I am in the process of re-education myself in the tube world. So I went back to square one and rebuilt my lab, set up some test jigs, and started to go through data sheets. To get a good grasp on how biasing works, I am reproducing the bias curves (for small signal tubes) that manufacturers create on the data sheets (using cathode biasing). To the most parts, I can mimic the same data. Since I used a cathode resister to bias the tubes, as the current goes up, the voltage across the cathode resistor goes up (ohms law). I tested first using a DC source, then switched to AC source to plot its characteristics (with and with out a cathode bypass cap). Then I decided to replace the cathode resistor with a constant voltage source, some diodes (1n7000x) and a resistor, across the diodes, rated above max diode forward voltage and tube current under test (I wanted to avoid the diodes slamming on). Note that I kept the resistor high enough not to affect the voltage across the diodes. I essence, a constant voltage source. I do understand that the diode is now action as a variable resistor, but I seem to get a more linear plot when I do the above tests. Also, it does not seem to require a cathode bypass cap. I do know there have been several postings on diode / resistors (and even LEDs, which did not seem to work as well),and how it affects / not affects the sound quality... Am I incorrect in exploring using a CV cathode biasing for small signal tubes? Any advantages / disadvantages?
Diode bias is a form of fixed bias, unlike using a cathode bias resistor. This does improve linearity, as the bias isn't moving with the signal. A bypass capacitor isn't required since the dynamic resistance of a forward biased Si diode is much lower than the rk of any VT, and so very little degeneration occurs.
It's a good idea if you're not using the cathode as a NFB summing node. As for sound quality, who knows? That's a matter of personal preference, and some like the quality of diode bias as opposed to the more common cathode resistor.bypass capacitor solution. Others would disagree. It's a case of listen and decide.
Most of tubes work with cathode resistor bias, some tubes - with -U on the grids. I don't know what kind of tube you used, but maybe it was wrong tube. Cathode res. works in almost situations. IMO, using diode for linearity, lead to pressing the scene of sound. /The same is with SS plate loads, linearity doesn't mean best sound/. And using and cat. caps allow to go down in LF.
I always use cathode res.....IMO. But the choice is Yours.
I always use cathode res.....IMO. But the choice is Yours.
Try the fixed bias voltage on the grid through a high value resistor, instead of in the cathode circuit.
Any nonlinear impedance at the cathode will cause distortion as it is a local feedback element.
In a practical circuit, there would have to be a coupling capacitor before the grid.
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