more voltage varies with current. If memory serves the IR I posted about earlier changed at most two or three percent with current from 10 ma to 15 ma. I'll have a better opportunity to compare like for like now because I found an IR LED with extremely low AC impendance - about 3 ohms @ 15 ma on a quick calc - but unlike the the other IR's the voltage drop is in the 1.6 Vdc range. The rest hovered around 1.1 Vdc. In DC terms this one should be almost identical to a red LED. Maybe later in the weekend I'll do a quick VI analysis of a few and post it.
I don't normally like to post details about sonic impressions because changes are so inter-related with the rest of the circuit you never know what they mean. The amp I used is an EL84 SE with a 6C45 front end, plenty of iron and oil caps, and the change went from top to bottom. I got the same impression mids to top but also heard the everything below open up. I'm really curious now what the new LEDs do.
I've found, just as the spec sheets reflect, the longer the LED's emitted wavelength the more voltage varies with current. If memory serves the IR I posted about earlier changed at most two or three percent with current from 10 ma to 15 ma. I'll have a better opportunity to compare like for like now because I found an IR LED with extremely low AC impendance - about 3 ohms @ 15 ma on a quick calc - but unlike the the other IR's the voltage drop is in the 1.6 Vdc range. The rest hovered around 1.1 Vdc. In DC terms this one should be almost identical to a red LED. Maybe later in the weekend I'll do a quick VI analysis of a few and post it.
I don't normally like to post details about sonic impressions because changes are so inter-related with the rest of the circuit you never know what they mean. The amp I used is an EL84 SE with a 6C45 front end, plenty of iron and oil caps, and the change went from top to bottom. I got the same impression mids to top but also heard the everything below open up. I'm really curious now what the new LEDs do.
I have on several amps now. The 6SN7 driver stage of my main amp has 4 LED's in series, but the total AC impedance goes up unfortunately. Still works well though! An alternative is to run higher current and have parallel chains of LED's, or use lower current LED's. This also worked very well in the EL84 output stage of my headphone amp, haven't experimented to see how important matching is though.... perhaps something else to investigate.
Steve
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So I made 2 CCS from the schema at tubelabs.com,changed the resistor so I got 15mA,and this was a bigger improvement to the sound than just the led´s.Wider soundstage,bigger separations between the instruments eventighter bass,now I hear instruments I havent heard before. I have been dead against mixing tubes and transitors before,well I guess I have to change my mind now.
By the way,what is the different in using a choke at the anode?would this be a better way?Can you compare this 2 metods?
Thanks for the tips!
I have been dead against mixing tubes and transitors before,well I guess I have to change my mind now. By the way,what is the different in using a choke at the anode?would this be a better way?Can you compare this 2 metods?
Thanks for the tips!
If you decide to build one of SY's LED CCS, heres a little tip. If you use the variable resistor to trim the current - make absolutely certain that you don't adjust this bellow about 100R . If you do you will ask the first transistor to conduct to much current and will send it out of its safe operating range - result quick death of the said transistor.
The CCS will probably still work as the second transistor will still function as a CCS, but you will get distortion and much reduced performance.
Shoog
The CCS will probably still work as the second transistor will still function as a CCS, but you will get distortion and much reduced performance.
Shoog
The color will affect the cathode voltage and can have a small effect on gain and distortion. You want the lowest impedance possible (yellows run 12-15 ohms, red runs 4-9 ohms) and a voltage appropriate for the stage you've got it in (yellow is about 2.0V, red about 1.65V).
It's also important to have at least 8-10mA running through the diode to minimize nonlinearity.
It's also important to have at least 8-10mA running through the diode to minimize nonlinearity.
Shoog said:
That's why R2 was included - to avoid accidental destruction of the silicon. It's usually the transistor nearesr the load that fails, not because Ic(max.) is exceeded, but because the combination of high current and standing voltage cause its power rating to be exceeded.
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You do it like this...
