How do the various resistor capacitor bias network combinations effect the tonal attributes of the V1a and V1b preamp input and first gain stage tubes? I have seen all sorts of values and type specifications.
Merlin has a Tube Preamp book. It just might require reading all that if you want an honest and complete answer (Disclaimer: I have read Merlin's book, but I assume he is thorough in his explanation).
How do the various resistor capacitor bias network combinations effect the tonal attributes of ...
What I want to know is: How is this affecting the tube. Is it the density of the electron cloud? The energy of the cloud? Harmonics? How does this relate to Fatness or purity of tone? Don't know. ...
Short answer:
The plate load resistor was probably selected to set the amount of voltage amplification for the stage, but may have been chosen to allow a certain maximum output voltage size in some designs/situations.
The cathode resistor is selected by the designer to set the chosen operating point of the tube, in conjunction with the chosen plate load resistor. It might be selected to bias to an operating point that results in maximum clean output voltage swing, or to minimize distortion, or to ensure distortion, or to change the nature of the distortion and/or which side of the output waveform clips first.
Most traditional guitar amps had values selected to provide a reasonable amount of voltage amplification, and something near maximum clean output voltage with the expected input signal voltage. Leo didn't pick 'em for "cloud energy" or "fatness" or "tone" unless that tone was loud and basically clean.
An unbypassed cathode resistor introduces local negative feedback, because the tube current with a signal causes a varying voltage across the cathode resistor, and therefore a varying bias. The cathode voltage variation impacts tube current in an opposite manner as the signal input that caused the current variation and therefore the cathode voltage variation.
Result: tube exhibits less voltage amplification as it would if the cathode voltage was constant.The amount of gain reduction due to the unbypassed cathode resistor depends on the value of the cathode resistor and the tube type being used. Rough rule of thumb with a 12AX7, 100k plate load and 1.5k cathode resistor would be gain is around half what it would be if the cathode voltage was constant.
Bypass cap is added to bypass the varying current around the cathode resistor, keeping the cathode voltage constant, and restoring voltage gain to the value allowed by the tube and plate load.
The cap stops looking like a piece of wire to some low frequency a.c., and starts looking like a bigger and bigger impedance. If you go low enough, the gain drops to an amount equal to that with an unbypassed cathode resistor.
You could tweak the value of the bypass cap up/down to cause the reduction of gain to happen at a frequency you choose. Exactly what value cap to use depends on cathode resistor value and the impedance seen by the cap when "looking into the cathode of the tube".
For you and me who know what frequency is right when we hear it, bigger cap equals full gain down to a lower frequency, smaller cap equals more bass is rolled off.That's the "short answer". A full detailed answer with all the how's and why's, including the background to explain the explanation, will easily fill several chapters or an entire book.
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