voltage drop across a CCS...

pedroskova · 8th August 2005, 04:08 PM

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.

EC8010 · 8th August 2005, 04:59 PM

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.

Yvesm · 8th August 2005, 05:05 PM

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>

tubelab.com · 8th August 2005, 05:14 PM

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.

pedroskova · 8th August 2005, 07:11 PM

Thanks EC, et.al., I now see the error of my ways.

8th August 2005, 04:08 PM	#1
pedroskova RIP Join Date: Dec 2002 Location: C'ville VA, USA	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.

8th August 2005, 04:59 PM	#2
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

8th August 2005, 05:14 PM	#4
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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8th August 2005, 05:05 PM	#3
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	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>

8th August 2005, 07:11 PM	#5
pedroskova RIP Join Date: Dec 2002 Location: C'ville VA, USA	Thanks EC, et.al., I now see the error of my ways.

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