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Old 16th June 2011, 04:07 AM   #1
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Default Turning up the heat: cathode bias

On a cathode bias amp like the simple se, if listening at a moderate level
(on efficient speakers) and using only a fraction of the handfull of watts available, raising the cathode bias resistor value, or lowering it shouldn't make much of a sonic difference should it?
Doesn't the value of Ip just determine the distortion. And distortion will increase with volume level?
So crank up the juice when cranking the volume?
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Old 16th June 2011, 04:24 AM   #2
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In combination with the B+ voltage, changing the quiescent current changes the output tubes load line, and could result in more or less distortion, depending on the spacing (linearity) of the grid voltage lines along the load line.

Cranking up the idle current will also reduce the life of the tubes. If you crank it up too much, you can exceed the power dissipation rating of the tubes, sometimes resulting in red plates, shorted tubes, and a few other bad things.

For the details, check out this link:


Also check out George's tables at the bottom of this page. Notice the relationship between Rk, B+, Pdiss, and distortion. The values in red are exceeding the nominal rated power values for each tube type.


Last edited by boywonder; 16th June 2011 at 04:31 AM.
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Old 16th June 2011, 10:13 AM   #3
DF96 is offline DF96  England
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So crank up the juice when cranking the volume?
Its a bit more complicated than that. You need to read up on load lines, grid current, grid cutoff, and distortion mechanisms.

For small signals, a small change in bias will not matter (assuming the original designer got it right). For large signals, correct bias is more important so you avoid distortion or premature clipping.
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Old 17th June 2011, 02:22 PM   #4
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Actually what I meant was keep the cathode resistor high (low bias) when listening to music ~ < 85 dB (most of my listening) and once in a while when I crank up the volume crank up the juice to a level still under plate dissipation maximum/plate glow.
I have a six positon selector switch running from 1 k ohm to 400 ohms for cathode bias.

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Old 17th June 2011, 02:36 PM   #5
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Location: South Wales

Trouble is, when you alter the bias current through the tube....

You're also altering the Plate Impedance of the tube....

Going too far from the optimum point will lead to excess distortion, unless you also alter the OPT ratio to compensate--even then, it still wont be good...

Personally, I cant see the point in what you're trying to do....
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Old 17th June 2011, 02:50 PM   #6
DF96 is offline DF96  England
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Increasing the quiescent current for large signals might or might not give you increased headroom. It all depends on the exact bias position. You might hit grid current earlier, which would reduce headroom. You might avoid cutoff, which would increase headroom. There is not a simple answer. In some cases the designer may have taken account of the bias shift which can happen when signals are present, in which case you may be defeating his careful design.
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