1.) Values of Uf vs. changing If for low currents, typical for many tubes (say 1-70 mA)

2.) Dynamic resistance rD (for AC) values for various If . We need really low numbers here, because anode resistance of triode will rise for ( μ + 1 ) x rD from ideal, Rk = 0 (Rk bypassed with Ck for AC) state. Especially when we`d need 2-3-4 (or even more) diodes in series, because of relative low Uf per diode.

For example, trioded D3a (μ = 70) with bypassed Rk has anode resistance of about 3 kΩ with Ia = 10-11 mA. Biased with two CSD01060 in series (Uk = 2 x Uf = 1,74 V), anode resistance is now larger, ra ~ 3,5 kΩ because of (unbypassed for AC) rD = 6,5 Ω in cathode path. If (and how much) it matters depends of implementation. One example where it is critical is RIAA stage.

Determining of Dynamic Resistance rD at a certain forward current, example 10 mA:

- At If = 10 mA (reads Uf = 0,87 V) point on If / Uf graph draw the tangent line

- Calculate rD = (ΔU_F)/(ΔI_F ) = (0,95-0,84)/(0,034-0) = 3,24 Ohms

Several rD values vs. If gives a new graph, where we can see how rD shifts with changing If . It is obvious that diode is quite capable of acceptable low rD when we are „away" from the „knee", about ≥ 10 mA.

Another example: Eleven diodes in series would be suitable for (cathode) bias PP EL84 output stage. About 58,5 mA DC standing current, 11 x 0,934 V = 10,27 V, where rD would be 11 x 0,5 = 5,5 Ohms. Large current „reserve" (2 A for CSD1060) is a another „bonus" on current peaks.

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