Can regulated B+ act like a CCS? - diyAudio
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Old 6th January 2016, 10:13 PM   #1
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Default Can regulated B+ act like a CCS?

I am pretty new to electronic design. Most of my experience has just been in troubleshooting and maintenance. I am trying to figure out the differences and benefits of CCS, CVS, and regulated B+, and have wound up with some questions that i could not get answered while searching.

Would a well regulated supply across a plate resistor not act like a CCS? I understand that as the resistor heats up, its resistance value changes, hence so does the current across it. But, wouldn't this eventually stabilize, or is the voltage swing across the grid force enough variation in current accross the plate resistor that it can not longer be considered constant?

How much difference in (audible) linearity would there be between a CCS and a well regulated B+?
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Old 6th January 2016, 10:40 PM   #2
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A plate load and a power supply are two separate functions.

A modern resistor might drift a percent or two in resistance value after a decade of use within specs.

Are you pondering plate loads?
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Old 6th January 2016, 11:48 PM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by winegamd View Post
Would a well regulated supply across a plate resistor not act like a CCS?
Actually yes, if the value of the AC resistance to ground (the tube circuit's effective AC output resistance)
is much smaller than the plate resistor in value. Triodes work best (with the lowest distortion) in this way.
That is, with both a high supply voltage and a high plate resistor value.

Last edited by rayma; 7th January 2016 at 12:03 AM.
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Old 7th January 2016, 05:33 AM   #4
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A CCS plate load presents the tube with a very high impedance load without the onerous B+ demands of a very large resistor.
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Old 7th January 2016, 05:37 AM   #5
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I am working with a power supply regulated by a bootstrapped LM10 opamp controlling the bias of two FET devices ( which I guess is a quasi CCS in a way). This is straight out of the TI LM10 design book p.21.

I am getting 227 volts before the anode resistor. I am using a 220ohm plate load resistor. Currently the cathode is running unbypassed with a 3.3k resistor, but I am pondering trying LED bias with an anode bypass resistor. Since everything I have seen says it works best with a CCS, this is where I came up with my line of questioning.
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Old 7th January 2016, 05:38 AM   #6
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Oh, the tube is a 12ax7.
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Old 7th January 2016, 07:33 AM   #7
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Not sure if this is what you're asking but if you understand how to draw loadlines for your 12AX7, guys here can help answer your question better.

Read about loadlines here: How to "Screw Around" Your Tube Load Line

Basically, the answer to your question:
1. CCS = the loadline will be horizontal
2. CVS = the loadline will be vertical
3. Regulated B+ with Resistor Load = the loadline will be tilted.. the tilt degree depends on your resistor load value. Higher resistor load value will tilt more towards horizontal. Very high resistor load value will eventually become a CCS. Of course B+ will have to be adjusted to keep sufficient plate current flowing.
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Old 7th January 2016, 07:41 AM   #8
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I did a bunch of research as I developed the design of a Hi-Fi tube amp I built (Two 6SN7s and two EL34s per channel), and ended up with a front end that consists of a 6SN7 with a CCS in place of the plate resistor. Everybody knows that using a CCS there improves linearity and increases gain somewhat, but what many apparently don't realize is that it also improves power supply rejection ratio a bunch (so significantly less hum caused by pwr supply) ( a good reason to use CCS by itself), and also if the tube ever shorts out internally, it limits current, so the burn out may well be less damaging to peripheral parts. My second stage is actually a follower with a CCS off the cathode to Gnd in place of that R. Single ended stages have very little Pwr supply rejection in their basic form. IXYS makes some CCS parts that are worth looking at.
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Old 7th January 2016, 02:40 PM   #9
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Ballpencil: I have played with load lines quite a bit and have read that article. To me it seams that, at least for the 12??7 valve types, a regulated supply would be the most linear. This is unless you don't need very much gain, your signal doesn't require much voltage swing, you can use maximum voltage, and use low current that is completely lacking in variance.

I can see in the latter case why an LED for cathode bias could be preferred over a resistor and bypass cap. With low gain and low signal (like in a phono stage) there is a higher chance for noise from the PS and components (at least audible noise).

Bob: I can see the benefit of replacing the plate resistor entirely. My only question is, if you are able to do so and still maintain a high voltage. Most low noise devices, at least that I have seen mentioned in my searches, can not support high voltage. I guess this does for CCS's in general.
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Old 7th January 2016, 02:43 PM   #10
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Bob: Nevermind on the high voltage. With the constant current, voltage becomes somewhat irrelevant (as long as you have enough to support the current required).
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