Depends on what the transistor nearest the load can withstand. If you were using a -15V supply to the CCS, each (red) LED would drop about 1.65V, so the voltage at the top of the upper LED would be -15 + 2 x 1.65 = -11.7V. The upper transistor will drop 0.7V across its base-emitter junction, so the emitter will be at -12.4V. Suppose you were using a valve with its cathode at +8V; that would mean that the upper transistor would have +8V - -12.4V = 20.4V across it. If the transistor was a BC549C and you wanted to limit its dissipation to 0.2W to keep it cool, then P/V = I = 0.2/20.4 = 9.8mA.
If you were using an MJE340 and wanted to keep dissipation to <2W, then P/V = I = 2/20.4 = 98mA. The lower transistor's conditions should then be checked. The lower transistor has 1.65V across it, so keeping power to 0.2W, P/V = I = 0.2/1.65 = 121mA, so that's not a problem. The BC549C is a 100mA device, so 98mA is within ratings. Just.
To calculate R2: The current programming resistance always has the LED voltage drop less the base-emitter voltage drop across it. For a typical red LED, that means 1.65V - 0.7V = 0.95V. If you knew you had to limit the current to 9.8mA with the variable resistor set to 0R, then R2 must set the current, so V/I = R = 0.95/0.0098 = 96.9R. Nearest value is 100R. Similarly, if you had to limit to 98mA, then 0.95/0.098 = 9.68R, and nearest value is 10R.
ryssen said:
Depends on what the transistor nearest the load can withstand. If you were using a -15V supply to the CCS, each (red) LED would drop about 1.65V, so the voltage at the top of the upper LED would be -15 + 2 x 1.65 = -11.7V. The upper transistor will drop 0.7V across its base-emitter junction, so the emitter will be at -12.4V. Suppose you were using a valve with its cathode at +8V; that would mean that the upper transistor would have +8V - -12.4V = 20.4V across it. If the transistor was a BC549C and you wanted to limit its dissipation to 0.2W to keep it cool, then P/V = I = 0.2/20.4 = 9.8mA.
If you were using an MJE340 and wanted to keep dissipation to <2W, then P/V = I = 2/20.4 = 98mA. The lower transistor's conditions should then be checked. The lower transistor has 1.65V across it, so keeping power to 0.2W, P/V = I = 0.2/1.65 = 121mA, so that's not a problem. The BC549C is a 100mA device, so 98mA is within ratings. Just.
To calculate R2: The current programming resistance always has the LED voltage drop less the base-emitter voltage drop across it. For a typical red LED, that means 1.65V - 0.7V = 0.95V. If you knew you had to limit the current to 9.8mA with the variable resistor set to 0R, then R2 must set the current, so V/I = R = 0.95/0.0098 = 96.9R. Nearest value is 100R. Similarly, if you had to limit to 98mA, then 0.95/0.098 = 9.68R, and nearest value is 10R.
Hi lndm. I observed an interesting effect on the bench with a LED biased 6C45-pe. Since a LED has an impedance greater than zero there will always be a some audio AC voltage generated across it when sinking AC current. It results in the LED generating a second harmonic distortion voltage across itself. Under normal bias conditions both the fundamantal voltage drop across the LED and its harmonics are so far down the effect on the tube's output appears to be well below the residual noise floor of any consumer audio source I know. However, as the DC current through a LED decreases its distortion increases. So now you have a tube generating 2nd harmonics and an LED in the cathode generating its own significant set of 2nd harmonics and (pending a second run on the bench for confirmation) 2nd harmonic cancellation occurs! The total distortion of tube + LED drops as current is reduced. Theoretically higher harmonics are being generated but if the tube is already substantially linear, such as the 6c45-pe where the second was 55 to 60 dB down, these higher harmoonics seem well below the capabilities of my 24-bit sound card to register.
In other words, depending of course on the load your 6sn7gt sees, raising the bias on the LED might actually increase the distortion. I would try varying the bias both ways. Eventually I'll return to the circuit I mentioned and couple it to a cathode follower for testing as a potential low-impedance, low-distortion, no global feedback driver.
