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Old 8th August 2005, 04:08 PM   #1
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Default voltage drop across a CCS...

Hi all,

I re-did my linestage over the weekend from PP to SE parallel feed, going from 5687's to 12B4A's. After an initial tail chasing session( one of the IXYS CCS's died, I got it up and running.

Now...the confusing part - B+ on top of the little 3-legged fuse (set for ~20mA) is 300V, while the voltage at the plate is....70V...

bias is currently ~-6.25V with a 330R cathode resistor.

I'm a little concerned as to why this voltage drop is so large. I've always been under the assumption that you set current, and the plate voltage settles in at a value dependant on the tube's characteristics. I've used the IXYS part on 6SN7's, 5687's, 6922's, etc., and have never seen this occur.

...should I think about futzing around with different bias/current values? Any help would be appreciated.

...hope my heat sinking is adequate.
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Old 8th August 2005, 04:59 PM   #2
EC8010 is offline EC8010  United Kingdom
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You have set 20mA through the 12B4 using the CCS, but you need to adjust your cathode resistor to set the anode voltage to your preferred value.
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Old 8th August 2005, 05:05 PM   #3
Yvesm is online now Yvesm  France
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Look at plate curves !
All seems correct.
The 12B4 with 6.25 bias MUST HAVE it's plate voltage at 70v in order to draw 20mA.
If you want more plate voltage for the same current, increase bias.
For exemple, you'll have 150v for about 18v bias.
To do that, rise cathode resistor to 900 Ohms.

Yves.

<cross post with EC8010>
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Old 8th August 2005, 05:14 PM   #4
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Default CCS question

EC8010 has it exactly right. The current is fixed by the CCS, but the plate voltage is now set by the tubes operating point. This is usually set by the cathode resistor, or fixed grid bias if used.

A non documented fact about the "little 3-legged fuse" (IXYS chip) is that they like to oscillate in the 1 to 10 MHz region causing distortion and dead chips. The fix is to add a gate stopper resistor just like you use on a tube. I use a 1K resistor in series, mounted right at the "G" pin of the chip.
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Old 8th August 2005, 07:11 PM   #5
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Thanks EC, et.al., I now see the error of my ways.
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