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In Reply to: RE: If you need precision current bias of power tubes... posted by PakProtector on June 03, 2017 at 19:12:36
"Current source my foot"Which aspect of its performance caused you to say that? I do see that its output impedance isn't very high at elevated current settings. Not sure how parts like the LM317 compare in this regard. Most published specifications for the LM317 are based on use as a voltage regulator, not current source.
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Buy Chinese. Bury freedom.
Edits: 06/04/17Follow Ups:
I wrote 'precision current bias of power tubes'.
A current source does not suit so fine in a (PP) power output stage.
Tube rectification always occurs when leaving class A operation and spice simulations show that a capacitor in parallel to the 'current source' (or the bias resistor) reduces total harmonic distortion.
I was simply considering to use this component for building pre-adjusted current generators to be eventually plugged in the power tube cathode(s) instead of a standard resistor.
This would put you in the position of totally eliminating the bias adjustment pots as you can rely on the pre-set current even when using limit tubes.
ecc230
"A current source does not suit so fine in a (PP) power output stage.
Tube rectification always occurs when leaving class A operation and spice simulations show that a capacitor in parallel to the 'current source' (or the bias resistor) reduces total harmonic distortion.
"
If you configure the output stage as a LTP with a current source "tail" it never leaves class A. Granted, matching tubes is a "bitch" but it can be done. Sonically the result can be quite surprising for those who have never heard it before. I'm currently running four triode connected KT-120s in PPP off of a single current source @290mA.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Class A operation with current source tail does not mean linear operation when tube rectification occurs.
Under these conditions some signal compression occurs.
I will certainly try your suggestion anyhow, I like the potential.
ecc230
I dislike the idea of applying this level of control with a current source at the cathode. There's no way to know what the grid-cathode voltage will actually be at the point where the source current setting is satisfied. It seems much more reasonable to apply a fixed voltage to the cathode with a regulator, and to adjust it within a known window for the correct bias current.
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Buy Chinese. Bury freedom.
You are certainly right if you are also looking for maximum output power.
I am rather focused on serviceability of the hardware.
Only a few tube experts are now able to carry out the periodic adjustments of power tubes.
So the lesser current controls are present, the better.
As I said, with 'current sources' you can eliminate adjustment pots.
Have a nice day.
Luca
ecc230
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