In other words, depending of course on the load your 6sn7gt sees, raising the bias on the LED might actually increase the distortion. I would try varying the bias both ways. Eventually I'll return to the circuit I mentioned and couple it to a cathode follower for testing as a potential low-impedance, low-distortion, no global feedback driver.
My personal experience is counter to what rdf suggests. I had a anode follower 6922 biased at about 4ma with a red LED. It sounded soft (the sort sound you would expect with second harmonic distortion). I supplemented the bias through the LED with about 6ma more current. The softness disappeared completely.
"In effect I did. The tube used a variable CCS as a plate load, the current adjustable via a ten-turn pot. As current went down so did the 2nd. I recall it being in the 6 ma range and a red cathode LED but not any details beyond that."
It sounds to me that you are trying to pass more or less current though your 6c45's than its bias point is asking the tube to pass. This would be different from adding supplementary current to the LED, so you are probably experiencing a different effect which is more dependent on under or over driving your tube.
My expereience with SY's CCS, as an anode load, was that the sound varied markedly if you altered the current through the tube whilst maintaning the same LED derived cathode bias point.
I believe this is the effect you are analysing. I would guess that what is happening is that the tube and the LED are generating 2nd harmonics in antiphase, which are cancelling and reducing overall 2nd harmonic distortion whilst each is infact distorting more.
My experience says it is definately worth boosting the current through the LED to a minimum of 10ma. This can be achieved with a simple circuit if the bias point of the tube is below 5Volts.
Rectify your heater supply and use a simple 7805 voltage regulator. Then work out a suitable resistor to form a voltage divider with the LED chain (ie for a bias point of 3V and a required supplementary current of 5ma 5-3=2V 2/0.005=400ohms). Couldn't be simpler than that really. An example of the circuit can be found on my thread "my version of the FVP5).
Shoog
"In effect I did. The tube used a variable CCS as a plate load, the current adjustable via a ten-turn pot. As current went down so did the 2nd. I recall it being in the 6 ma range and a red cathode LED but not any details beyond that."
It sounds to me that you are trying to pass more or less current though your 6c45's than its bias point is asking the tube to pass. This would be different from adding supplementary current to the LED, so you are probably experiencing a different effect which is more dependent on under or over driving your tube.
My expereience with SY's CCS, as an anode load, was that the sound varied markedly if you altered the current through the tube whilst maintaning the same LED derived cathode bias point.
I believe this is the effect you are analysing. I would guess that what is happening is that the tube and the LED are generating 2nd harmonics in antiphase, which are cancelling and reducing overall 2nd harmonic distortion whilst each is infact distorting more.
My experience says it is definately worth boosting the current through the LED to a minimum of 10ma. This can be achieved with a simple circuit if the bias point of the tube is below 5Volts.
Rectify your heater supply and use a simple 7805 voltage regulator. Then work out a suitable resistor to form a voltage divider with the LED chain (ie for a bias point of 3V and a required supplementary current of 5ma 5-3=2V 2/0.005=400ohms). Couldn't be simpler than that really. An example of the circuit can be found on my thread "my version of the FVP5).
Shoog
Shoog said:
That's exactly what I mean! I think it works especially well with the 6C45-pe because it's a low distortion and an incredibly versatile tube - everything from 40 ma bias in a spud amp to low bias phono stages judging from the projects on line - and invariant. Time and again in different circuits I've swept the standing bias through a CCS plate-loaded 6c45 and seen little or no change in 2nd, 3rd and 4th harmonic content from below 10 ma to 25 plus. Since the LED will have minimal variation in voltage drop across that range I suspect by varing the current it's really the 2nd harmonic distortion of the LED being tuned against the tube's.
You're also right that I made it sound a bit too simple. No doubt a number of factors have to line up for this to work, including the LED's voltage drop, the 2nd harmonic levels of tube and LED, the tube's gain, etc.. Also at such low currents the tube's arguably no longer suitable as a driver (my application, both EL84 and KT100 two stage amps. Now that's versatility) which is why I'll investigate coupling it to a cathode follower. Regrading how it sounds, no idea yet. Future project. I still suspect it's very worth trying.
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