LED's in the cathode.. - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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
Reply
 
Thread Tools Search this Thread
Old 9th April 2012, 06:19 PM   #11
diyAudio Member
 
yagoolar's Avatar
 
Join Date: Oct 2004
Location: Mazowieckie, Jozefoslaw
Send a message via ICQ to yagoolar Send a message via Skype™ to yagoolar
I have used LED matrix in my SE amp based on 6B4G equivalent (Russian 6C4C) replacing RC bias. The sonic difference is huge and clearly audible. My first try was SY's RLD. Now there is ongoing push-pull project with LEDs.

Oh, pardon me. You asked about *pre* stages.
  Reply With Quote
Old 9th April 2012, 06:30 PM   #12
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Quote:
Originally Posted by dgta View Post
Osvaldo, the LEDs are used in the cathode, for bias. Not in the anode.
ANODE CURRENT IS THE SAME AS CATHODE CURRENT (except in grid leak), so if the cathode current is distorted, the anode also will be, and there, the anode AC voltage to ground will be also distorted.
__________________
LW1DSE
  Reply With Quote
Old 9th April 2012, 06:43 PM   #13
diyAudio Member
 
Join Date: Oct 2011
Location: Tauberbischofsheim, Germany
Here's the thread, TheGimp, posting #33.

Best regards!
  Reply With Quote
Old 9th April 2012, 06:51 PM   #14
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by Osvaldo de Banfield View Post
ANODE CURRENT IS THE SAME AS CATHODE CURRENT (except in grid leak), so if the cathode current is distorted, the anode also will be, and there, the anode AC voltage to ground will be also distorted.
And yet, experimentally the distortion of circuits using LED bias is quite low (assuming competent design). My phono preamp, for example, has 0.03% distortion at full output, almost entirely second harmonic, with no harmonics detectable past third, and it uses LED bias for two of the three amplification stages.

If you think about the actual AC impedance, how it varies over the current swing of the stage, and the effect on circuit operation, it becomes more evident why the cathode LED is not a significant source of non-linearity.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 9th April 2012, 06:54 PM   #15
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by SY View Post
And yet, experimentally the distortion of circuits using LED bias is quite low (assuming competent design). My phono preamp, for example, has 0.03% distortion at full output, almost entirely second harmonic, with no harmonics detectable past third, and it uses LED bias for two of the three amplification stages.

If you think about the actual AC impedance, how it varies over the current swing of the stage, and the effect on circuit operation, it becomes more evident why the cathode LED is not a significant source of non-linearity.
SY,

Do you have any thoughts on the effect of LED cathode bias in preamp tubes with CCS or Gyrator anode load?

Just an observation OK the circuit I describe should have a cap across the cathode load to prevent drift however here is a tail...I used resistive load in place of a ccs with no cathode bypass cap.. the volume of the amp altered with the increase in temp over about an hour (on its own). Replaced the resistor on the cathode with an LED and 1N4148 for the set point..Guess what no drift..and better drive from the stage..

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 9th April 2012 at 07:03 PM.
  Reply With Quote
Old 9th April 2012, 07:04 PM   #16
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Since that's what I do in most of my designs, I'm clearly in favor. Take a look at His Master's Noise to see one implementation.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 9th April 2012, 07:20 PM   #17
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
so if the cathode current is distorted, the anode also will be
Yes the anode and cathode currents are essentially equal, otherwise the split load PI wouldn't work. The difference is that a tiny voltage change on the anode wouldn't be noticed. A tiny voltage change on the cathode will affect the bias on the tube causing a bigger than normal change in current through the tube.

Quote:
And yet, experimentally the distortion of circuits using LED bias is quite low (assuming competent design)......The LED acts as a constant voltage source. Or close to it, depending on the particular LED's dynamic impedance. Whether it's appropriate and of benefit for any given stage depends on the design.
I have also noticed some very low distortion readings in LED biased stages. Replacing the LED with a resistor and cap adjusted for the same operating conditions did not yield the same low distortion. Adding a bypass cap across the LED actually increased the distortion. The LED improvement varies with the LED and the tube being used, even among batches of inentical LEDs and tubes. The tubes were CCS loaded 12AT7's and the LED's were from a bag of 1000 surplus green LED's.

it is my belief that the nonlinearities in the 12AT7's and the LED's were cancelling to some extent, but I have not tried to prove this theory yet.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 9th April 2012, 07:24 PM   #18
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
For the lowest THD in a triode input stage, I use a constant current source on the anode and LEDs in the cathode. For best results use LEDs with a low dynamic impedance. The red and green ones seem to be the best with a dynamic resistance under 10 ohm in most cases.

I have verified my claims that the lowest THD is obtainable with a CCS and LED biasing using an HP 8903A distortion analyzer. I have verified this with several triodes - 6J5, 5842, d3A, to name a few.

I realize that for some, this may be too much sand in the glass, but ultimately it comes down to your design goals. For me, the goal is to achieve the best sound quality.

~Tom
__________________
Modulus-86: Composite amp achieving 0.00018 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 9th April 2012, 07:30 PM   #19
mogliaa is offline mogliaa  United Kingdom
diyAudio Member
 
mogliaa's Avatar
 
Join Date: Dec 2009
Location: London
I personally achieved great results on a 6j5 driver with a Mu-follower gyrator achieving less distortion than CCS and very happy with sound, of course: LED bias
__________________
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
http://www.bartola.co.uk/valves/
  Reply With Quote
Old 9th April 2012, 07:34 PM   #20
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Looks like another stage of LED bias testing on the cards..

Just got to crack the gyrator set up....
One other thing I did notice was how much better a range of CD's and material sounded with the LED bias...some being harsh on some systems..

Regards
M. Gregg
__________________
What is the sound of one hand clapping?

Last edited by M Gregg; 9th April 2012 at 07:36 PM.
  Reply With Quote

Reply


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
LED's jim5763 Swap Meet 3 21st February 2010 02:04 AM
300B Cathode Feedback + Cathode Bias korneluk Tubes / Valves 13 7th January 2009 09:07 PM
UL to triode mode conversion on cathode bias amp-different cathode resistors needed? bigwill Tubes / Valves 6 30th December 2007 01:37 AM


New To Site? Need Help?

All times are GMT. The time now is 11:12 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2