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Old 17th April 2012, 02:33 PM   #1
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Default I have 2 choices for resistive load, need opinions.

I have a 2C22 triode I am using as a preamp. I remember reading somewhere, either in these forums or a book, that having a larger change in current is better to reduce noise. Here are 2 load lines, both produce the required 12.5 Volts peak from a 1 Volt peak input. Which one would you use and why ?
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Old 17th April 2012, 03:09 PM   #2
Yvesm is offline Yvesm  France
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Yeap, but what load are you trying to drive ?

Yves.
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Old 17th April 2012, 03:53 PM   #3
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Image 1 is a 10K resistive load, Image 2 is a 39K resistive load. This preamp stage is capacitor coupled to a pentode stage.

The question is, when designing a preamp stage, is it better to have a bigger plate current swing ?
Image 1 has a peak of 1.2 mA and draws over 1 Watt at Q point, image 2 has a peak of 0.3 mA and draws 1/4W. Which is better and why ? Or, does it even matter ?
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Old 17th April 2012, 05:03 PM   #4
kevinkr is offline kevinkr  United States
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Take a look at the rp of your 2C22 and the steepness of the loadline into 10K, a few simple calculations will tell you what sort of thd this is likely to produce. IMO the 39K load line is likely to result in considerably better linearity. Usual rule of thumb is you want a load that is at least 3 - 4X greater than rp for good linearity. A CCS will give you the best possible linearity and a gain approaching mu without the need for an obscenely high supply voltage.

I would run these at 5 - 6mA with something like 27 - 30K plate load resistors...
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Old 17th April 2012, 05:12 PM   #5
Yvesm is offline Yvesm  France
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Hi,
My question was: What do you intend to drive with this preamp, that is the "external load", not the "internal" one.
AC speaking they are in parallel.
I do prefer the one that draw the higest current and I would probably tried to draw even more but to do so you need an higher plate supply voltage.
This will put the tube in a more linear region.

Yves.
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Old 17th April 2012, 06:30 PM   #6
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Preamp is driving a 12AB5 Pentode, 100K grid resistor, 0.1uF coupling cap.
What I really need is 12.5Vpk output for a 1Vpk input. Was just wondering if I should create a large current swing or a small current swing.
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Old 17th April 2012, 09:30 PM   #7
DF96 is offline DF96  England
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You need to include the effect of the 100k grid resistor. For AC this is in parallel with the anode load resistor so it rotates the loadline towards the vertical a little.

Look at the distortion effect of a bigger signal. Choose the one with lower distortion for bigger signals, as this is likely to give less distortion for smaller signals too.
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Old 17th April 2012, 09:44 PM   #8
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For triodes small current swing is prefferable, hence the CCS recommendation. However the most linear region tends to be in a higher current region.
So basically you want as large load resistor as possible while still keep your idle point in the linear region.
You also need to realise that the 100k grid resistor of the suceeding stage will be in paralell with the load resitor at AC. In your 39k case the effective loading at AC will be approx 28k.

/Olof
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Old 18th April 2012, 12:30 AM   #9
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Thank you for the answers. I'm still a bit of a noob, what is CCS?
I'll probably up the value of the grid resistor to 470K and set my load resistor somewhere between 10K and 39K. This is only a 3+Watt amp and I'm not looking for hi-fi.
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Old 18th April 2012, 02:45 AM   #10
rongon is offline rongon  United States
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Quote:
Originally Posted by EddieRich View Post
Thank you for the answers. I'm still a bit of a noob, what is CCS?
Constant current source, or constant current sink.
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