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Old 26th January 2013, 06:36 PM   #21
DF96 is offline DF96  England
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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.
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Old 26th January 2013, 08:49 PM   #22
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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?
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Old 26th January 2013, 09:33 PM   #23
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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?
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Old 27th January 2013, 12:17 PM   #24
Ketje is offline Ketje  Belgium
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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
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Old 27th January 2013, 02:19 PM   #25
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Quote:
Originally Posted by Ketje View Post
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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Old 27th January 2013, 03:53 PM   #26
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by merlin el mago View Post
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
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Old 27th January 2013, 04:00 PM   #27
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Sorry Mona, wich resistor have I to replace for 330R:

-470R or 560R?
-or both?
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Old 27th January 2013, 04:01 PM   #28
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Actually I'm using two red leds instead 470R // 22uF
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Old 27th January 2013, 04:30 PM   #29
DF96 is offline DF96  England
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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).
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Old 27th January 2013, 04:37 PM   #30
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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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