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LED tube biasing, pros and cons
LED tube biasing, pros and cons
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Old 8th July 2018, 07:52 AM   #21
artosalo is offline artosalo  Finland
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Quote:
TL431 has a dynamic impedance of 0.2...0.5 ohms...
It sounds as horrible as a piece of wire ?
Then all fixed bias circuits should sound too, but why they do not ?
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Old 8th July 2018, 08:13 AM   #22
trobbins is offline trobbins  Australia
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I guess the worry with TL431 type circuits is if they unknowingly move in to either non-linear extremity region for part of a waveform. Some may not realise how a supposed class A action of output stage devices can deviate, especially if the common cathode current includes grid and screen currents, mu imbalance, and non-ideal transformer windings.
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Old 8th July 2018, 09:36 AM   #23
dreamth is offline dreamth  Romania
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a resistor paraleled by a capacitor is also providing a quasi-fixed bias at ac , that is why many people would prefer less gain and better distortion figures by giving up the capacitor.Better S/N ratio and higher gain is not allways what we need in a valve amp.Way too many possibilities outhere!
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Old 8th July 2018, 05:01 PM   #24
20to20 is offline 20to20  United States
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Originally Posted by DAK808 View Post
I was wondering if there is any reason to try LED biasing of a 6j5 in a SE amp where this tube is the driver for a 6550? I am using a 1w resistor now. If i put the LED in how do i set it up other than just replacing the resistor? I know different LED colors correspond to different voltage requirements. Does that mean i need to select the color according to my voltage drop? Thanks for all the help.
I don't see any recommended cathode resistor values under 1.3K. How do you control the tube current to a specific value to keep it under max dissipation if an LED has no set resistance? What supply voltage will the LED see on the cathode? You have to know that to restrict the current through any LED. Will you use an LED and a current limiter resistor? What's the point of the LED then?

Last edited by 20to20; 8th July 2018 at 05:18 PM.
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Old 8th July 2018, 05:35 PM   #25
DAK808 is offline DAK808  United States
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LED tube biasing, pros and cons
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Originally Posted by 20to20 View Post
I don't see any recommended cathode resistor values under 1.3K. How do you control the tube current to a specific value to keep it under max dissipation if an LED has no set resistance? What supply voltage will the LED see on the cathode? You have to know that to restrict the current through any LED. Will you use an LED and a current limiter resistor? What's the point of the LED then?
not sure what you are getting at. The led will have a fixed current use. The led determines the voltage thru the cathode. The resistor if you use both is to provide degenerative feedback. But i had not tried that yet.
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Old 8th July 2018, 05:40 PM   #26
20to20 is offline 20to20  United States
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Originally Posted by DAK808 View Post
not sure what you are getting at. The led will have a fixed current use.
Can you calculate what the current will be?
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Old 8th July 2018, 05:57 PM   #27
euro21 is offline euro21  Hungary
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LED tube biasing, pros and cons
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Originally Posted by 20to20 View Post
Can you calculate what the current will be?
Yes.
If you want to use tube (for example D3a as triode), at the required operating point (for example 175V anode voltage, 10mA current) the grid voltage is -2V (from datasheet).

You can use R//C cathode complex (2V/10mA=200R paralleled with appropriate capacitor), or any device, which is produce 2V if 10mA current flowing through it.

The old type 5mm green LED is such a device....if the current swing not too much.
If you use CCS as anode load, the current is virtually invariable, so reverse voltage of LED is stable.
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Old 8th July 2018, 06:02 PM   #28
DAK808 is offline DAK808  United States
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LED tube biasing, pros and cons
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Originally Posted by rongon View Post

If your 6J5 has a 1k cathode bias resistor and is drawing 4mA plate current, then you need -4V grid bias, or +4V at the tube's cathode. To achieve that, use two green LEDs in series from the tube's cathode to ground to get +4V at the tube's cathode.


Note that if the 6J5 in your circuit uses its 1k resistor with no bypass capacitor, then it's using current feedback from the unbypassed cathode resistor. Putting an LED in there will ruin that. (Gain will go up quite a bit.)
In the first statement why not use a single blue led which will be close to the 4v drop?
In the second statement someone mentioned using a led followed by a resistor or is it a resistor followed by a led to have some feedback. Is that applicable here?
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Old 8th July 2018, 06:10 PM   #29
gabdx is offline gabdx  Canada
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I like led biasing for 6j5 etc, even use it in pre-amps.

They should be an improvement for small tubes over the traditional resistor/cap setup.
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Old 8th July 2018, 06:11 PM   #30
merlin el mago is offline merlin el mago
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There are LEDs 2.8Vf 1.5mA?
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