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LED bias indicator
LED bias indicator
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Old 30th December 2017, 08:48 PM   #21
MarcelvdG is offline MarcelvdG  Netherlands
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You may not need the Zener; a 4049 can handle a 12 V supply without any problem, a 4049B can even work on 15 V (just don't use a 74HC4049).
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Old 30th December 2017, 10:46 PM   #22
PRR is offline PRR  United States
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Quote:
Originally Posted by denny9167 View Post
I found this circuit online....
You would not use '741 because...

Smart modern people have forgot how to use '741
The Vos is a bit high for this application
The input stops working ~~2V above the negative supply rail (we want less)
You can hardly get a '741 today; the few left should be saved for people who need them
There are cheaper better modern parts

The LM339/LM393 is perfect. Costs pennies, current production (DIP may be old stock). Inputs go all the way to the negative rail. Output is open-collector which allows logic tricks.

The rig below shows red if too hot, turn-down now! Yellow if too cool, no panic, turn up for best sound. When right, it is dark, except signal peaks may flicker.
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Old 30th December 2017, 10:53 PM   #23
globalplayer is offline globalplayer  Germany
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Why not engineer a complete regulated bias adjustment?
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Old 30th December 2017, 11:04 PM   #24
PRR is offline PRR  United States
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Originally Posted by globalplayer View Post
Why not engineer a complete regulated bias adjustment?
It is better to have knobs and lights.
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Old 30th December 2017, 11:05 PM   #25
globalplayer is offline globalplayer  Germany
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Originally Posted by PRR View Post
It is better to have knobs and lights.


You can use a on/off switch and use leds anyway.
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Old 2nd January 2018, 01:46 PM   #26
denny9167 is offline denny9167  United States
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A better solution

An Accurate Bias Meter for Tube Output Stages
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Old 2nd January 2018, 06:49 PM   #27
6A3sUMMER is offline 6A3sUMMER  United States
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If we are having to set the fixed bias on tubes, don't we need to check them as the tube ages.
And if we have to set them to less negative as the tube ages, don't we have less max power from the amp, and potentially more distortion. Is it not just time to replace the tube(s)?

Self Bias requires a resistor (or current source) for each tube, and a bypass capacitor for each tube, unless we are cathode coupling with a current source. We can have a test point for each cathode, or for parallel cathodes.
This way, we can check the cathode voltage easily over time (unless we are too lazy).
It takes a $10 DMM.

You might even find it varies due to time of day, etc., according to the electric power delivered to your power outlet (unless everything is regulated, B+ and filaments).
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Old 20th January 2018, 10:42 PM   #28
denny9167 is offline denny9167  United States
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I’ve drawn up a circuit that works really well, it indicates low, high and balanced
Bias. There are two 10k pots that calibrate the window comparators, will have its own power supply, and can connect from a tap off heater winding.

YouTube
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Last edited by denny9167; 20th January 2018 at 10:46 PM.
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Old 21st January 2018, 05:37 AM   #29
MarcelvdG is offline MarcelvdG  Netherlands
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Looks good! I see only a part of the schematic, but if I understand it correctly, Ref is the upper threshold and WV the lower. Depending on the supply voltage, R23 and R24 may have to be changed to stay below the LM339's maximum output current.

Last edited by MarcelvdG; 21st January 2018 at 05:49 AM.
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Old 21st January 2018, 09:06 AM   #30
denny9167 is offline denny9167  United States
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Quote:
Originally Posted by MarcelvdG View Post
Looks good! I see only a part of the schematic, but if I understand it correctly, Ref is the upper threshold and WV the lower. Depending on the supply voltage, R23 and R24 may have to be changed to stay below the LM339's maximum output current.
Yes, In the YouTube video I posted I was using a 5volt DC Power source,so I used 330R for the limiting resistor. The neat thing is even if the upper and lower window are very close together
Let’s say 650mV and 600mV are my window, the green LED doesn’t shut completely off but the High and Low LEDs are completely off when the target voltage is reached.

Last edited by denny9167; 21st January 2018 at 09:21 AM.
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