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LED tube biasing, pros and cons
LED tube biasing, pros and cons
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Old 12th July 2018, 01:36 AM   #101
jackinnj is offline jackinnj  United States
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LED tube biasing, pros and cons
Quote:
Originally Posted by trobbins View Post
Thanks
For viewers without calculators, could provide those test signal levels in mA rms or pp?
dBm = power (milli-Watts) relative to 50 ohm impedance. 0 dBm= 0.2236 Vrms, -30dBm =0.00707 Vrms
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Old 12th July 2018, 02:45 AM   #102
trobbins is offline trobbins  Australia
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Ta, so the 0dB level will force a very large current swing to achieve 0.6Vpp swing across a typical LED, where you have a DC bias voltage set up for those 2 test idle currents. Even the - 30dB signal is likely to force a substantial current swing for 20mVpp.

The low freq level should equate to an incremental resistance taken from static measurements.

The high freq response is likely related to effective jnction capacitance in that forward bias region.

The mid-band response will need some explaining.
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Old 12th July 2018, 02:51 AM   #103
rongon is offline rongon  United States
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LED tube biasing, pros and cons
Quote:
Originally Posted by 20to20 View Post
The 12au7 I referrenced was just another example of 10uA being cutoff, it was on the top of my head at the time. I could have used 6sn7, too. I was adressing PRR in establishing what type of current level was considered effectively 0, because he didn't want to accept the term I used, of nearly 0 amps, when an LED was shut off. In your post above this, asking how often does it happen, I think the question is, when will it happen. When does a diode LED become reversed biased? If the diode has 1.5v across it, and the signal (-) cycle has (-)2v, is it reversed biased? Or is there still 1.5v on the cathode no matter how large the signal goes (-) on the grid, and still flowing forward current? Does the tube never see cutoff until it goes to (-)18v on the grid, (if the plate voltage is 250v.?) Never mind the grid current clipping on the (+), it's a given.
I'm not sure I know enough about the physics of the situation, but my understanding is that LED bias is best used with small signal stages, such as in phono preamps or hi-fi line level stages. One would not use LED bias in a power stage that would be expected to clip often.

Similarly, a CCS or gyrator plate load has a sharp cutoff between safe operation area and overload. While a CCS in the plate load of a triode will reduce distortion and maximize gain, it will also make the transition to overload (grid current or cutoff) very abrupt, with harsh clipping.

So while I think you're correct that using LED cathode bias would create complications as that stage approaches overload conditions, I don't think it's good practice to use LED bias in a stage that's likely to be overloaded in normal operation. That's why you'll never see LED bias used in a guitar amp. The clipping would be gnashy and nasty, which is the opposite of what you want from a guitar amp (and helps explain why most tube guitar amps employ basic circuitry made with nothing but tubes, Rs, Cs, and Ls).

I hope that's relevant. If not, please excuse the interruption.
--

Last edited by rongon; 12th July 2018 at 02:55 AM.
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Old 12th July 2018, 04:12 AM   #104
PRR is offline PRR  United States
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Quote:
Originally Posted by 20to20 View Post
...He didn't get it.
The tube will "cut off" LONG before the LED will.

Try it. Any tube you like. Any circuit which will pass audio (we agree that picoAmps is not a happy place for audio).
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Old 12th July 2018, 04:12 AM   #105
daqvin_carter is offline daqvin_carter  United States
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You can get strings of LEDs with adhesive backing if you are biasing a power amplifier.
They are made for 12 volts with three LEDs and a resistor per segment. Cut 3-4 segments and put them in series for 12 volt increments.

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Old 12th July 2018, 04:15 AM   #106
PRR is offline PRR  United States
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Quote:
Originally Posted by rongon View Post
...One would not use LED bias in a power stage that would be expected to clip often.... --
Actually, this one would.

Also if this is a Hi-Fi thread, the amps should not "clip often".

In guitar-amp context, where clipping is fun, it needs study. Historically we have slammed the cathode R-C network for dynamic change of timbre. LED will do it different. However the grid R-C has been a more prolific source of dynamic change.

ALSO: LED bias is very little different from Fixed Bias. And fix-bias has been a mainstay of higher power output stages since the 1930s. One issue is that tube power stages need more V and I of bias than 10/$1 LEDs will support. However high-power LEDs may be stacked as needed. No more or less wasteful than Zener bias (which I have seen done). Free light.

Last edited by PRR; 12th July 2018 at 04:18 AM.
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Old 12th July 2018, 07:49 AM   #107
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by PRR View Post
Actually, this one would.
I'm surprised SY hasn't stuck his head in to pimp the Red Light Disctrict again?
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Old 12th July 2018, 01:49 PM   #108
jackinnj is offline jackinnj  United States
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LED tube biasing, pros and cons
Quote:
Originally Posted by trobbins View Post
Ta, so the 0dB level will force a very large current swing to achieve 0.6Vpp swing across a typical LED, where you have a DC bias voltage set up for those 2 test idle currents. Even the - 30dB signal is likely to force a substantial current swing for 20mVpp.
Did this run of various stimuli -- from -30dBm to +10dBm -- with 1.25mA flowing through a red HLMP 1340 LED:
Attached Images
File Type: png LED_Impedance_Stimuli.png (68.0 KB, 136 views)
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Old 12th July 2018, 03:06 PM   #109
trobbins is offline trobbins  Australia
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Perhaps there is waveform distortion occuring in the test signal, or some other effect related to the test jig? I just doubt the LED is the cause of that response.
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Old 13th July 2018, 11:51 AM   #110
jackinnj is offline jackinnj  United States
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LED tube biasing, pros and cons
Some effect of the test jig? I have placed an inquiry. The baseline 50 ohm Caddock is ruler flat when using the J2130A DC Bias Injector.

Using the "Gain" method, I get consistentcy -- Z at -10dBm, 0dBm and +10dBm:
Attached Images
File Type: png Led_Impedance_Gain_Jig.png (13.4 KB, 90 views)
File Type: png LED_Impedance_GainMethod.png (68.9 KB, 93 views)
File Type: png LED_Current_Injector.png (68.1 KB, 91 views)
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