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Old 19th January 2011, 07:22 PM   #1
M Gregg is offline M Gregg  United Kingdom
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Default Fisher bias

Just for interest,

In post #4 this bias technique

Cathode bias resistor calculation - AMPAGE Archive

old Fisher hi-fi bias adjustment trick

What "if any" problems can you see with this?

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M. Gregg
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Old 19th January 2011, 10:32 PM   #2
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This "trick" works just fine, although I would add two comments:

1. I would also put a bypass cap from the wiper of the new bias control to ground to keep the grid return resistors grounded AC wise regardless of the control setting.

2. Increasing the bias resistor to increase the range of the bias control also acts to reduce power output, so do not increase the bias resistor anymore than necessary (if necessary), to bring the range of the control in line with the typical bias needs for the tubes employed.

Overall however, a very simple and easy way to adjust cathode bias current draw.

Dave
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Old 20th January 2011, 11:50 AM   #3
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by dcgillespie View Post
This "trick" works just fine, although I would add two comments:

1. I would also put a bypass cap from the wiper of the new bias control to ground to keep the grid return resistors grounded AC wise regardless of the control setting.

2. Increasing the bias resistor to increase the range of the bias control also acts to reduce power output, so do not increase the bias resistor anymore than necessary (if necessary), to bring the range of the control in line with the typical bias needs for the tubes employed.

Overall however, a very simple and easy way to adjust cathode bias current draw.

Dave
What are your thoughts if the tube has cathode feedback IE. " the O/P winding of the O/P tx in the cathode path?
(Cathode resistor - O/P winding - Gnd)

Would the High resistance (R + pot) go across the cathode resistor or across the cathode to Gnd?

The reason I ask: To set bias in previous post for KT66 / EL34 / 6L6.

Would it be better to have max value for EL34 cathode resistor /Current and adjust down for KT66 or set for KT66 and adjust for EL34? Or at mid point? trying to get max out from each tube.

See post #56
Any thoughts on this circuit?

I have never tried this so any info would be good.

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M. Gregg
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Old 20th January 2011, 12:08 PM   #4
DF96 is offline DF96  England
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Applying positive grid bias and then compensating by increasing the cathode resistor makes an existing problem worse: the shift in bias with signal caused by the second-order distortion of the valve. If at all possible, provide a little negative bias (voltage doubler off the heater supply?) and reduce the cathode resistor. You get the same adjustability but a smaller bias shift with signal.
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Old 20th January 2011, 12:40 PM   #5
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To apply this bias trick with cathode feedback, you would ideally need to place the output winding between the cathode, and top of the cathode bias resistor and cap, which themselves are then grounded. Then, the bias control could be placed across the cathode bias resistor/cap in the normal fashion. However, this places the output common above ground potential, and prevents any common output connections to real world components. Therefore, I would leave the configuration as shown in the post (output winding grounded), and connect the bias control between the cathode and ground. The problem with this configuration however, is that now the FB voltage is also appearing across the bias control. Therefore, not only is the idea of adding a bypass cap at the bias control wiper as I previously suggested absolutely necessary, but now, I would also provide one at the top of the bias control as well. These two caps will then help form a filter to keep the AC off the grid.

As to which tube to set the resistor for, you would want to set the bias resistor to accommodate whichever tube will require the greatest bias voltage. Then, the bias can be adjusted down from there for the other tubes. It would likely be the KT66 that requires the most bias.

Dave
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