Self Bias and Fixed Bias for Big Tubes? - 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 20th November 2006, 03:29 AM   #1
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
Default Self Bias and Fixed Bias for Big Tubes?

Most of the design for the bigger tubes i searched used the fixed biasing, eg. Ongaku 211, Ankoru 845, and many others. Is there any specific reasons why self-bias using high power resistors at the DHT cathode is not use, be it for sonic or safety reasons? can anyone enlighten?
  Reply With Quote
Old 20th November 2006, 03:52 AM   #2
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Large tubes like these require higher voltages to operate correctly. In order to use self (cathode) bias, a large value resistor would be needed which would seriously hurt efficiency. So it's a matter of practicality. It also gived better control.

Victor
  Reply With Quote
Old 20th November 2006, 04:28 AM   #3
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Actually I know of one 845 SE design that uses a kind of cathode bias in a grounded grid arrangement. The cathode is driven from a triode below which is current sourced through a transistor. If I can find the diagram, I'll try and post it.

Victor
  Reply With Quote
Old 20th November 2006, 04:44 AM   #4
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
thanks for the reply. the design u mentioned sounds interesting...awaits the schematic...
  Reply With Quote
Old 20th November 2006, 06:22 AM   #5
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
the design u mentioned sounds interesting
It makes me dizzy just to think about it

To give a bit more input on fixed vs. cathode bias, as HollowState says, efficiency has a lot to do with it. Another is the class of operation and a third is consideration of B+.

At first glance, cathode bias appears to suit Class A perfectly well; however, it does have its downside. You need a higher B+ because some of it is taken up with the cathode bias voltage. Another disadvantage is the need for cathode bypass caps. These need to be of high capacitance and are usually electrolytic type, which do not give the best sound.

You can avoid using a bypass cap if you use Class A push-pull with a single shared bias resistor (or a CCS); however, the OP tubes must be really well-matched or else you will get a DC offset in the transformer primary, causing some DC saturation of the core. Separate cathode bias resistors give a useful degree of automatic balancing, but then you need bypass caps.

In any (PP) class other than A, fixed bias is called for. Sure, mild Class AB1 (close to Class A) can use cathode bias but separate, bypassed cathode resistors are essential and efficiency is impaired. With deeper Class AB1 and with Class AB2 or Class B, cathode bias is not viable. Fixed bias is the only solution.
  Reply With Quote
Old 20th November 2006, 01:00 PM   #6
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Other aspects:

Self bias normally alows a higher value grid-leak resistor, so driver requirements are relaxed.

Fixed bias gets you more output power.
  Reply With Quote
Old 21st November 2006, 03:19 AM   #7
diyAudio Member
 
mitwrong's Avatar
 
Join Date: Oct 2006
Send a message via AIM to mitwrong Send a message via MSN to mitwrong Send a message via Yahoo to mitwrong
Default Bias

Commstech:
About tube bias, there are 3 types:
1, fixed bias
2, self bias
3, provided bias

Fixed bias is derived from power source, a negative voltage rectifier and trimmed to a desired voltage and feed to the grid leak resistor.

self bias is a positive voltage appears at the cathode when plate current flows, a voltage dropped on the cathode resistor. this voltage with respect to grid, it just turns up side down and become a negative bias voltage. the number of voltage appears at cathode, the same voltage bias.

provided bias is used in transmitter driver or final stage, usually transmitter final stage works on class C, no external bias were supplied, the tube gets it's bias at the driver stage, the coupling resonance coil. at RF positive cycle, the tube flows at max current, and cuts off at negative cycle, then the negative cycle is the bias of the tube. C type amplifier can deliver the highest power than ever been. For details, refer to RF amplifier app. note.

One can shorted the plate to ground, ( with plate load ) and flow up the cathode with a negative voltage supplied, what happen??? The tube still at normal working condition. This is similar to self bias.





we switch around the voltage, then becomes bias
  Reply With Quote
Old 21st November 2006, 06:35 AM   #8
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: Self Bias and Fixed Bias for Big Tubes?

Quote:
Originally posted by commstech
Most of the design for the bigger tubes i searched used the fixed biasing, eg. Ongaku 211, Ankoru 845, and many others. Is there any specific reasons why self-bias using high power resistors at the DHT cathode is not use, be it for sonic or safety reasons? can anyone enlighten?
Those "big tubes" mentioned there are low u triodes. That means a relatively high negative bias to set a Q-Point. Cathode bias would rob Vpk. The high wattage resistors tend to be more inductive, and really should not be used if fidelity is the design goal. Even if you parallel up a whole bunch of smaller resistors, you still have the heat problem. Besides, fixed bias tends to produce less distortion, sometimes much less.

Other times, the "big tubes" were originally designed as RF finals. In these cases, the amplification factors are so high that they require positive grid bias to set any sort of practical Q-Point. Here, cathode bias isn't possible anyway.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 21st November 2006, 11:39 AM   #9
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
Default Re: Re: Self Bias and Fixed Bias for Big Tubes?

Quote:
Originally posted by Miles Prower


.....really should not be used if fidelity is the design goal. .
...
Ahhhh......this is the motherhood statement that helped me in the decision for what method of biasing...

  Reply With Quote
Old 23rd November 2006, 11:56 PM   #10
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Here's a diagram (I hope) of the (AC) grounded grid SE 845. Although it's not really a cathode bias type as I had thought.
The amp is pictured on the next page.

Victor
Attached Images
File Type: jpg quadric diagram1.jpg (81.0 KB, 537 views)
  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
Fixed bias vs. auto bias HFGuy Tubes / Valves 30 14th October 2011 10:45 AM
Fixed bias: Pots near supply or tubes? DrewP Tubes / Valves 10 23rd July 2009 01:17 AM
Finding bias point in PP fixed bias beamnet Tubes / Valves 14 7th December 2008 02:25 PM
Battery grid bias to fixed bias in Toccato? w00t Tubes / Valves 3 2nd December 2007 01:34 AM
Fixed or self bias? satelitis Tubes / Valves 33 5th July 2005 07:16 PM


New To Site? Need Help?

All times are GMT. The time now is 01:46 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