How biasing mA in tubes - Page 3

DF96 · 26th January 2013, 06:36 PM

LTspice won't design anything for you. It may tell you whether your design will work. Choosing the current through a valve is part of design. Obtaining that current is another part of design.

In general, it is unwise to rely too much on statements like "the ECC81 should have 5mA current". Instead, study the datasheet and find out why people recommend a particular working point. Bear in mind that some of them may be wrong, or just repeating what someone else told them.

merlin el mago · 26th January 2013, 08:49 PM

How can understand the datasheets to study? wich parameters I have to look for? how can be sure I'm not wrong like the others?

merlin el mago · 26th January 2013, 09:33 PM

Let's see if I understand something of the ECC81 datasheet applicated to posted mu-follower schematic:

Schematic uses upper anode 280V the max anode dissipation is 2.5W
Looking at curves shows 9mA for upper anode
Grid voltage aprox -2,66V

Schematic uses lower anode 85V the max anode dissipatios is 2.5W
Looking at curves shows aprox 27mA for lower anode
Grid voltage aprox. +3V

Are my calculations OK?

Ketje · 27th January 2013, 12:17 PM

If you try to turn up the current to 5mA the voltagedrop at 33k resistor is 165V.Starting with 280V that leaves 280-165=115V for two tubes.But with 115/2=57.5V on a ECC81 -Vg is almost zero to reach 5mA.
Best stick with 3mA.Data ECC81 ;Va=85V Ia=3mA at -Vg=1 so Rk=330ohm ,not 470 nor 560.
Mona

merlin el mago · 27th January 2013, 02:19 PM

Quote:

Originally Posted by Ketje

If you try to turn up the current to 5mA the voltagedrop at 33k resistor is 165V.Starting with 280V that leaves 280-165=115V for two tubes.But with 115/2=57.5V on a ECC81 -Vg is almost zero to reach 5mA.
Best stick with 3mA.Data ECC81 ;Va=85V Ia=3mA at -Vg=1 so Rk=330ohm ,not 470 nor 560.
Mona

If I understood correctly I have to change 560R & 470R for 330R in both places?

If I do this how many mA will be biased in both triodes?

Thanks
Felipe

Ketje · 27th January 2013, 03:53 PM

Quote:

Originally Posted by merlin el mago

If I understood correctly I have to change 560R & 470R for 330R in both places?

If I do this how many mA will be biased in both triodes?

Thanks
Felipe

Best stick with 3mA.Data ECC81 ;Va=85V Ia=3mA at -Vg=1 so Rk=330ohm ,not 470 nor 560.

Mona

merlin el mago · 27th January 2013, 04:00 PM

Sorry Mona, wich resistor have I to replace for 330R:

-470R or 560R?
-or both?

merlin el mago · 27th January 2013, 04:01 PM

Actually I'm using two red leds instead 470R // 22uF

DF96 · 27th January 2013, 04:30 PM

Quote:

Originally Posted by merlin el mago

How can understand the datasheets to study? wich parameters I have to look for?

Read some books. RDH4 is available free on the internet. Morgan-Jones is available from bookshops, both real and virtual. Also try Valve Wizard site (Merlin Blencowe).

Chris Hornbeck · 27th January 2013, 04:37 PM

The part you want to learn about is called "load lines". Valves' characteristic curves are drawn with anode current on the Y axis and anode voltage on the X axis, for various grid 1 voltages.
With a little familiarity you'll soon see what is a good and what is a bad load line for your circuit and use. Well worth a small learning curve for a deeper knowledge.

All good fortune,
Chris

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26th January 2013, 06:36 PM	#21
DF96 diyAudio Member Join Date: May 2007	LTspice won't design anything for you. It may tell you whether your design will work. Choosing the current through a valve is part of design. Obtaining that current is another part of design. In general, it is unwise to rely too much on statements like "the ECC81 should have 5mA current". Instead, study the datasheet and find out why people recommend a particular working point. Bear in mind that some of them may be wrong, or just repeating what someone else told them.

26th January 2013, 08:49 PM	#22
merlin el mago diyAudio Member Join Date: Sep 2009 Location: Catalonia new Europe state	How can understand the datasheets to study? wich parameters I have to look for? how can be sure I'm not wrong like the others?

26th January 2013, 09:33 PM	#23
merlin el mago diyAudio Member Join Date: Sep 2009 Location: Catalonia new Europe state	Let's see if I understand something of the ECC81 datasheet applicated to posted mu-follower schematic: Schematic uses upper anode 280V the max anode dissipation is 2.5W Looking at curves shows 9mA for upper anode Grid voltage aprox -2,66V Schematic uses lower anode 85V the max anode dissipatios is 2.5W Looking at curves shows aprox 27mA for lower anode Grid voltage aprox. +3V Are my calculations OK?

27th January 2013, 12:17 PM	#24
Ketje diyAudio Member Join Date: Dec 2012 Location: Flanders	If you try to turn up the current to 5mA the voltagedrop at 33k resistor is 165V.Starting with 280V that leaves 280-165=115V for two tubes.But with 115/2=57.5V on a ECC81 -Vg is almost zero to reach 5mA. Best stick with 3mA.Data ECC81 ;Va=85V Ia=3mA at -Vg=1 so Rk=330ohm ,not 470 nor 560. Mona

27th January 2013, 04:00 PM	#27
merlin el mago diyAudio Member Join Date: Sep 2009 Location: Catalonia new Europe state	Sorry Mona, wich resistor have I to replace for 330R: -470R or 560R? -or both?

27th January 2013, 04:01 PM	#28
merlin el mago diyAudio Member Join Date: Sep 2009 Location: Catalonia new Europe state	Actually I'm using two red leds instead 470R // 22uF

27th January 2013, 04:37 PM	#30
Chris Hornbeck diyAudio Member Join Date: May 2011 Location: Little Rock	The part you want to learn about is called "load lines". Valves' characteristic curves are drawn with anode current on the Y axis and anode voltage on the X axis, for various grid 1 voltages. With a little familiarity you'll soon see what is a good and what is a bad load line for your circuit and use. Well worth a small learning curve for a deeper knowledge. All good fortune, Chris