The Red Light District - another PP EL84 amp - Page 61

SY · 21st November 2009, 02:32 PM

Ahh, I see what you guys are getting at. Yes, once the conventional RC cathode bias has been removed, and the grid current limited, you can then build a negative supply, regulate it, then attach the bottom of the grid leaks to it. Or, for five bucks and a free light show, you can get the same instantaneous bias voltage recovery and no more impedance penalty than the usual idle current sensing resistor.

jackinnj · 21st November 2009, 02:51 PM

Quote:

Originally Posted by GK

Won't work. An LM317 has a single NPN series pass element, so, as a voltage regulator, it can only source current, not sink it. In other words, the cathode current of the tube will reverse bias the pass device and the voltage will shoot up to whatever.

It is fine to use an LM317 here as a current regulator/sink as it will need to be bypassed with big electro anyway. This makes the HF performance of the current regulator/sink irrelevant.

But anyway, all this still looks quite pointless to me. I'd just connect the cathodes to ground and use (adjustable) fixed bias on the grids.

Further to your point -- Walt Jung's 2 part series on current sources is available on the AudioXpress website for free. Well worth the reading.

TheGimp · 23rd November 2009, 12:33 AM

Quote:

Originally Posted by GK

Won't work. An LM317 has a single NPN series pass element, so, as a voltage regulator, it can only source current, not sink it. In other words, the cathode current of the tube will reverse bias the pass device and the voltage will shoot up to whatever.

It is fine to use an LM317 here as a current regulator/sink as it will need to be bypassed with big electro anyway. This makes the HF performance of the current regulator/sink irrelevant.

But anyway, all this still looks quite pointless to me. I'd just connect the cathodes to ground and use (adjustable) fixed bias on the grids.

Yup, I wasn't thinking. LM 317 is a source not a sink.

I have both output tubes biased up with the LED stacks now. I trimmed the screen supply resistor to get it to 200V and all is up and running nicely.

I'll work on a couple screen supplies next, then I have something in mind to try in place of the diodes, but will refrain from sticking my foot in my mouth again till I do some more research.

GK · 23rd November 2009, 12:43 AM

Quote:

Originally Posted by TheGimp

Yup, I wasn't thinking. LM 317 is a source not a sink.

Come to think of it, you could make it work if you really wanted to by loading the LM317 voltage regulator with a resistor that draws more than the maximum cathode current.

TheGimp · 23rd November 2009, 03:49 PM

I was thinking of using a string of LEDs to drive the base of a NPN with it's emitter tied to ground and a resistor from base to ground to set the bias current of the LED string, collector tied to top of LED string.

Would the transistor amplify the noise of the LEDs and would result in more noise than the LEDs alone.

Bias the LEDs for 10mA.

Now I need 40mA from the collector so base current will be 40ma/Beta.

Most of the LED current and noise would be in the bias leg. The only portion in the collector of the transistor would be the amplified base drive. Both the noise and dc current should be beta times that injected in the BE junction.

This should not end up much noisier than the LED strings together should it?

Or do the values of the parallel strings of LEDs form a Square root of the sum of the squares function?

Yvesm · 23rd November 2009, 05:35 PM

I've routinely used that configuration as shunt regulator, wich is nothing else than a voltage source or a "Zener Booster"

And with Varistor as voltage reference:

Should work with LED string also but I can't tell about noise . . .

Yves.

TheGimp · 06:48 PM

I've used suppression diodes like the 1.5KE350 as a HV ref diode for regulation and had thought about MOVs (Metal Oxide Veristors) that are used across power lines for noise suppression, but thought they would have too loose of tolerance. I'll look at the L14K60 series.

Does anyone have a link to Bas's article? the link http://basaudio.net/diymag.htm
from post 86 is 86'd.

I'm trying to find out how Sy measured the z of the LEDs adn can't find it in this thread so I presume it is outlined in the article.

Yvesm · 23rd November 2009, 06:57 PM

Yes, tolerance is wide, but thermal drift is much lower than a zener !

SY · 23rd November 2009, 07:04 PM

Quote:

Originally Posted by TheGimp

I'm trying to find out how Sy measured the z of the LEDs adn can't find it in this thread so I presume it is outlined in the article.

SYclotron Audio » The Red Light District

TheGimp · 23rd November 2009, 07:16 PM

Thanks Sy.

21st November 2009, 02:32 PM	#601
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Ahh, I see what you guys are getting at. Yes, once the conventional RC cathode bias has been removed, and the grid current limited, you can then build a negative supply, regulate it, then attach the bottom of the grid leaks to it. Or, for five bucks and a free light show, you can get the same instantaneous bias voltage recovery and no more impedance penalty than the usual idle current sensing resistor. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

23rd November 2009, 06:21 PM	#607
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	I've used suppression diodes like the 1.5KE350 as a HV ref diode for regulation and had thought about MOVs (Metal Oxide Veristors) that are used across power lines for noise suppression, but thought they would have too loose of tolerance. I'll look at the L14K60 series. Does anyone have a link to Bas's article? the link http://basaudio.net/diymag.htm from post 86 is 86'd. I'm trying to find out how Sy measured the z of the LEDs adn can't find it in this thread so I presume it is outlined in the article. Last edited by TheGimp; 23rd November 2009 at 06:48 PM. Reason: request for link.

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23rd November 2009, 03:49 PM	#605
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	I was thinking of using a string of LEDs to drive the base of a NPN with it's emitter tied to ground and a resistor from base to ground to set the bias current of the LED string, collector tied to top of LED string. Would the transistor amplify the noise of the LEDs and would result in more noise than the LEDs alone. Bias the LEDs for 10mA. Now I need 40mA from the collector so base current will be 40ma/Beta. Most of the LED current and noise would be in the bias leg. The only portion in the collector of the transistor would be the amplified base drive. Both the noise and dc current should be beta times that injected in the BE junction. This should not end up much noisier than the LED strings together should it? Or do the values of the parallel strings of LEDs form a Square root of the sum of the squares function?

23rd November 2009, 05:35 PM	#606
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	I've routinely used that configuration as shunt regulator, wich is nothing else than a voltage source or a "Zener Booster" And with Varistor as voltage reference: Should work with LED string also but I can't tell about noise . . . Yves.

23rd November 2009, 06:57 PM	#608
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	Yes, tolerance is wide, but thermal drift is much lower than a zener !

23rd November 2009, 07:16 PM	#610
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Thanks Sy.

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