Tube DIY Asylum

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Rosenblit Also Got This Wrong

Posted by Triode_Kingdom on June 13, 2021 at 08:34:04:
According to Bruce Rozenblit ...

"If the current through V1 were identical to that through V2, then every increase in current flow through Rk caused by V1 would be met with a corresponding and equal decrease in current flow in Rk caused by V2. If the current is increased and decreased by the same amount at the same time, the net change is zero, and no signal would be present across Rk to drive V2. The only way the circuit can work is when the currents through the two tubes are unequal by a small amount." (Bold font is mine.)

Absolutely not true. The first sentence - on which every subsequent conclusion is based - is fundamentally incorrect. The two equal/opposite currents in each tube do not simply meet and cancel each other at Rk as Rozenblit suggests. Rather, the current flow between the two cathodes "seesaws" back and forth. This action occurs regardless of whether the currents are identical (such as when a CCS is used). When a tail resistor is used instead of a CCS, each triode attempts to create current flow independently according to its gain and drive level, and the difference passes through the tail resistor.

I'll just add that the error should have been caught when that last conclusion was put to paper. If it were true, it should be possible to null the output of V2 by moderate adjustment of the anode resistor ratio. The output of V2 should also null when currents are equalized by means of a CCS in the tail. Of course, neither scenario occurs, because the basic hypotheses regarding the necessity of unequal currents is wrong.