6L6 Push pull -- Power tranny question

guitvinny · 15th May 2006, 04:54 PM

Take a look at this amp : http://www.geocities.com/mydesign_99/PP6L6.JPG

- Does it realy going to draw 200 mA over the power transformer ? Cause I would like to use a edcor transformer for 2 channels of this amp... and I think a 500 mA is a biiiit overpower.

SemperFi · 15th May 2006, 06:49 PM

Yes.
Since the diff stage already has a negative supply for its ccs bias, why not use the same supply for the 6L6 bias and not use R11 and C3?

refference · 15th May 2006, 10:56 PM

Hi quitvinny ,

Except for the input stage , the output schematic PP 6L6 Class A
is a classic , and it REALLY DRAWS about 170 - 200 mA at the full
power . and for two channels , you will really need about 400 -
450 mA of useful current .
The total output power is 18 Wats of pure Class A per channel .
But the sound is very amazing , very smooth and I'm sure that you will like it .

Regards

Carlos

guitvinny · 16th May 2006, 01:08 PM

Ok, I`ll check with edcor if 1x450 mA or 2x250 mA is less costy. For the output tranny, who can I calculate the primary impedance ? Is 7600 ohms will be allright like the CXPP25-8-7.6K ?

guitvinny · 16th May 2006, 01:40 PM

For the output tranny, I check on http://tdsl.duncanamps.com/show.php?des=6L6
Zout is the impedance right ? do I look in the good column : Class : A PP Va : 270 .....Zout 5000 ??

mod_evil · 16th May 2006, 02:22 PM

Can I use a 6sn7 in the place of 12AT7?

What is the sound of this amp?

refference · 17th May 2006, 01:19 AM

Hi quitvinny ,

As I said , this 6L6 PP Class A schematic is a classic and the correct Zout , that is the total load ( plate to plate ) is 5000 ohms , and the +B correct value is
270 – 300 Volts . Use a output tranny that can “hand” 25 Watts or more .

Preferably use a matched pair or a same brand / same lot pair of 6L6’s in each channel , because the quiescent current is high ( about 170 mA total or aprox. 85 mA per tube ) and if the current through one tube is very diferent from the current through another tube , this fact will cause a lot of distortion at the output.
( don’t forget : the output tubes are cathode coupled by the 125 ohms resistor )

Don’t try to change the cathode resistor , the correct value is on the schematic
( 125 ohms ) , and this value will give an aprox. grid bias value of ( - ) 20 Volts .

Don’t worry about the high value of quiescent current , because the tubes are
connected in the push-pull way , and the magnetic field created by one tube is
entirely canceled by the opposite magnetic field created by another tube .

The circuit is highly stable , and you won’t have any kind of trouble .

Good luck ,

Carlos

refference · 17th May 2006, 01:37 AM

Hi mod_evil

No you can’t use the 6SN7 in the place of 12AT7 in anyway , they are totally
different . The 6SN7 is a medium mu and the 12AT7 is a high mu dual triode .

The 6SN7 Class A grid bias is ( - ) 8 Volts and the 12AT7 Class A grid bias is
( - ) 2 Volts at 250 Volts plate voltage , the plate resistance and transconductan-
ce are different too .

If you follow the above gave schematics , will result a very amazing and a very
smooth sound .

Carlos

ray_moth · 17th May 2006, 05:15 AM

The reason the 12AT7 was chosen for the first stage, I suspect, was that it can draw enough current to drive the LM317 and also has reasonably high gain. A 12AX7 has more gain but would draw too little current. 12AU7 (or 6SN7) would be the opposite.

If I were you, I would not use the LM317 as a CCS - it's reputed to have a poor high frequency response. Choose a better CCS, maybe using a couple of transistors of appropriate type. If you do that, it would also allow you to use a higher gain but lower current tube for the first stage, such as a 12AX7 or, better still for sound quality, a 6SL7.

If you want to make it sound even nicer, use a choke in place of the 100 ohm resistor (R12) in the power supply, especially if you're planning a common supply for both channels.

guitvinny · 17th May 2006, 02:06 PM

Hey, thank you all,

For CCS, where can I find good desing information ? or if you have shematic lying around send it to me.

And if I do that, can ou tell me ray_moth what I will have to change to use 6SL7 ?

Last question : I saw silver-steel bar on ebay (dia : 1 mm) does anyone have ever work with this material ? I would like to use it as a bus bar.

15th May 2006, 04:54 PM	#1
guitvinny diyAudio Member Join Date: Nov 2005 Location: Québec	6L6 Push pull -- Power tranny question Take a look at this amp : http://www.geocities.com/mydesign_99/PP6L6.JPG - Does it realy going to draw 200 mA over the power transformer ? Cause I would like to use a edcor transformer for 2 channels of this amp... and I think a 500 mA is a biiiit overpower.

