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Old 6th August 2010, 09:23 PM   #1
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Smile 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?
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Old 6th August 2010, 09:49 PM   #2
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Search on "fixed bias".
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Old 7th August 2010, 08:16 AM   #3
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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".
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Old 7th August 2010, 05:04 PM   #4
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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?
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Old 7th August 2010, 05:24 PM   #5
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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.
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Old 7th August 2010, 05:27 PM   #6
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Quote:
Originally Posted by DualTriode View Post
There seems to be a hole in the collective knowledge base.
Drama major?

Quote:
Originally Posted by DualTriode View Post
Any experience doing “fixed bias”, any examples?
How about the most ubiquitous tube amp of all:
Click the image to open in full size.
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Old 7th August 2010, 05:30 PM   #7
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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.
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Old 7th August 2010, 06:29 PM   #8
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Hello Brain Trust,
Now we are talking.
Thank You
DT
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Old 7th August 2010, 09:13 PM   #9
DF96 is offline DF96  England
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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!
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Old 8th August 2010, 11:00 AM   #10
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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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