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 11th August 2019, 12:31 PM   #21
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
An amp staying purely in Class A will still have shifts in average current caused by second-order distortion in the output stage. With fixed bias this does not matter. With resistor bias you get a shift in quiescent current in the opposite direction, but this is not too serious and you can compensate by biasing the valves a little 'hotter' than is ideal. With CCS bias (however achieved) you get a bigger shift in quiescent current so you need either to bias even hotter or accept some crossover distortion on sustained loud passages. Hence resistor bias is the best option if you want to avoid user adjustments, and fixed bias is the best option if the user is happy to fiddle; CCS bias is never best.
  Reply With Quote
Old 11th August 2019, 01:36 PM   #22
gabdx is offline gabdx  Canada
diyAudio Member
 
Join Date: Jan 2008
cathode bias lose a lot of power and resolution.

to me it is unacceptable for a hifi amplifier.

DF96 is right that bias is higher for cathode bias, but max anode dissipation is lower.

I use a separate transformer and passive supply with individual capacitors to each grid.

The danger with fix bias is on shut down, if the (-) drops too fast you end up pushing for 1 second at maximum dissipation your tubes, especially with big capacitor banks.

so the trick is to have a minimum set up bias if the trimpot fails with a resistor in // then an individual charging capacitor after the trimpot.

Also another mistake with fix bias, especially with big tubes like kt150 is to have too much resistance to ground.

I use grid bias with battery in the phono stage, and grid bias in all stages of my amplifiers, or led bias too is super good.
__________________
amplificateurslegrand.ca
  Reply With Quote
Old 11th August 2019, 01:43 PM   #23
pblix is offline pblix  United States
diyAudio Member
 
pblix's Avatar
 
Join Date: Apr 2011
Location: USA
Quote:
Originally Posted by kodabmx View Post
Maybe not, but deliberately wasting heat in a resistor is different IMHO.
I never built amps with Triode connected output stages for the same reason, i.e. deliberately wasting power. Different strokes for ditto folks I suppose
  Reply With Quote
Old 11th August 2019, 02:00 PM   #24
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by gabdx
cathode bias lose a lot of power and resolution
Typically less than 10% power loss, so much less than the output stage heaters. If you are worried about efficiency you would not use valves.

Resolution? No idea what you mean.
  Reply With Quote
Old 11th August 2019, 02:33 PM   #25
kodabmx is offline kodabmx  Canada
Spreading triode love.
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
Autobiasing output tubes (6L6)?
Quote:
Originally Posted by pblix View Post
I never built amps with Triode connected output stages for the same reason, i.e. deliberately wasting power. Different strokes for ditto folks I suppose
Fair, but at least you get something for the power loss in triode mode.
  Reply With Quote
Old 11th August 2019, 09:06 PM   #26
gabdx is offline gabdx  Canada
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by DF96 View Post
Typically less than 10% power loss, so much less than the output stage heaters. If you are worried about efficiency you would not use valves.

Resolution? No idea what you mean.
10% power loss, of what decibels, voltage output?

i worry a lot about losing precious power that the output stage can develop, it sound a lot better with grid bias.

resolution I mean that the local feedback or the capacitor color the sound and it is absolutely not needed.
__________________
amplificateurslegrand.ca
  Reply With Quote
Old 11th August 2019, 10:27 PM   #27
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
You are worried about Efficiency, Right? Do not take the following too seriously, but do think about it: Tubes are not efficient in most of the configurations and topologies of audio amps. Output transformers have a power loss, so do not use an OPT. OTL has a large power loss due to impedance mismatch, so do not use OTL. LED power dissipation is the same as resistor power dissipation (V x A). Battery Bias is great for safe power up (cold tubes are already biased); and for safe power down (hot tubes are biased during power down in case B+ is still there).

A 2A3 consumes 6.25W filament, 15W plate, 21.25W total. 6.25/21.25 = 29.4% filament, the rest 70.6% is the plate.

Don't use DHT unless it is a Thoriated filament (some are a little higher efficiency). Push Pull AB1 amp: do not use Individual LED for cathode bias, the LED will turn on and off, and will be very non-linear over the large changes in current. Use a common LED array.

