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

quick question on fixed bias
quick question on fixed bias
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
Thread Tools Search this Thread
Old 6th August 2007, 06:01 AM   #1
sorenj07 is offline sorenj07  Germany
diyAudio Member
sorenj07's Avatar
Join Date: May 2006
Location: Berlin
Default quick question on fixed bias

hi, first post in a while. I'm working on a couple of monoblocks and recently decided that I'd like to go the fixed-bias route in lieu of cathode biasing, both to get rid of the bypass electrolytics and to get a bit more power. my B+ will probably be around 425V - one 200mA 1000VCT choke-input rectified with a NOS 5U4GB into either an 80 or 100uF motor run cap. I'd rather avoid buying a separate bias transformer for each monoblock (stupid hammond 2xx series doesn't have the windings).

my idea is this: connect a UF4007 to one of the 500V legs to get a half-wave-rectified negative voltage source. where I go from there, though, is fuzzy. should I tag a smoothing capacitor right there, then use a voltage divider to get negative voltage down to something more manageable, like -70VDC? I'd like to leave some room for one pot to control overall voltage, and one pot to balance the tubes. I'm shooting for around 50mA per tube, for around 21W of dissipation. theoretically around 55mA should net me 24 or so watts, which is around 80% of the 6L6GC's max dissipation of 30, so I might try that also.

another question: when figuring out resistance values, it helps to know how much current each 6L6 (Sov 'WXT+, in this case) will draw.. I figured that a 10K pot in between the tubes might work, and maybe a 50K pot to control overall voltage? if current is low enough I'd love to use some ultra compact 21-turn trimpots to do this adjustment. also, how important is capacitance in filtering this bias supply?

thanks for all the help!
Edit/Delete Message
  Reply With Quote
Old 6th August 2007, 08:07 AM   #2
lcsaszar is offline lcsaszar  Hungary
diyAudio Member
Join Date: Mar 2004
Location: Budapest, Hungary

I tried both fixed bias and cathode bias in 807 monoblocks with exactly the same B+ voltage and current. My experience was that fixed bias was not stable, sometimes the tubes run overcurrent. I used separate variable resistors for each grid setting. The cathode resistor (560 ohms in my case) blocked with 2 x 10 uF PP capacitor gave a stable solution.
If you still want to try fixed bias, use a separate small transformer. Since there is no significant current drawn, a 470 uF puffer will do.

  Reply With Quote
Old 6th August 2007, 08:44 AM   #3
ray_moth is offline ray_moth  Indonesia
diyAudio Moderator Emeritus
ray_moth's Avatar
Join Date: Jan 2004
Location: Jakarta
You could try using capacitors to drop the voltage to the bias supply, using full-wave rectification, as Gary Pimm did. This link is to an article where Gary describes the development of his 47 PP. You have to scroll a long way down to see where he introduces capacitive droppers into his power supply.
  Reply With Quote
Old 6th August 2007, 02:26 PM   #4
sorenj07 is offline sorenj07  Germany
diyAudio Member
sorenj07's Avatar
Join Date: May 2006
Location: Berlin
thanks everyone! the capacitor/resistor divider method as shown in the diytube eiclone schematic seems pretty simple, so I'll give it a go.
  Reply With Quote


quick question on fixed biasHide 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
Fixed bias design question Rob11966 Tubes / Valves 15 2nd May 2009 11:30 PM
Battery grid bias to fixed bias in Toccato? w00t Tubes / Valves 3 2nd December 2007 12:34 AM
p3a bias trouble, quick question :) Mattyo5 Solid State 13 13th May 2004 08:03 PM
quick bias tolerance question...across source resistors Mattyo5 Solid State 6 12th October 2003 11:09 AM

New To Site? Need Help?

All times are GMT. The time now is 09:00 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio