The decoupling cells allow to isolate the various amplification stages from each other and from the power supply common impedance.
Purists love also these decoupling caps as, when set close to the tube, they give a plus to the dynamics of the stage.
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Stage without a decoupling cell. |
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Stage with a decoupling cell. |
Three-stages amp without decoupling cells. A strong current appeal from the power stage provoques a drop in the supply voltages because of the power supply impedance. As this impedance is common with the previous stages, the voltage drop occurs also in the supply of these stages which will make the output signal vary and go straight to the output stage ...
The decoupling cells isolate each stage from the power supply and avoid a reaction through its common impedance.