Then use 2 elevated voltage 1 Ohm cathode resistors to monitor the cathode currents (and you may find that your "Matched" output tubes are not so well matched. These amps are big and heavy, they waste power being transported across the country or the ocean. The massive parts also take lots of energy to produce, so use solid-state amps. Linear solid state amps are too heavy. Use solid state switcher amps. All designs have tradeoffs, live with it.

Last edited by 6A3sUMMER; 11th August 2019 at 10:29 PM.
  Reply With Quote
Old 11th August 2019, 10:58 PM   #28
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Addendum to #27 above:

Do not use Tube Rectifiers, they waste filament power and plate power.

Last edited by 6A3sUMMER; 11th August 2019 at 11:01 PM.
  Reply With Quote
Old 11th August 2019, 11:14 PM   #29
gabdx is offline gabdx  Canada
diyAudio Member
 
Join Date: Jan 2008
well, i know already about all this, nothing new here, I already use many of your 'suggestions' because power is important to me and has audible benefits. (edit: my conscience require that I build something which uses the resources to their full potential, I feel compelled to get that 10% or whatever it is power out of those output tubes.)

It is still fun to build amps with triodes and tube rectifiers.

maybe you didn't like what I said about cathode bias.

I was truthful to the question about his amplifier autobiasing, which many of you said that it was a cathode resistor, which he could modify to fix bias and enjoy it even more.
__________________
amplificateurslegrand.ca

Last edited by gabdx; 11th August 2019 at 11:16 PM.
  Reply With Quote
Old 11th August 2019, 11:42 PM   #30
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Please do not get me wrong, Fixed bias is a good thing. Like all other topologies there are tradeoffs.

Output stages for example: You can use fixed bias and interstage transformers. You can use fixed bias and RC coupling. You can use fixed bias and DC coupling.

If you use RC coupling, keep the signal below the point of drawing grid current, or you have the same effect as cathode R plus bypass cap self bias.

If you use interstage transformers, for a push pull amp, be sure to use matched tubes, or separate grid windings for individually adjusted biasing. Note that drawing grid current will change the load to both the interstage and the driver, account for that.

If you use DC coupling, be sure to deal with mis-matches, and drift of all causes. Again take account of any grid current.

If you use fixed bias, do notice what was said (before my posts): take care of power-up, and of power-down. I am sure lots of people have heard very excellent systems that used fixed bias, self bias, battery bias, and whatever.

Some topologies get a bad reputation, because they are not done properly. That is why I and 2 others did over 600 extensive measurements, multiple listening venues with different audiences and different loudspeakers, math proofs, and physics proofs. Then we wrote the results about parallel tubes for the last issue of Glass Audio, "Paralleling Tubes Effects" Volume 12, Number 5, 2000. All started as a lark, when we asked the question about the sound of parallel tubes: Why? The Michelson-Morely Experiment (they tried to prove the existence of Ether; but proved it did Not exist). We started with the premise that parallel tubes sound Bad, but proved with correct design and attention to details, that parallel tubes sound Good. Hmm.

"For every engineering problem there are 100 solutions, of which at least 3 will work . . . but only if they are implemented properly" - Me

More power sounds better, because of the Fletcher Munson effect. Fletcher's Munson effect causes extreme frequencies not to be heard by our ears when they are played at low levels.

Over 40 years ago, I took a friend to a Stereo warehouse. He was interested in those original Bose 901 speakers. We listened to music, including some low frequency organ music. Sounded pretty good.

Then I asked the salesman to play that organ music again, but through a pair of AR3a Acoustic Research speakers. Suddenly we FELT the Bass notes. My friend brought the AR3a speakers home with him.

In reality both the Bose 901s are not very efficient, because the low frequencies are boosted in order to level the apparent frequency response (and a majority of the power in music is quite often in the bass frequencies. Of course, the very high frequencies are also boosted, again to make up for the driver rolloff (but music highs require lower power than bass frequencies).

AR3a speakers are not noted for efficiency either.

Perhaps we should be using the 'last design' of the EV patrician with horn loading, 30 inch woofers, 12 inch bass, 8 inch lower mid, horn higher mid, and T350 horn tweeter. Then we can all use 45 amplifiers, which take a lot less power than many solid state non-switching amps.
  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 02:30 PM.


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