voltage drop across a CCS... - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 8th August 2005, 04:08 PM   #1
pedroskova's Avatar
Join Date: Dec 2002
Location: C'ville VA, USA
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.
  Reply With Quote
Old 8th August 2005, 04:59 PM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
EC8010's Avatar
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...
  Reply With Quote
Old 8th August 2005, 05:05 PM   #3
Yvesm is offline Yvesm  France
diyAudio Member
Yvesm's Avatar
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.


<cross post with EC8010>
  Reply With Quote
Old 8th August 2005, 05:14 PM   #4
diyAudio Member
Tubelab_com's Avatar
Join Date: Jul 2005
Location: West Virginia panhandle
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.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote
Old 8th August 2005, 07:11 PM   #5
pedroskova's Avatar
Join Date: Dec 2002
Location: C'ville VA, USA
Thanks EC, et.al., I now see the error of my ways.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
rectifier voltage drop dsavitsk Power Supplies 21 25th November 2015 05:01 PM
Effects of voltage drop??? kyrie48 Solid State 1 25th October 2007 08:29 AM
Rail voltage drop in preamp mmerig Solid State 4 25th May 2006 04:12 AM
voltage drop ThSpeakerDude88 Chip Amps 30 19th July 2005 09:00 PM
Great voltage drop... drob Car Audio 9 9th July 2003 12:21 PM

New To Site? Need Help?

All times are GMT. The time now is 01:28 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2