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 26th January 2013, 06:36 PM #21 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 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 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 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
diyAudio Member

Join Date: Sep 2009
Location: Catalonia new Europe state
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

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Join Date: Dec 2012
Location: Flanders
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

 27th January 2013, 04:00 PM #27 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 diyAudio Member     Join Date: Sep 2009 Location: Catalonia new Europe state Actually I'm using two red leds instead 470R // 22uF
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Join Date: May 2007
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).

 27th January 2013, 04:37 PM #30 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

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