Self Bias and Fixed Bias for Big Tubes? - 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 24th November 2006, 12:00 AM   #11
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
This is the actual amplifier.
Attached Images
File Type: jpg quadric amp1.jpg (27.5 KB, 380 views)
  Reply With Quote
Old 24th November 2006, 04:45 AM   #12
diyAudio Member
 
Join Date: Oct 2002
Location: SINGAPORE
that looks cool!
  Reply With Quote
Old 24th November 2006, 03:17 PM   #13
Jaap is online now Jaap  Netherlands
diyAudio Member
 
Jaap's Avatar
 
Join Date: Dec 2002
Location: Netherlands
Default fixed bias in IT coupled designs

What is better in an amp that is transformer coupled to the power tube. Fixed bias into the secondairy of the transformer and a grid resistor to ground. Or fixed bias into the grid resistor and the center tap of the transformer attached to ground ?
  Reply With Quote
Old 27th May 2013, 02:21 AM   #14
jerrys is offline jerrys  United States
diyAudio Member
 
Join Date: Mar 2011
Location: Spokane, Washington, USA
Quote:
Originally Posted by HollowState View Post
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
not exactly. Cathode bias works fine with the 6L6 family. IF you want to get crazy with the plate voltage, then you have an incentive to use fixed bias. It also bears nothing that the screen grid (and thus the voltage applied to the screen) actually exerts more control over the tube than the plate. Normally, in pentode mode, 250 ohms is good on a 6L6 (or member of the 6L6 familyi) and you waste very little power in cathode-bias mode UNLESS you push the plates beyond 400 volts AND raise the screen voltage as well. Then the difference in power output between cathode and fixed bias will become relevant. Look at the tube data sheets, study them a bit. The whole story is right there. But, it really depends on the TUBE, and not necessarily how powerful the tube is. The 7591 is normally run in fixed bias for a reason and it's LESS powerful than the 6L6GC. Bottom line, in a 6L6 with plate voltage of 400 or maybe a bit more and a reasonably sane screen voltage (say, less than 350), you have no incentive to use fixed bias over cathode bias.
  Reply With Quote
Old 27th May 2013, 04:58 AM   #15
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Yes exactly. We were not talking about 6L6s or 7591s. These are not "large tubes". The original poster was refering to 211s and 845s. These tubes work at 1000 plate volts (or more) and negative 80 and 150 grid volts respectively because of their low . Under these conditions fixed bias is far more pragmatic.

With smaller tubes like the 6L6 etc, I tend to agree with you. Self bias can be more easily implemented because the losses are smaller. Both bias systems have their pros and cons with these smaller tubes. However, I am of the opinion that fixed bias is generally superior for all but the lowest power output tubes. Self bias is popular with some DIY's because it is simpler, and it raises the bottom line for manufactures. So here, it comes down to designer's choice.
__________________
"The first principle is that you must not fool yourself, and you are the easiest person to fool." ~Richard P. Feynman
  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 11:20 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