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RE: What value should an EL34 Cathode bypass capacitor be in a CCS?

Setting aside the level of bias current, if the cathode has a capacitor bypass then there is an AC path between the output tubes plate and its cathode. With a PP pair, in a sense then, they are in parallel. One tube can be pushed into cutoff while the other conducts (AB1). With a high impedance CCS "tail" and no capacitor the only available AC path for the signal is plate to plate and cathode to cathode. In a sense they are in series. Like the old style Christmas tree lights where if one burns out the whole string goes down. Sense it is impossible for one tube to conduct with the other cutoff under these conditions you have imposed Class A operation. The higher in impedance the CCS the smaller any imbalance AC currents traipsing though the PS picking up PS colorations. With a bypass cap there will be no suppression of the AC imbalance currents.

FWIW my mono-blocks output stage is PPP differential with four triode KT120s running off a single cathode CCS with no bypass cap. The purity of forced Class A is something one should hear before considering the trade-offs (less power etc.) compared to Class AB1.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

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