How does this pentode power amplifier (old technology, 1960s) work?

Question

This is a diagram of a pentode power amplifier:

This amplifier is used to control an electro-pneumatic actuator. Let me describe it a little bit:

• the input voltage (the control signal) is V3 (-30V < V3 < +30 V)
• the output is at two pins of C8 (or two anodes of P3 and P4);
• There are two stages: stage 1 includes P1 and P2, stage 2 includes P3 and P4. When the input V3 changes, the voltage difference between two anodes of P1 and P2 changes correspondingly. The signal is sent to stage 2 through voltage dividers R6-R7 and R8-R9.
• Z1 and Z2 represent two coils inside the electro-pneumatic actuator. When the actuator moves, it constantly feeds a singal back, represented by V4.
• V1 and V2 are constant voltage sources, V1 = -150V ; V2 = +150V ;

The whole purpose of this amplifier is to keep V4 = V3. In the other word, for example, if we set V3 = 10V, then the actuator will move so that its feedback V4 = 10V.

The pentodes used in the diagram are 6Ж5Б-B, which has following properties:

I would like to know how this circuit works. I put this on LTSpice but it doesn't work because LTSpice does not have pentode models, just icons. I put some pentode models that I downloaded from Internet but it still doesn't work, one of the reasons is the models are not 6Ж5Б-B.

So I decide to understand this by hand-calculation using the characteristic curves of 6Ж5Б-B. For example, what are the current in the anodes of P1 and P2 when V3 = 10V? Even more, what are the purpose of C3 ; R12 and R13 ; R29 and C9 ; C8...?

Answer 1

You can probably substitute just about any pentode model (with anode current rating at least 32mA) and have it work reasonably well. Valve circuits are usually quite tolerant of parameter variations; by design, since characteristics drift as the valves age.

So go ahead and try the 6K5B. (And something else, maybe EL84 for luck, and compare the results) (EDIT : my Brimar book lists the 6K5G as a triode, with 1mA anode current, but the 6K6G is a power pentode with 32mA anode current so a pretty good match. Other possibilities : 6V6, 38mA max anode current, 6L6 even more)

Some basic analysis : the first stage is a long tailed pair, acting both as a gain stage and a phase splitter. Note the high values of R1,R3,R4 : anode current must be in the range 0 to 200 uA, suggesting Vg-k for each will be around -4.5V.

With Vin +ve, P1 will be turned ON, and P2 (with its grounded grid) will be OFF : following their anode voltages to the output stage, P3 will be OFF and P4 ON, pulling Z1 low and providing NFB via R12,R25. So far, so good... Then the actuator will presumably drive V4 until P1 grid matches P2 grid (0V). Just like an opamp.

So... in balance, R4 and the cathodes must be around +4.5V to +5V (-Vg), giving us the cathode current (155V/0.82Meg = 190uA), split between P1,P2 cathodes, 95 uA each.

Anode load is 1Meg to 150V (R1) in parallel with 3Meg (R6,R7) to -150V, or by Thevenin, 0.75Meg to 75V. But the cathode current is split between anode and screen grid (and the screen grid voltage is set by R2,R5 to about 25V max, further reduced by screen grid currents). So if we assume that half of the cathode current becomes anode current; say 50uA, the anode voltage would be 37.5V below the 75V Thevenin source, i.e. 37.5V. Pretty low.

Then we can establish Vg of P3,P4 at -37.5V, and their cathode voltage somewhere around -35V, giving 115V across R10,R24 (4.36K) or around 25mA shared between P3,P4 to drive the actuator.

Once the basic amplifier is understood and working, the negative feedback by R12,R13,R25 appears to control the P term (if you view it as a PID controller), R29,C9 provide some sort of I term, and C3 across R8 may provide a D term. These should allow tuning the controller for the (unknown) characteristics of the actuator.