Fixed bias supply problems... - 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 31st August 2004, 10:08 AM   #1
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Angry Fixed bias supply problems...

Hi, I am trying to rig up a fixed bias stereo 807PP amp using diode rectifiers. I have wired the amp so that negative bias/g1 voltage is switched on with the filaments, and then the B+ and screen/g2 voltage is turned on with a second (standby) switch.

With roughly 550V on the anodes, 295 on the screen (g2) and -33V on the grids it works fine, solid AB1, no problems.

The problem i encounter is turnon surge of the bias voltage: When I turn the B+ on, the grid voltage briefly rises from -38V to -10V before settling back to -33V. As a result, the plate current of each 807 rises to about 100mA at turn on, settling back to 35mA. This is pretty high, and I am a bit worried about the cathode stripping effect of this! The 807s are NOS and it happens with a different (good) set as well.

My bias supply is like this: 100VAC secondary winding -> 1N5404 -> 220uF -> 2.2K -> 22uF -> variable resistor to ground (bias taken from wiper). The other leg of the 100V winding is connected to ground. I can adjust the current quite nicely, it works fine, just the massive turnon surge.

I am guessing I need better regulation in this bias supply, anyone got any schematics for this type of thing?
  Reply With Quote
Old 31st August 2004, 10:41 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Have you checked that there is no oscillation as you bang the tube with its B+? That the B+ (or even more important, the screen supply) isn't overshooting? And that the secondary bias winding voltage isn't dipping at turn-on?

One easy thing to try is to temporarily connect a really big cap across the 22uF, like 4700uF or so, then see if the circuit still behaves the same way. If it does, look somewhere else than the bias supply. Also, 100mA isn't really all that huge- I wouldn't worry much about cathode stripping. If it's only running that high for a couple of seconds, dissipation won't be a problem either.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 31st August 2004, 10:44 AM   #3
MIKET is offline MIKET  United States
diyAudio Member
 
MIKET's Avatar
 
Join Date: Jul 2003
Location: Maine
Send a message via Yahoo to MIKET
Default Other possible solution

If you use an RC filter in the bias supply fairly large resistor in series with a cap which feeds the bias voltage the bias voltage will rise slowly causing a soft start effect. This should help eleviate the turn on issue.
  Reply With Quote
Old 31st August 2004, 11:22 AM   #4
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Quote:
When I turn the B+ on, the grid voltage briefly rises from -38V to -10V before settling back to -33V.
Most probably the time that link caps charge trhu the grid leak resistor.
Use no larger value than necessary.

You may also try to delay (big RC) the 807' G2 voltage.

Regards, Yves.
  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
Self Bias and Fixed Bias for Big Tubes? commstech Tubes / Valves 14 27th May 2013 03:58 AM
Fixed bias: Pots near supply or tubes? DrewP Tubes / Valves 10 23rd July 2009 12:17 AM
Finding bias point in PP fixed bias beamnet Tubes / Valves 14 7th December 2008 01:25 PM
DHT's, fixed bias, sharred filament supply audiowize Tubes / Valves 2 22nd October 2008 08:32 AM
Battery grid bias to fixed bias in Toccato? w00t Tubes / Valves 3 2nd December 2007 12:34 AM


New To Site? Need Help?

All times are GMT. The time now is 07:32 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