The SOHA's input stage is a simple grounded cathode amplifer with a CCS plate load. We designed three different CCSs so that at least one of them could be built by anyone around the globe from available parts. Even so, there were some problems finding components. Since the CCS sets the plate current in the tube and since we wanted 40V on the plate, it was necessary to provide a cathode bias trimpot for adjustment to set the plate voltage. This trimpot is necessary because tubes vary and because they seem to vary more at low voltages.
The goal for the SOAH II input stage is to eliminate both the CCS complexity and the trimpot. I tried a number of designs using the grounded cathode configuration (single triode). They all involved an opamp servo and I didn't like any of them because they were too complicated. One of the other ideas for the SOHA II had used a differential pair for the input stage. I decided to have a look a this again. After some thought and a few sims, the input stage took shape.
Using A Common Cathode Amp
The first thing to do with a pair of triodes with common cathodes is to put a CCS load on the tail. This reduces distortion. It also tends to make the voltage gain of each half equal, but in this case this doesn't matter because the input stage is not really a differential pair. It's actually a common cathode amplifier where the first triode operates as a cathode follower driving the second triode in grounded grid mode. The basic pair looks like this:
Simple Common Cathode Amplifier
Notice that the first triode has no plate load, forcing it to operate like a follower. There are still two important problems to solve with this configuration: 1) How to set the plate voltage of the second triode to 40V with a B+ of 60V and 2) How to provide a very high RL when there is only 20V drop across it. These problems necessitate some kind of active solution.
Adding A Necessary Current Mirror
The first step is to solve the RL problem. A simple CCS for RL leave us with the same problems as the SOHA. No good. To make this work we use another technique, fairly commonly known, of using a current mirror to load the pair of triodes. The first triode gets the control transistor. Like this:
Current Mirror Load on the Triode Pair
The current mirror forces the currents in the triodes to be the same and provides a high RL for the second triode. Unfortunately, it doesn't completely solve the problem because the plate voltage of the second triode falls close to the B+ and, thus, the triode can't swing positive voltage. We still have to get the plate voltage down to 40V. The quietest way to do this is to insert a resistor between the mirror and the first plate. Like this:
Current Mirror and Resistor Load
The resistor value is chosen to drop about 20V (including the BE diode drop and the emitter resistor drop) down from the B+. If the tail current is set to 2mA then the triode current is 1mA and the closest standard resistor value is 18k which will drop about 19V from the B+. This is close enough for tubes.
Now how does this work? One step at a time.
- The tail CCS sets the total current flowing through the triode pair
- The current mirror forces each triode to have the same current, each half the total set by the tail CCS
- The resistor, BE junction of the BJT, and emitter resistor force the plate of the first triode to be, approximately, 19V less than the B+, about 41V
- The tail CCS adjusts the cathode voltage of the first triode so that, given its plate current forced by the CCS and mirror, its plate voltage will be 41V. The is no other option providing that the triode is conducting and not in positive grid mode
- The second triode sees exactly the same cathode bias as the first triode. And because of the mirror, it also has exactly (or almost exactly) the same plate current
- Since the second triode has the same grid bias and plate current as the first triode, it MUST set its plate voltage to be the same as the first triode - 41V
How close the second triode's plate is to 41V depends on how closely the triodes match. Generally, good working triodes in the same envelope are reasonably well matched. But, beyond this, we don't need exactly 41V at the plate. We just need something close to this. A few volts either way won't matter. A 20% mismatch in the triodes tends to be a few volts either way.
Note that because the tail CCS maintains a constant total current and because the mirror equally divides this current between the two triodes, the current through the plate resistor is constant for any type of tube. Thus, the first triode will always sit at about +41VDC no matter which twin triode is rolled in.
Finishing the Tail CCS
Now, how to do the tail CCS? Once again the simplest and most widely available CCS will be the trusty ring-of-two BJTs. The final input stage, then, is this:
Complete SOHA II Input Stage
The trimpot (which we worked so hard to eliminate) is needed only to set the reference triode's plate voltage one time. No adjustment is needed when tubes are changed. A fixed 2mA is achieved with a single 330Ω resistor in place of the fixed resistor/trimpot combination, but having the flexibility of an adjustment to get 2mA is a better option. See Amplifier Schematic section.
With these values and devices the current mirror load on the second triode is about 3MΩ. This resistance will be in parallel with the load provided by the buffer.