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Old 27th December 2010, 03:13 PM   #1
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Default 6CG7 Bias Current?

I plan to use a 6CG7 with an Ebb of 240v as the gain stage for a SE 6BQ5 amp. I'm having trouble determining what bias current to use.

I have seen schematics with it as low as 2.3 mA. The datasheet (GE) lists 9 mA as "Typical Operation. The Resistance Coupled chart has Ebb 300v and a R L of .1 meg. That's 3 mA.

When I model it in Tube Cad at about 2.3 mA it says the current is to low and will distort.

What should it be?
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Old 27th December 2010, 03:22 PM   #2
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I think you mean "idle current" or "quiescent current," not "bias current." In any case, you want at least 6-7 mA running through the tube to achieve linearity. That will take some playing with load lines, especially with your relatively low available B+.

If it were me, I'd look at higher B+ or the use of CCS loading.
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Old 27th December 2010, 04:20 PM   #3
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Sorry "Idle Current" Newbie mistake. Thanks for the imput. 6-7 mA does seem more logical.

Right now I'm working with the voltages that are calculated when I model the PS in PSU2. I expect (and hope) when I get the amp on the borad, voltages will be higher.

What is CCS Loading? I have seen the term before but haven't researched it. By the way, I'm a tube and electronic Newbie.
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Old 27th December 2010, 04:23 PM   #4
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Quote:
Originally Posted by Troncones View Post
Newbie mistake.
Don't worry, a lot of people who aren't exactly newbies are pretty sloppy with those terms as well.

STRONG recommendation: Get a copy of Morgan Jones's "Valve Amplifiers." He covers all of these issues (and many more) very thoroughly and in an extremely readable style. It's absolutely the best investment you can make at this stage in your progress.
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Old 27th December 2010, 04:53 PM   #5
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Thanks for the book tip SY.
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Old 27th December 2010, 06:12 PM   #6
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Download the data of General Electric 6CG7 here:

http://www.mif.pg.gda.pl/homepages/f...093/6/6CG7.pdf

There are ready circuit and component values given for different supply voltages (page 3 ).

You do not need to play with load lines. GE has done it for the users.
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Old 27th December 2010, 06:35 PM   #7
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Quote:
You do not need to play with load lines. GE has done it for the users.
But learning to work with loadlines teaches you a heck of a lot...
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Old 28th December 2010, 07:50 PM   #8
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I referenced the GE data sheet in my first post. Using their chart the highest plate current is 3mA (300V/100K)=3mA. The data sheet also shows 9mA as "Typical Operation". So what should I design to 3mA or something closer to 9mA?

See my first post regarding Tube Cad resaults.
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Old 28th December 2010, 08:12 PM   #9
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Quote:
So what should I design to 3mA or something closer to 9mA?
It is most obvious that GE has very carefully considered what data they publish about their products. Therefore I am sure that the data is valid and correct, irrespective of what Tube Cad says.

But what is your idea about your amplifier, how should it perform ?
(gain, input level, used load impedance etc.)
All these have effect to used circuit and bias/current value.
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