Fixed Autobias? - 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 26th December 2011, 01:19 PM   #1
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Default Fixed Autobias?

Simple question,

I have never tried this I guess someone has..

Is it possible to fix bias an output tube close to bias point and then autobias it to the correct value? So you have fixed bias not adjustable and its auto biased into correct operation?
I guess this dosen't work or I would have seen it done?

Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  Reply With Quote
Old 26th December 2011, 01:32 PM   #2
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
I am told it works..LOL

OK how low can you get the cathode resistor and still have effective auto bias?


Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  Reply With Quote
Old 26th December 2011, 02:00 PM   #3
diyAudio Member
 
hidnplayr's Avatar
 
Join Date: Sep 2008
I guess you need a servo..
__________________
Real tubes have top-caps
  Reply With Quote
Old 26th December 2011, 02:17 PM   #4
diyAudio Member
 
Join Date: May 2010
Location: Ball Ground, GA
The result would be to effectively and significantly reduce the value of the cathode bias resistor over that traditionally used for pure cathode bias. From a pure performance standpoint, fixed bias usually delivers the best overall performance, although cathode bias can deliver very good performance as well when designed correctly. However, if any deviation from pure fixed bias represents a move from optimum, then the combination of the two bias schemes would have to represent something less than optimum conditions.

One way to easily try your idea would be to combine my Enhanced Fixed Bias (EFB)(tm) design with the addition of an appropriately small cathode bias resistor between the output of the EFB regulator and the cathodes. In this scenario, the EFB regulator would be set for (possibly) a couple of volts less bias than required, and then the small cathode resistor could make up the last few volts.

In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!

Dave
  Reply With Quote
Old 26th December 2011, 02:17 PM   #5
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by hidnplayr View Post
I guess you need a servo..
I don't want to use a servo....I am wondering about Grid bias and standard auto bias combined..

Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  Reply With Quote
Old 26th December 2011, 02:27 PM   #6
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by dcgillespie View Post

In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!

Dave
If I can get a reduction of resistor to quarter of the value or even half the value it will be useful..

Regards
M. Gregg
__________________
What is the sound of one hand clapping?
  Reply With Quote
Old 26th December 2011, 03:22 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
This is sometimes known as mixed bias. I have used it for an EL34 UL PP output. My 'fixed' bias was variable. If it is really fixed then you can't take it too far as it won't adjust for valve variations and ageing. Halving the cathode resistor value is about as far as you can go, unless you can test and select the valves and hope they don't age too quickly.
  Reply With Quote
Old 26th December 2011, 03:23 PM   #8
diyAudio Member
 
hidnplayr's Avatar
 
Join Date: Sep 2008
Oh, so you mean a plain combination of cathode resistor & fixed bias? It will work yes, but regulation will obviously be not as good as with simple auto bias..
__________________
Real tubes have top-caps
  Reply With Quote
Old 26th December 2011, 03:32 PM   #9
M Gregg is offline M Gregg  United Kingdom
diyAudio Member
 
M Gregg's Avatar
 
Join Date: Jun 2010
Location: UK
Quote:
Originally Posted by DF96 View Post
This is sometimes known as mixed bias. I have used it for an EL34 UL PP output. My 'fixed' bias was variable. If it is really fixed then you can't take it too far as it won't adjust for valve variations and ageing. Halving the cathode resistor value is about as far as you can go, unless you can test and select the valves and hope they don't age too quickly.
looks interesting, I guess it could be one dual pot to just shift the Fixed value..(If needed)
Just reducing the resistor would reduce the discharge time of the capacitor after transient..worth a try...

Thank's for your thoughts...

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

Last edited by M Gregg; 26th December 2011 at 03:35 PM.
  Reply With Quote
Old 26th December 2011, 08:18 PM   #10
diyAudio Member
 
Join Date: Apr 2010
Fixed bias with cathode bias.

Prowess Amplifiers - Fender - Schematics - Bandmaster ac568
  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
Autobias OTL....Any thoughts? Alastair E Tubes / Valves 47 17th January 2012 02:47 PM
EL84 fixed fixed bias razorrick1293 Tubes / Valves 3 20th November 2011 03:33 PM
KT90 to KT88 with autobias pof@ Tubes / Valves 5 23rd August 2011 09:51 AM
Filament Autobias kenpeter Tubes / Valves 18 6th September 2010 03:03 PM
Autobias for 6L6/EL34/KT88 SWMoore Tubes / Valves 10 2nd April 2003 07:26 AM


New To Site? Need Help?

All times are GMT. The time now is 06:19 PM.


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