variable bias versus fixed bias guitar amp

If the bias shift is severe, the tube will go into cutoff, and large amounts of 'crossover' distortion will occur. This bias shift can be audible, but is sometimes desirable for guitar amp use, as it adds varying harmonic overtones to the sound. This causes a bias shift, usually in the direction of a colder bias point. 'The problem comes in when there are large signal level changes, and the average DC level of the cathode voltage changes. If the capacitor is removed, the cathode DC voltage will have a signal voltage superimposed on it, which will subtract from the grid-to-cathode signal voltage, and reduce the gain of the stage. This capacitor effectively 'shorts' the AC signal component to ground, while allowing the DC voltage to remain relatively constant. It turns out that in order to keep the DC bias supply voltage at the cathode constant while the input signal is changing, the cathode resistor must be bypassed with a large capacitor. 'Since cathode biasing eliminates the need for a special negative DC bias supply, why don't all amplifiers use cathode biasing? Well, cathode biasing is not without its faults. 'When do you use fixed biasing instead of cathode biasing? Regardless, it won't affect his plans for MIXED (switchable?) 'self' and 'fixed' bias, because that's isolated BEHIND (after?) the PI.and the coupling caps isolate the negative DC bias-voltage to the output grids from the PI circuits.Īllow me to quote from Randall Aiken's wonderful Tech Notes: The circuit is technically called a 'side-branch phase inverter.' For 'best' (can never get perfect) output signal symmetry, the opposite plate resistor should equal the sum of the two branch side resistors ANY changes in branch circuit resistances or branch tube gain result in mis-matched output. You are correct, the 6.8K resistor is the 'signal' generating resistance, with tube gain making up the difference. Oops! I just noticed the cathode modulation 'DEPTH' feeding into the PI 12AX7 and, that's a 10Meg plate resistor, not 10K like I 'thought' I saw. Maybe we're not talking about the SAME schematic? I'm referring to the Model 5E9-A 'Tremolux' schematic shown on page 464 of Asspin Sh-tman's book.

Variable bias versus fixed bias guitar amp