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

Autobiasing output tubes (6L6)?
Autobiasing output tubes (6L6)?
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 9th August 2019, 03:02 AM   #1
buffstereo1 is offline buffstereo1
diyAudio Member
 
Join Date: Sep 2015
Cool Autobiasing output tubes (6L6)?

My friend bought a "Dared" 6L6 PP amp. He says it's Class A AND Auto-biasing. What is the methodology of "Auto-Biasing." Anyone heard of "Class A" Dared PP amps? Yes I could look up the Class A part, but I'm truly curious about this Auto-Biasing, and I don't want to read a vendor's description! Thanks in advance for your time answering my questions!
  Reply With Quote
Old 9th August 2019, 03:17 AM   #2
gabdx is offline gabdx  Canada
diyAudio Member
 
Join Date: Jan 2008
its a current sense feedback to the voltage bias... very simple.

however the more i understand output transformers and output valves, the more I think a perfect balanced bias does not matter.

The tolerance depends on the transformer,

the most important is not to have 0 null current at the B+ tap.

What is important is to keep the transformer inductances (both phases) to the maximum values.

Any drop in the OT inductance caused by some effects of current is very detrimental to the whole THD, sound and bass accuracy.

if the windings require -1.5mA bias or 2mA as a resultant, then it is the best bias.

Note too that as things warm up, the optimal bias point can shift somewhat.

An auto-bias should take the warm up in consideration which is around 15-30 minutes on a well ventilated amplifier.
__________________
amplificateurslegrand.ca
  Reply With Quote
Old 9th August 2019, 05:06 AM   #3
jjasniew is offline jjasniew  United States
diyAudio Member
 
Join Date: Feb 2002
Location: Roy, WA
There's multiple methods I suspect. One is the so called "servo - amplifier" which is basically an op-amp set up with a very long time constant, usually implemented using a capacitor. There's controversy whether that capacitor is in the audio path - and I recall a friend 20+ years ago ripping out all the caps and replacing them in his servo amp designs.

There's other methods where the website selling them puts in all caps NOT IN SIGNAL PATH IN ANY WAY or some such claim - maybe they use a microprocessor or something to create the instrument that does this. So I guess this part of whatever implementation is bothersome and I'm sure there's 101 ways to skin that cat.

I'm finding out that the output tube bias isnt so critical in terms of precision - it's like the inflation on your cars tires, where getting all 4 to within a fraction of a psi automatically is more of a convenience, than something that's going to blow you away in terms of how the car drives with such a gizmo installed.
__________________
Joe Jasniewski
  Reply With Quote
Old 9th August 2019, 05:51 AM   #4
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
I use individual self bias resistors and caps for each tube, and professionally matched output tubes. The plate currents match very well. That way, the OPT gets "babied". I do not use regulated filament supplies, and do not use regulated B+. But as the mains range from 117V to 123V, the plate currents still do match. Simple and effective. The match stays good as the tubes age together. My tube vendor, eurotubes.com, re-tests the JJ tubes (multiple tests for noise, match, etc.). They are local, and I drive over and get what I want.

Last edited by 6A3sUMMER; 9th August 2019 at 05:54 AM.
  Reply With Quote
Old 9th August 2019, 06:15 AM   #5
Kay Pirinha is offline Kay Pirinha  Germany
diyAudio Member
 
Kay Pirinha's Avatar
 
Join Date: Oct 2011
Location: Somewhere in Germany
Yes, I also think your friend just might find the usual cathode resistor/capacitor combo per each output tube if he opens his Dared »Autobias« amplifier.
Best regards!
__________________
"Bless you, Sister. May all your sons be bishops." (Brendan Behan)
  Reply With Quote
Old 9th August 2019, 09:51 AM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
'Autobias' can mean anything from a bypassed resistor to a microprocessor output stage management system. If the output uses Class A then a bypassed resistor may be the best option, as it is also the cheapest option and automatically adjusts as the valve ages or gets swapped.
  Reply With Quote
Old 9th August 2019, 09:54 AM   #7
Kay Pirinha is offline Kay Pirinha  Germany
diyAudio Member
 
Kay Pirinha's Avatar
 
Join Date: Oct 2011
Location: Somewhere in Germany
I severly assume that this »Autobias« is just audiophoolish babble .
Best regards!
__________________
"Bless you, Sister. May all your sons be bishops." (Brendan Behan)
  Reply With Quote
Old 9th August 2019, 06:09 PM   #8
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Some do not like to use a self bias resistor and bypass cap, because they are worried about the time constant, and the resultant bias shift with very large and or long transients . . .

But some of those same people really like the idea of a microprocessor system that uses a current sense resistor (which requires a capacitor or other form of integrator). Well, to that I say . . . there is a time constant there too, and you will get the same bias shift results.

Same results, which is easier?
  Reply With Quote
Old 9th August 2019, 10:20 PM   #9
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
> What is the methodology of "Auto-Biasing."

It can be many things.

But this is an 18 Watt 2x6L6 amplifier with a huge resistor under the 6L6es. Almost certainly it is plain old cathode bias (a fine plan).
Attached Images
File Type: jpg strip1.jpg (55.7 KB, 410 views)
  Reply With Quote
Old 9th August 2019, 11:15 PM   #10
Wavebourn is offline Wavebourn  United States
Anatoliy Lisovskiy
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Autobiasing output tubes (6L6)?
I could not find the better way to auto-bias than bypassed cathode resistors. If to use CCS it is the best for the convenience, but the worst for the sound. It changes bias voltage following by the sound envelope. Fixed bias is the best for the sound, but the worst for convenience.
__________________
!!!Warning!!!
Single Ended Class-A-Holism is addictive!!!
  Reply With Quote

Reply


Autobiasing output tubes (6L6)?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
What tubes with 3.3K p2p output? no gas Tubes / Valves 11 27th November 2010 06:29 PM
TV Horz. Output Tubes used as Output Audio ppl Tubes / Valves 90 27th August 2009 02:21 PM
Which of these output tubes? jnb Tubes / Valves 1 21st June 2007 03:49 PM
max. output for two 5881 tubes? tonskulus Tubes / Valves 7 21st January 2007 11:00 PM
Any other sub for KT 88 output tubes? Burnedfingers Tubes / Valves 13 22nd January 2005 10:28 PM


New To Site? Need Help?

All times are GMT. The time now is 08:12 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki