EL84 SE auto bias

alexmoose · 31st March 2007, 09:48 PM

Thats a weird glich, no wonder no one was responding......

okay, well I am currently constructing an EL84 SE amplifier, and I am using an LM317 as a CCS for biasing each tube (one LM317 per channel) and I wanted to know what would happen if I didn't bypass them with capacitors, I have seen several CCS bias designs that do not use bypass capacitors (none of them SE), and I wanted to know what the DIY Audio community thought.

-Moose

d2134 · 31st March 2007, 10:10 PM

Quote:

I have seen several CCS bias designs that do not use bypass capacitors

Please post some schematics or links.

alexmoose · 31st March 2007, 11:39 PM

The design I am making is this

I have mostly seen non-bypassed designs on Push-pull stuff, but I did not know if it would carry over, here are some designs that do not include it

http://www.diyparadise.com/buildel84b.html

and

EL84 Amp - Baby Huey

d2134 · 31st March 2007, 11:55 PM

In the case of Baby-Huey:
On general schematic is drawn a "bias block". On the second page we notice the bias block in detail. It has the actual CCS (the 2 tranzistors + rezistors) and the bypass capacitors (4700uF+470uF+1uF) parallel for good frequence response. This is a good practice.

In your case you have to bypass the LM317 based CCS in a similar way

d2134 · 1st April 2007, 12:02 AM

In the case of LED array:
it is NOT a CCS. The LED array is a fix voltage biasing element. Like a zener diode with hight current.

d2134 · 1st April 2007, 12:09 AM

The case of Simple EL84 is a litle special. We have an auto phase invering final stage. We notice one valve grid is grounded. In this case the CCS must be NOT bypassed. The signal for the lefthand pentode is applied on grid and for the righthand pentode is applied in cathode. Search the net for "long tail pair" for more details. http://www.aikenamps.com/LongTailPairDesign.htm

alexmoose · 1st April 2007, 03:04 AM

wow, thank you, i'll be sure to bypass those now

-Moose

kooltubes · 5th April 2007, 11:46 AM

I have just built a El84SE using an LM317 in lieu of the cathode bias resistor.
The voltages are correct, but the amp is mute...any idea?

Pin 1(adj) is grounded; 23.2 ohms from Pin 2 (Vout) to ground; Pin 3(Vin) is tied to the El84 cathode.
I have read in this thread that the LM317 needed to be bypassed, what is the cap value and do I connect it from Vin (pin 3) to ground?

Your help is much appreciated.

SINGLEENDED · 5th April 2007, 02:50 PM

Quote:

Originally posted by kooltubes
I have just built a El84SE using an LM317 in lieu of the cathode bias resistor.
The voltages are correct, but the amp is mute...any idea?
Pin 1(adj) is grounded; 23.2 ohms from Pin 2 (Vout) to ground; Pin 3(Vin) is tied to the El84 cathode.
I have read in this thread that the LM317 needed to be bypassed, what is the cap value and do I connect it from Vin (pin 3) to ground?

Your help is much appreciated.

Your ccs is correct, you also have 50ma .As to the bypass caps. U can try it out any value from 47 -470 . U may also want to try higher current, its sound sweeter. Try 12R its tasty.

kooltubes · 7th April 2007, 11:50 AM

Thanks SINGLEENDED.

Amp works fine. I have used different cap values but 100mf sounds better to my ears.
I have also added 22mf betwwen Vout (pin 2) to ground, the soundstage seems to be scaled down.

I am confused about your suggestions to run the El84 at 104ma using a 12R resistor

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31st March 2007, 09:48 PM	#1
alexmoose diyAudio Member Join Date: Mar 2006 Location: Michigan	Thats a weird glich, no wonder no one was responding...... okay, well I am currently constructing an EL84 SE amplifier, and I am using an LM317 as a CCS for biasing each tube (one LM317 per channel) and I wanted to know what would happen if I didn't bypass them with capacitors, I have seen several CCS bias designs that do not use bypass capacitors (none of them SE), and I wanted to know what the DIY Audio community thought. -Moose

31st March 2007, 11:39 PM	#3
alexmoose diyAudio Member Join Date: Mar 2006 Location: Michigan	The design I am making is this I have mostly seen non-bypassed designs on Push-pull stuff, but I did not know if it would carry over, here are some designs that do not include it http://www.diyparadise.com/buildel84b.html and EL84 Amp - Baby Huey

31st March 2007, 11:55 PM	#4
d2134 diyAudio Member Join Date: Mar 2007	In the case of Baby-Huey: On general schematic is drawn a "bias block". On the second page we notice the bias block in detail. It has the actual CCS (the 2 tranzistors + rezistors) and the bypass capacitors (4700uF+470uF+1uF) parallel for good frequence response. This is a good practice. In your case you have to bypass the LM317 based CCS in a similar way

1st April 2007, 12:02 AM	#5
d2134 diyAudio Member Join Date: Mar 2007	In the case of LED array: it is NOT a CCS. The LED array is a fix voltage biasing element. Like a zener diode with hight current.

1st April 2007, 12:09 AM	#6
d2134 diyAudio Member Join Date: Mar 2007	The case of Simple EL84 is a litle special. We have an auto phase invering final stage. We notice one valve grid is grounded. In this case the CCS must be NOT bypassed. The signal for the lefthand pentode is applied on grid and for the righthand pentode is applied in cathode. Search the net for "long tail pair" for more details. http://www.aikenamps.com/LongTailPairDesign.htm

1st April 2007, 03:04 AM	#7
alexmoose diyAudio Member Join Date: Mar 2006 Location: Michigan	wow, thank you, i'll be sure to bypass those now -Moose

5th April 2007, 11:46 AM	#8
kooltubes diyAudio Member Join Date: Sep 2004 Location: Paris	I have just built a El84SE using an LM317 in lieu of the cathode bias resistor. The voltages are correct, but the amp is mute...any idea? Pin 1(adj) is grounded; 23.2 ohms from Pin 2 (Vout) to ground; Pin 3(Vin) is tied to the El84 cathode. I have read in this thread that the LM317 needed to be bypassed, what is the cap value and do I connect it from Vin (pin 3) to ground? Your help is much appreciated.

7th April 2007, 11:50 AM	#10
kooltubes diyAudio Member Join Date: Sep 2004 Location: Paris	Thanks SINGLEENDED. Amp works fine. I have used different cap values but 100mf sounds better to my ears. I have also added 22mf betwwen Vout (pin 2) to ground, the soundstage seems to be scaled down. I am confused about your suggestions to run the El84 at 104ma using a 12R resistor

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