Grid Bias provided by transformer secondary winding, what is this stuff about?

DualTriode · 6th August 2010, 09:23 PM

Hello,
I recently bought a couple of power transformers with a winding dedicated to grid bias. I have never biased a grid this way. I have used autobias with a cathode resistor, LED’s, CCS’s and the like.
Will someone explain the methods using a dedicated secondary winding, perhaps examples? Are there options including old school without solid state devices and the new hybrid methods of doing things?
DT
All just for Fun!

leadbelly · 6th August 2010, 09:49 PM

Search on "fixed bias".

mnturner · 7th August 2010, 08:16 AM

Instead of raising the cathode above the grid with a resistor/LED/CCS to make your grid negative relative to the cathode, you ground the cathode and use a voltage divider or something similar (after a coupling cap) to apply a negative DC bias voltage to the grid.
As leadbelly said, search for "fixed bias".

DualTriode · 7th August 2010, 05:04 PM

Hello Brain Trust,
To bump this back up on the list.
I have searched; I did search “fixed bias” prior to posting this thread.
There seems to be a hole in the collective knowledge base. In my searches there are many glancing references to “fixed bias”. I did not find anything substantive. Some days I am a nuts and bolts hands on practitioner. I did not find any examples of how it is done. Any experience doing “fixed bias”, any examples?
DT
All just for Fun!

Miniwatt · 7th August 2010, 05:24 PM

Last time I used it I had a separate winding for bias, followed by a 'upside down' power supply to get a negative voltage. It is connected to the grid after the coupling capacitor through a pot and a resistor.
A 10 ohm cathode resistor is handy for measuring bias.

Hope this helps.

leadbelly · 7th August 2010, 05:27 PM

Quote:

Originally Posted by DualTriode

There seems to be a hole in the collective knowledge base.

Drama major?

Quote:

Originally Posted by DualTriode

Any experience doing “fixed bias”, any examples?

How about the most ubiquitous tube amp of all:

Eli Duttman · 7th August 2010, 05:30 PM

DT,

Many "classic" amps use "fixed" bias O/P tubes. The H/K Cit. 2 is but 1 example. Check the schematic of the "Duece" out.

DualTriode · 7th August 2010, 06:29 PM

Hello Brain Trust,
Now we are talking.
Thank You
DT
All just for Fun!

DF96 · 7th August 2010, 09:13 PM

You won't find much "substantive" on fixed bias because it is actually conceptually simpler than cathode bias. You want -X volts on grid? - then make -X volts power supply and connect to grid. That is it!

ray_moth · 8th August 2010, 11:00 AM

I would just add that the negative voltage is applied to the grouded end of whatever is acting as the "grid leak" of the tube (usually a resistor), as in that ST70 schematic. It's more usual, though, to provide a separate negative voltage source (a potentiometer network) for each tube in a push-pull circuit, to allow the currents to be balanced.

Also, the maximum value of the grid leak resistor with fixed bias is generally less than with cathode bias. This can make driving a fixed bias output circuit more difficult.

6th August 2010, 09:23 PM	#1
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Grid Bias provided by transformer secondary winding, what is this stuff about? Hello, I recently bought a couple of power transformers with a winding dedicated to grid bias. I have never biased a grid this way. I have used autobias with a cathode resistor, LED’s, CCS’s and the like. Will someone explain the methods using a dedicated secondary winding, perhaps examples? Are there options including old school without solid state devices and the new hybrid methods of doing things? DT All just for Fun!

6th August 2010, 09:49 PM	#2
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	Search on "fixed bias". __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

7th August 2010, 05:30 PM	#7
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	DT, Many "classic" amps use "fixed" bias O/P tubes. The H/K Cit. 2 is but 1 example. Check the schematic of the "Duece" out. __________________ Eli D.

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7th August 2010, 08:16 AM	#3
mnturner diyAudio Member Join Date: May 2010	Instead of raising the cathode above the grid with a resistor/LED/CCS to make your grid negative relative to the cathode, you ground the cathode and use a voltage divider or something similar (after a coupling cap) to apply a negative DC bias voltage to the grid. As leadbelly said, search for "fixed bias".

7th August 2010, 05:04 PM	#4
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello Brain Trust, To bump this back up on the list. I have searched; I did search “fixed bias” prior to posting this thread. There seems to be a hole in the collective knowledge base. In my searches there are many glancing references to “fixed bias”. I did not find anything substantive. Some days I am a nuts and bolts hands on practitioner. I did not find any examples of how it is done. Any experience doing “fixed bias”, any examples? DT All just for Fun!

7th August 2010, 05:24 PM	#5
Miniwatt diyAudio Member Join Date: Nov 2004 Location: Den Haag	Last time I used it I had a separate winding for bias, followed by a 'upside down' power supply to get a negative voltage. It is connected to the grid after the coupling capacitor through a pot and a resistor. A 10 ohm cathode resistor is handy for measuring bias. Hope this helps.

7th August 2010, 06:29 PM	#8
DualTriode diyAudio Member Join Date: Aug 2009 Location: Sacramento	Hello Brain Trust, Now we are talking. Thank You DT All just for Fun!

7th August 2010, 09:13 PM	#9
DF96 diyAudio Member Join Date: May 2007	You won't find much "substantive" on fixed bias because it is actually conceptually simpler than cathode bias. You want -X volts on grid? - then make -X volts power supply and connect to grid. That is it!

8th August 2010, 11:00 AM	#10
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I would just add that the negative voltage is applied to the grouded end of whatever is acting as the "grid leak" of the tube (usually a resistor), as in that ST70 schematic. It's more usual, though, to provide a separate negative voltage source (a potentiometer network) for each tube in a push-pull circuit, to allow the currents to be balanced. Also, the maximum value of the grid leak resistor with fixed bias is generally less than with cathode bias. This can make driving a fixed bias output circuit more difficult.

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