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LED Bias Modifications

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dsavitsk

dsavitsk

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Joined 2005
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I have a circuit that uses an LED to set a tube's bias. The amp, with a basic schematic, can be seen at http://www.ecp.cc/semha.html

The current tube is I am using is a 7788, and I have it biased with a single LED at about 2.35V. With a B+ of 160V, this is leading to a current draw ~30mA ... maybe a tad more. I would like to reduce this slightly by upping the bias to closer to 3V. So, the question is, should I pick up some 3V LEDs, or will a Schottky diode in series with the currently used LED work just as well? Anyone with any reason one solution is better than the other?
 
S

Sheldon

Member
Joined 2003
You can do as 8010 suggests, but no reason not to try a Schottky or other diode and see how it sounds to you. The dynamic impedance of the Schottky will not be as low as the LED and will cause some distortion, but don't know how audible it will be. Another option, if you can give up a little gain, to try just a small series resistor, say 100r or so. A little degeneration will reduce distortion, might sound better, might not.

Sheldon
 
kevinkr

kevinkr

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dsavitsk said:
I have a circuit that uses an LED to set a tube's bias. The amp, with a basic schematic, can be seen at http://www.ecp.cc/semha.html

The current tube is I am using is a 7788, and I have it biased with a single LED at about 2.35V. With a B+ of 160V, this is leading to a current draw ~30mA ... maybe a tad more. I would like to reduce this slightly by upping the bias to closer to 3V. So, the question is, should I pick up some 3V LEDs, or will a Schottky diode in series with the currently used LED work just as well? Anyone with any reason one solution is better than the other?
Click to expand...

Hi Doug,
I've found certain red leds with a 1N4148 in series to be a reasonable compromise. (Larger diodes tend to sound distorted.)
 
mach1

mach1

Banned
Joined 2004
How do you know the AC resistance of the different LED's?
Click to expand...

Measure the slope of the line at your chosen operating current on the datasheet forward current / forward voltage graph (if available).

BTW, very few of the commonly available red diodes produce a 1.6.- 1.7 forward voltage - most are now in the region of 2.1V. If you want a 1.7V drop red diode with a guaranteed low dynamic impedance pay a bit extra and get HLMP6000.
 
kevinkr

kevinkr

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mach1 said:


Measure the slope of the line at your chosen operating current on the datasheet forward current / forward voltage graph (if available).

BTW, very few of the commonly available red diodes produce a 1.6.- 1.7 forward voltage - most are now in the region of 2.1V. If you want a 1.7V drop red diode with a guaranteed low dynamic impedance pay a bit extra and get HLMP6000.
Click to expand...


I can confirm this, all of the red leds I have are hovering around 1.95V - 2.1V at 10mA. (I even have a bunch of jumbo red leds from about 1982 which fall into this range.)

You can also measure two or more points on the curve directly with your chosen LED type. Simply measure the difference in mA between two "operating" points and simultaneously measure the voltage across the LED at each of those operating points, then just take the difference in voltage and R just becomes E/I. (More correctly delta of E over delta of I)

I would recommend doing the measurements using the expected range of current in the amplifier stage.

You can take several sets of points (Just 3 points is actually enough) to make sure that R is not too dynamic over the current range you intend to operate. (R varying with current is a distortion generating mechanism.)

Using this technique should allow you to choose diodes with the lowest dynamic resistance variation of the lot(s) you have in addition to determining the VF.
 
richwalters

richwalters

Member
Joined 2003
dsavitsk said:
I have a circuit that uses an LED to set a tube's bias.
Click to expand...


I've never found this an method ideal to set up an output stage, estimating LED brightness variations versus actual current is not a sensitive method for the human eye. And yet alot of manufacturers do it. I suppose it's just bog simple, but it isn't if one is slightly 3 colour blind.

richj
 
dsavitsk

dsavitsk

diyAudio Member
Joined 2005
diyaudio.ecp.cc
Re: Re: LED Bias Modifications

richwalters said:



I've never found this an method ideal to set up an output stage, estimating LED brightness variations versus actual current is not a sensitive method for the human eye. And yet alot of manufacturers do it. I suppose it's just bog simple, but it isn't if one is slightly 3 colour blind.

richj
Click to expand...

I don't know what this response means at all? I determined the current and voltage by measuring them.
 
kevinkr

kevinkr

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Hi Doug,
I would hazard a guess that Rich thought you were talking about using a led as a bias setting indicator, not as the actual source of bias.

OT:
ARC of course used comparators that would light the led when the bias was correct, hyper critical to set. The comparators failed regularly when inappropriately chosen output tubes arced over.:smash:
 
rcavictim

rcavictim

Member
Joined 2004
What about tube diodes?

This is related to cathode bias but from a different angle. Instead of using a cute glowing LED (or several in series) some have used the plate- cathode junction of a small indirectly heated thermionic diode such as a 6AL5 or 6H6 in small signal stage applications, and larger current capability diodes such as 6AX4 or their cousins which are single diodes designed for use as damper tube diodes good for over 100 mA.

Anyone here have any experience with the tube diode technique, or better yet with both techniques and comments? I have experimented with neither yet.

edit to fix typo.
 
kevinkr

kevinkr

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richwalters said:
Yup Kevin, youre right.. I was a bit hasty...I should have really mentioned to an earlier post that using a 4148 in series ruins all the impedance advantages as diode introduces a wild temp cooef going the wrong way.
Click to expand...

Um, LOL, that post would have been mine. :D And yes you are right, although the impedance is still far lower than could typically be achieved with a very good electrolytic at LF.. The variation in bias voltage 10mVdegreeC over the temperature range this thing operates has not proven to be problematic so far. I needed about 2.7V for bias with a high transconductance triode. The variation works out to be < 3% typically over the entire temperature range this amplifier is operating at. Less in fact than some of the ever popular CC resistors some people still seem to espouse.. LOL
 
S

Sheldon

Member
Joined 2003
Re: What about tube diodes?

rcavictim said:
This is related to cathode bias but from a different angle. Instead of using a cute glowing LED (or several in series) some have used the plate- cathode junction of a small indirectly heated thermionic diode such as a 6AL5 or 6H6 in small signal stage applications, and larger current capability diodes such as 6AX4 or their cousins which are single diodes designed for use as damper tube diodes good for over 100 mA.

Anyone here have any experience with the tube diode technique, or better yet with both techniques and comments? I have experimented with neither yet.

edit to fix typo.
Click to expand...

http://members.aol.com/sbench/dio_bias.html
 
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