Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki 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

Fixed Bias: How to build it conveniently to use?
Fixed Bias: How to build it conveniently to use?
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
Reply
 
Thread Tools Search this Thread
Old 7th February 2018, 02:23 PM   #11
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
Using a TL783 to set current in place of that resistor making balancing easy, and is just as reliable in most cases.
  Reply With Quote
Old 7th February 2018, 03:22 PM   #12
goldenbeer is offline goldenbeer  Germany
diyAudio Member
 
Join Date: Jan 2014
A CCS instead of cathode resistors has issues with bias shift and class AB operation.
  Reply With Quote
Old 7th February 2018, 04:03 PM   #13
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
Only if you like to clip your amp You bypass it with a large cap as you would with a resistor, and IMHO what you gain from being able to balance current across the OPT allows the use of toroidal power transformers as OPT outweighs the odd bias shift. They also tend to save the tubes if the grid leak resistor somehow becomes disconnected (ask me how I found out...)


Plus you can design it more elaborately to effectively get a class A system with odd bursts of peak current. I have used this version several times with great success.

Click the image to open in full size.

Broskie has some interesting ideas here:
Cathode Bias with a Counstant Current Source

Last edited by kodabmx; 7th February 2018 at 04:07 PM.
  Reply With Quote
Old 7th February 2018, 04:28 PM   #14
goldenbeer is offline goldenbeer  Germany
diyAudio Member
 
Join Date: Jan 2014
Quote:
Originally Posted by kodabmx View Post
Only if you like to clip your amp You bypass it with a large cap as you would with a resistor, and
Benign clipping behavior is a major trait which separates world class designs from mediocre amps.
The bias shift is more severe with a CCS than with a resistor, even if we're staying in class a.
If we're moving deeper into class ab, it doesn't even work, except for Broskie's kludge, which is a sonic mess.
  Reply With Quote
Old 7th February 2018, 04:30 PM   #15
audiowize is offline audiowize  United States
diyAudio Member
 
Join Date: Aug 2005
Location: Seattle
Send a message via AIM to audiowize
Quote:
Originally Posted by PRR View Post
Use Self-Bias Whenever Possible!

When fixed-bias works you can get some/lots more power out of a pair of tubes. But when it does not work things can get very ugly. A 250 Ohm 10W cathode bias resistor is very reliable. After decades of blowing-up tubes (I used to do that a lot in my job) I have mellowed and will accept a reliable 25 Watts rather than 50 Watts on good days and major repairs on other days. (My last major P-P project was down-rating an unreliable 60W amp to a 20W which may out-live me.)
I wanted to formulate a rebuttal to this, but what I ended up writing was me agreeing with you...

So +1!
  Reply With Quote
Old 7th February 2018, 06:37 PM   #16
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
Quote:
Originally Posted by goldenbeer View Post
Benign clipping behavior is a major trait which separates world class designs from mediocre amps.
The bias shift is more severe with a CCS than with a resistor, even if we're staying in class a.
If we're moving deeper into class ab, it doesn't even work, except for Broskie's kludge, which is a sonic mess.
That means all self biased designs are marde compared to fixed bias? We're talking about circuit behavior when the circuit is abused, not when used as designed. If I build an amplifier using any number of auto bias schemes, and it puts out 15W of clean sine wave power, I rate it as a 10W amp. Furthermore in actual testing, not SPICE, the amp doesn't exhibit drift until it's severely overdriven. That might be a typical operation for a guitar amp, but those operating conditions don't usually happen in hifi.
  Reply With Quote
Old 7th February 2018, 06:50 PM   #17
goldenbeer is offline goldenbeer  Germany
diyAudio Member
 
Join Date: Jan 2014
We were talking specifically ccs cathode bias, and yes, it's only acceptable in class a. Which can be push pull class a. With all it's shortcomings.
Occasional clipping & overload is pretty normal use in reproduction of dynamic and / bass heavy audio, especially for low powered (class a) tube amps.
  Reply With Quote
Old 7th February 2018, 07:18 PM   #18
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
What you say is true of truly low powered class designs (<5W), however I find that since 95% of listening happens within the first watt of power (unless you like to crank it, but you wouldn't be using a low powered amp for that) coupled with the fact that my designs using CCS cathodes are usually at least 10W it's usually a non issue. I've even used the CCS scheme pictured about on a push pull KT120 triode amp biased at 90ma with 485V B+. 33W of clean 1kHz sinewave as measured. I've since changed it into a 100W tetrode amp with the auto bias board from audioamp.eu with regulated screens though. This bias doesn't drift very much at all. See this thread for details: Automatic bias board.
Click the image to open in full size.

Last edited by kodabmx; 7th February 2018 at 07:20 PM.
  Reply With Quote
Old 8th February 2018, 07:25 AM   #19
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Quote:
Originally Posted by kodabmx View Post
listening happens within the first watt of power coupled with the fact that my designs using CCS cathodes are usually at least 10W it's usually a non issue.
Transients can demand a hundred times more power than the average level. A 10W amplifier will clip fairly often.
  Reply With Quote
Old 8th February 2018, 01:15 PM   #20
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
I've never seen a 100V peak on a 1V RMS signal but I'm sure they can if it's one of those test tracks of someone dropping a piano or something. Considering in most music, the RMS is only a couple of db less than peak (don't you just love modern digital engineering, compression, and brick wall limiters?) it's a non issue for me. Those peak transients are so short in duration that the bypass caps and resevoir caps provide the added kick needed in most situations. Just my experience...
  Reply With Quote

Reply


Fixed Bias: How to build it conveniently to use?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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Fixed Bias (no bias control) and Upgrading Output Transistors timber wolf Solid State 12 16th June 2015 12:01 AM
DC Automatic Bias Control - Auto fixed bias Ceglar Tubes / Valves 21 11th October 2014 09:21 AM
EL84 fixed fixed bias razorrick1293 Tubes / Valves 3 20th November 2011 03:33 PM
Finding bias point in PP fixed bias beamnet Tubes / Valves 14 7th December 2008 01:25 PM
Battery grid bias to fixed bias in Toccato? w00t Tubes / Valves 3 2nd December 2007 12:34 AM


New To Site? Need Help?

All times are GMT. The time now is 05:18 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki