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LED tube biasing, pros and cons
LED tube biasing, pros and cons
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Old 6th July 2018, 09:44 PM   #1
DAK808 is offline DAK808  United States
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LED tube biasing, pros and cons
Default LED tube biasing, pros and cons

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.
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Old 7th July 2018, 04:54 AM   #2
rongon is offline rongon  United States
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LED tube biasing, pros and cons
I know this is covered in many previous threads and articles, as well as a few books. Quick answers:

Installing an LED for cathode bias is as simple as it gets. Just install from cathode of tube to ground, with the LED's cathode connecting to ground (LED anode to tube's cathode). The LED simply replaces the resistor and its bypass capacitor.

Different color LEDs exhibit different forward voltage drop. Rough estimates of color vs. V drop:

1N4148 diode = 0.7V (approx)
Infrared = 1.2V
Red = 1.8V to 2.0V
Green = 1.9V to 2.2V
Yellow = c. 2V
White = 3.2V (roughly)
Blue = 3.6V to maybe 4V

Note that there are kazillions of different LEDs made, each with slightly different forward voltage drop. They can vary quite a bit. You may need to buy a batch and test them for yourself.

The voltage across the LED goes up slightly with increasing current.

Most standard package LEDs have max current capacity of 20mA.

You can put LEDs in series to achieve higher voltage. They simply add.

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.

The desirable quality of LED cathode bias is that you get the desired voltage drop with very little internal resistance. It's like a cathode resistor with a large value bypass capacitor in one easy to use part, which lights up to tell you it's working.

Some people say using an LED for cathode bias makes the tube 'sound more solid-state.' No comment from me on that.

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.)

I think the best discussion of using LEDs for cathode bias of tubes is found in Morgan Jones' "Valve Amplifiers" (3rd Edition, also in the 4th Ed.).
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Old 7th July 2018, 06:24 AM   #3
jackinnj is offline jackinnj  United States
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LED tube biasing, pros and cons
Just because an LED has a certain forward voltage doesn’t mean that it is low impedance - and a high Z LED militates against its use.
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Old 7th July 2018, 06:35 AM   #4
juanitox is offline juanitox  France
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everything put on the cathode have a big influence on the sound, unbypassed resistor , resistor/cap, led , schottky diodes, CCS , battery ,..

my own choice will be fixed bias at first but if i can't i will take unbypassed resistor ( with R under 200ohm)
than schottky diodes , resistor /cap led will be the last
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Old 7th July 2018, 07:03 AM   #5
rongon is offline rongon  United States
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LED tube biasing, pros and cons
Quote:
Originally Posted by jackinnj View Post
Just because an LED has a certain forward voltage doesn’t mean that it is low impedance - and a high Z LED militates against its use.
Example of a high Z LED?

According to Morgan Jones (in his book "Valve Amplifiers") a blue LED he measured gave Vdrop of 3.7V with internal resistance of 30 ohms. Is that considered 'high'?
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Old 7th July 2018, 10:28 AM   #6
artosalo is offline artosalo  Finland
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TL431 would be excellent replacement for a LED and adjustable between 2.5 V up to 37 V.

General features:

Quote:
• Low Dynamic Output Impedance, 0.22 ? Typical
• Sink Current Capability of 1.0 mA to 100 mA
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Old 7th July 2018, 11:47 AM   #7
artosalo is offline artosalo  Finland
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Like this. The desired bias voltage is adjusted with R2 and R5.
The reference voltage at TL431 is always 2.5 V.
Attached Images
File Type: png 6SN7_TL431_bias.png (16.7 KB, 529 views)
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Old 7th July 2018, 12:09 PM   #8
dreamth is offline dreamth  Romania
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LED's will get you better S/N ratio, higher gain and higher distortion figures as the valves biased that way will saturate easily, no resistor-no feedback...I usually go with a combination of resistor//capacitor+resistor or resistor + led to get some feedback.
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Old 7th July 2018, 12:20 PM   #9
rongon is offline rongon  United States
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LED tube biasing, pros and cons
Quote:
Originally Posted by artosalo View Post
TL431 would be excellent replacement for a LED and adjustable between 2.5 V up to 37 V.
Good one! And its data sheet says it can sink up to 100mA. I wonder why we don't see this mentioned more often.

However, wouldn't TL431 have the same problem at low voltages that something like an LM317 (used as a ccs) would have? TL431 reference voltage is 2.5V, so at least that plus a little more must be dropped across the device for it to function. Let's say you use it in this hypothetical 6J5 drawing 4mA plate current, with +4V at the 6J5 cathode. Doesn't that mean the resulting 6J5 with Ip=4mA using TL431 for cathode bias can only accept 1.5V of input signal before the TL431 is in danger of dropout?

But for a driver stage with a 'tall' bias, like a 6J5 with more like Vk=6V and Ip=8mA, a TL431 looks like a great option. Thanks for the tip!
--

Last edited by rongon; 7th July 2018 at 12:41 PM.
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Old 7th July 2018, 01:27 PM   #10
artosalo is offline artosalo  Finland
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Quote:
However, wouldn't TL431 have the same problem at low voltages that something like an LM317 (used as a ccs) would have?
No it does not, because it is not connected as a CCS. It is a CVS and keeps the grid to cathode voltage fixed as long as some (less than) 0.5 mA current flows thru it (and the tube).

Last edited by artosalo; 7th July 2018 at 01:38 PM.
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