Mirko,
the only valve measurements in which the R10 shunt resistor is involved are mutual conductance and anode current (amplifier and rectifier valves). There are two potentially critical issues:
- Clipping by limited shunt amplifier output swing. The RC low pass input filter transforms the half wave rectified sinus input into a triangular shaped symmetric signal (0 V DC) which appears superimposed on the DC output. Its amplitude can be given by
triangular symmetric Uapeak)/(half wave rectified Uepeak) = 0.55/(2*pi*fRC) = 0.012 (50 Hz, 68 Kohm, 2.2 uF)
130 mA mean DC half wave rectified input (10 mA above the 120 mA for the biggest testable rectifier) would cause an equivalently shaped 26 V mean DC output voltage caused by a single peak amplitude of pi*26 V = 81.7 Vpeak per 20 ms (50 Hz) period. This is transformed by the low pass network into 0.012*81.7 = 0.98 V ~ 1 V peaks on top of the 26 V mean output. To avoid clipping in this extreme case the output must go up to 26 + 0.98 >= 27 V above virtual ground. For 120 mA this maximum drops to 24*(1+0.012*pi) = 24.9 V which is well within the output span. The ripple amplitude is 0.012*pi = 3.8 % of the mean DC value.
Meanwhile I have increased V+ to 32 V and replaced the 68 Kohm filter resistor by 20.5 Kohm to improve transient performance. This increases the ripple amplitude to 12.3 %.
- Clipping by the panel meter protection diodes. When measuring rectifier diodes the shunt voltage (and ripple amplitude) is reduced to the 97.5 mV range of the meter, so the ripple stays well below diode forward voltage. When backing off anode current during the measurement of amplifier valves the maximum ripple at 50 mA mean DC (100 mA on the backing off controls) is 10 V * 3.8 % = 380 mV. This is reduced to 73 % = 277 mV by the backing off resistor network. Measuring gm adds up to 97.3 mV giving max. 374 mV peaks across the meter. Problems might begin here when Schottky protection diodes are used. Simply increase the shunt amplifier time constant in that case.
Panel meter clipping is not an issue with the meter amplifier described earlier.