94 Valve Amplifiers +285V 100k 151.5V 2M2 E88CC 72.5V 75V V out 0V E88CC 2.5V 1k8 750k 330n 0V FIGURE 2.21 The cascode. This is the additional noise, compared to the triode, that is generated by the electron stream splitting to pass either to the anode, or to g 2 . This addi- tional noise is related to the ratio of anode to screen grid current and to the mutual conductance of the screen grid; typically, this makes a given pentode 6À14 dB noisier than the pentode connected as a triode. (Connected as a tri- ode, the EF86 is actually quite a good triode.) Even worse, partition noise has a 1/f frequency distribution, which means that its amplitude rises as fre- quency falls, which has been found to be particularly irritating to the ear. THE CASCODE What we would like is a valve, or a compound device, that gives all the advantages of the pentode with none of its disadvantages À this compound device, known as the cascode [5], was invented for use as a high-gain error amplifier in power supply regulators but quickly became very popular as a pentode substitute (see Figure 2.21). The cascode bears considerable similarity to the pentode in that there is an arrangement of components (R 1 , R 2 and C 1 ) that looks very much like a screen grid bias supply, and, indeed, this is what it is. The device has a very