Improved Anode Current Measurement
Anode current is measured as the voltage drop across a 200 ohm shunt resistor. The shunt increases the internal resistance of the C.T. 160 anode circuit which can cause considerable error when measuring mutual conductance of valves with low dynamic resistance delta Ua/delta Ia.
The shunt resistance and anode circuit internal resistance can be greatly reduced by replacing the 200 ohm shunt with e.g. 4.7 ohm followed by an op amp with V=200/4.7=42.5. The output voltage swing of this circuit must go up to >24 V in order to measure the 120 mA maximum in rectifier mode correctly.
Circuit description
Fig. 1 shows the circuit diagram. The circuit operates from a 29 V isolated supply and uses a standard LM358 dual op amp. Virtual ground is at 880 mV. At this level the amplifier output has sufficient current sinking capability. Max. output voltage is >26 volts above virtual ground, limiting measurable current to approx. 130 mA. This is sufficient as the overcurrent relay will trip at slightly above 120 mA.
The 68 kOhm / 2.2 uF network smoothes anode current pulses to values within the 26 V output swing. Individual trimpots on the amplifier output enable independent and precise adaption to the A and D measuring circuits.
Together with this circuit the original power supply for the backing off resistor network should also be replaced by a supply feeding well smoothed DC to the network. This greatly reduces dancing of the panel meter needle particularly when measuring mutual conductance.
Installation
Fig. 2 is a section from the C.T. 160 circuit diagram (Fig. 2a: modified original by Martin Forsberg) together with the rather simple and straightforward wiring modifications. Fig. 3 is the realization on a breadboard. The photo also shows the new RV4 A circuit balancing potentiometer. The worn out original was replaced by a robust 100 Ohm military type which was shunted to align it with the mA scale on the panel.
Alignment
A. Basic alignment
Connect a DC voltmeter between amplifier output (pin 1) and – input.
- Set Null adjust trimpot to 0 V on output.
- Apply 120 mA DC between + and – inputs. Set V adjust trimpot to 24 volts on output.
- Increase DC input to 130 mA. Output should read 26 volts.
B. Adaption to A measuring circuit
Set Anode current controls to 0 (fully counterclockwise), Anode volts to 20 and Electrode Selector to A1. Connect a 25 kOhm resistor (variable from 15 to 25 kOhm in series with a DC mA meter and a silicon diode (anode to A1) between A1 and cathode. Alternatively use a suitable valve and proceed as below.
- Turn Circuit Selector to Test and adjust current to 0.5 mA. Now advance mA/V disc to "Set Zero" and adjust A1,A2 trimpot to 1 mA/V mark on AVO meter scale (center of green "good" area).
C. Adaption to D measuring circuit
Set Anode current control to 1 mA (inner dial) and Electrode Selector to D1. Connect a DC mA meter in series with a silicon diode (anode to D1) between D1 and cathode.
- Turn circuit selector to Test. Measured current will be close to 1 mA. Adjust D1,D2 trimpot until AVO meter reads 72*(measured current). Example: measured current = 1.05 mA => AVO meter should read 72*1.05 = 75.6.
Results
The following internal resistances of the anode circuit were measured:
Anode / Internal
Volts / Ohms
400 / 150
300 / 108
250 / 85
200 / 69
100 / 63
Method: for each anode volts setting vary anode current and plot measured values (Ua, Ia). The internal resistance is the linear regression gradient of the plotted curve.
The C.T. 160 manual states 375 Ohm internal resistance for Va = 400 V. This complies quite well with the now measured 150 Ohm of the improved circuit: 375 Ohm – 200 Ohm (deleted shunt resistor) + 4.7 Ohm (new shunt resistor) = 180 Ohm.
A graph of the approximate correction factor for a measured mutual conductance can be drawn for these results. As can be seen from the graph in Fig. 4, mutual conductance values for valves with Ra >3000 Ohm need not be corrected even when measured at 400 V anode voltage.