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15th May 2006, 06:49 PM	#2
SemperFi diyAudio Member Join Date: Jan 2002 Location: nowhere	Yes. Since the diff stage already has a negative supply for its ccs bias, why not use the same supply for the 6L6 bias and not use R11 and C3?

15th May 2006, 10:56 PM	#3
refference diyAudio Member Join Date: Jan 2006 Location: SAO PAULO - SP	Hi quitvinny , Except for the input stage , the output schematic PP 6L6 Class A is a classic , and it REALLY DRAWS about 170 - 200 mA at the full power . and for two channels , you will really need about 400 - 450 mA of useful current . The total output power is 18 Wats of pure Class A per channel . But the sound is very amazing , very smooth and I'm sure that you will like it . Regards Carlos

16th May 2006, 01:08 PM	#4
guitvinny diyAudio Member Join Date: Nov 2005 Location: Québec	Ok, I`ll check with edcor if 1x450 mA or 2x250 mA is less costy. For the output tranny, who can I calculate the primary impedance ? Is 7600 ohms will be allright like the CXPP25-8-7.6K ?

16th May 2006, 01:40 PM	#5
guitvinny diyAudio Member Join Date: Nov 2005 Location: Québec	For the output tranny, I check on http://tdsl.duncanamps.com/show.php?des=6L6 Zout is the impedance right ? do I look in the good column : Class : A PP Va : 270 .....Zout 5000 ??

16th May 2006, 02:22 PM	#6
mod_evil diyAudio Member Join Date: Dec 2004 Location: Rio	Can I use a 6sn7 in the place of 12AT7? What is the sound of this amp?

17th May 2006, 01:19 AM	#7
refference diyAudio Member Join Date: Jan 2006 Location: SAO PAULO - SP	Hi quitvinny , As I said , this 6L6 PP Class A schematic is a classic and the correct Zout , that is the total load ( plate to plate ) is 5000 ohms , and the +B correct value is 270 – 300 Volts . Use a output tranny that can “hand” 25 Watts or more . Preferably use a matched pair or a same brand / same lot pair of 6L6’s in each channel , because the quiescent current is high ( about 170 mA total or aprox. 85 mA per tube ) and if the current through one tube is very diferent from the current through another tube , this fact will cause a lot of distortion at the output. ( don’t forget : the output tubes are cathode coupled by the 125 ohms resistor ) Don’t try to change the cathode resistor , the correct value is on the schematic ( 125 ohms ) , and this value will give an aprox. grid bias value of ( - ) 20 Volts . Don’t worry about the high value of quiescent current , because the tubes are connected in the push-pull way , and the magnetic field created by one tube is entirely canceled by the opposite magnetic field created by another tube . The circuit is highly stable , and you won’t have any kind of trouble . Good luck , Carlos

17th May 2006, 01:37 AM	#8
refference diyAudio Member Join Date: Jan 2006 Location: SAO PAULO - SP	Hi mod_evil No you can’t use the 6SN7 in the place of 12AT7 in anyway , they are totally different . The 6SN7 is a medium mu and the 12AT7 is a high mu dual triode . The 6SN7 Class A grid bias is ( - ) 8 Volts and the 12AT7 Class A grid bias is ( - ) 2 Volts at 250 Volts plate voltage , the plate resistance and transconductan- ce are different too . If you follow the above gave schematics , will result a very amazing and a very smooth sound . Carlos

17th May 2006, 05:15 AM	#9
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	The reason the 12AT7 was chosen for the first stage, I suspect, was that it can draw enough current to drive the LM317 and also has reasonably high gain. A 12AX7 has more gain but would draw too little current. 12AU7 (or 6SN7) would be the opposite. If I were you, I would not use the LM317 as a CCS - it's reputed to have a poor high frequency response. Choose a better CCS, maybe using a couple of transistors of appropriate type. If you do that, it would also allow you to use a higher gain but lower current tube for the first stage, such as a 12AX7 or, better still for sound quality, a 6SL7. If you want to make it sound even nicer, use a choke in place of the 100 ohm resistor (R12) in the power supply, especially if you're planning a common supply for both channels.

17th May 2006, 02:06 PM	#10
guitvinny diyAudio Member Join Date: Nov 2005 Location: Québec	Hey, thank you all, For CCS, where can I find good desing information ? or if you have shematic lying around send it to me. And if I do that, can ou tell me ray_moth what I will have to change to use 6SL7 ? Last question : I saw silver-steel bar on ebay (dia : 1 mm) does anyone have ever work with this material ? I would like to use it as a bus bar.