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it provides better signal integrity because the output impedance is really low so it can drive the tone stack better), or a recovery stage after the TMB tonestack in a blackface amp. That is why there is a DC coupled pair (with a cathode follower) driving a TMB tone stack in a tweed bassman (i.e. A TMB tone stack does this more than a simple vol control (or a simple interstage voltage divider). They load down the stage and affect the integrity of the signal. The vol and tone controls in a pre-amp are an elaborate example of contribution to the output impedance for the stage in front of them. Conversely, the lower the output impedance is of the stage before, compared to the input impedance of the stage after, the more the signal will be preserved. Typically the higher the output impedance of the previous stage etc is compared to the input impedance of the next stage, the more AC gain will be attenuated. These interact in conjunction with (other) loads from (either) your guitar pickup, or the stage in front, or the stage behind, or the output transformer load (which also incorporates the speaker impedance), depending on where the stage is in the amp.

The amount that a stage amplifies is also affected by the stage's input impedance and its output impedance. Transconductance (how much the plate current varies with variations in the grid voltage) andĪmplification factor (how much the plate voltage changes with variations in the grid voltage) and is the product of the plate resistance and the transconductance. Plate resistance - the resistance that the triode presents to AC (determined by how much the plate current varies with variations in the plate voltage) and The amplification factor of the triode itself is determined by: The HT sets how much overall power the stage will have, and to a point the plate resistor and cathode bypass cap set how much gain the stage will have, and the cathode resistor sets the operating point/bias of the stage. The the plate resistor, cathode resistor (and bypass cap), and HT voltage, are means to an end.