Hybrid Amplifiers

It seems to me that the solid-state amplifier would still be doing too much, that the same 25mV-range-of-linearity sound that the transistor-based differential input stage exhibits would still be in charge of the input signal. How much can we expect the triode to do, considering that it does not amplify the signal in any way? And what happens at start up, when the tube is cold and not conducting? Or when the tube is wiggled in its socket? Does the solid-state amplifier slam to one rail, while the woofer smolders until the triode conducts or reconnects?

But before dismissing this design altogether, let have some fun. Couldn't a vacuum tube signal diode be used in place of the triode, as it too has an rp?

Or, what if we use a triode as the bottom resistor of the two-resistor voltage divider that sets the negative feedback ratio of an amplifier? Both resistors influence the sound out of the amplifier. The schematic above right makes the point. The triode’s cathode resistor is being shunted by the triode’s rp/mu (the inverse of its gm) and the amplifier is safe in terms of DC offset, even at start up. In fact, the triode could be pulled from its socket, while the amplifier was in use, with the only result that there would be a momentary popping noise and the gain would fall off a bit.

(By the why, when you see any bipolar power supply, imagine that it implies an additional power supply rail voltage three times the single rail voltage, as a voltage tripler can easily be added to the bipolar power supply. In the circuit below, we see a conventional bipolar power supply with four extra diodes and three more caps. This voltage tripler is a full-wave type that won't need too much filtering.)

Still, what bothers me is the idea that the throwing the ball to the solid-state amplifier first. Is this a good idea? If the solid-state amplifier fumbles, we have no hope of winning. So, how do we reorder the amplifier so that the tube gets first crack at the input signal? In my posting on simple amplifiers ( http://www.tubecad.com/page12.html) I displayed a topology that might prove to be a real sleeper, shown below, to the left.

The idea here is that the tube and solid-state amplifier form a simple two-stage amplifier, with the tube in charge of the input signal and the global negative feedback. Yet, the loudspeaker is never at risk, as the solid-state amplifier’s feedback is never left open. Both the tube and the solid-state provide voltage gain, so a low-mu triode, such as the 5687 could be used. If more second harmonic coloration is needed, then the schematic above right might be the way to go. Once again the triode is charge of both the input and the feedback; but as the triode undergoes greater current swings than the triode in the circuit to the left, it will produce more distortion.

The circuit at the right uses the triode’s impedance at its cathode added to its cathode resistor to define the input resistor of an inverting amplifier. The solid-state amplifier inverts the signal and is in charge of the negative feedback loop this time. Either a resistor or a constant-current source can be used to provide a current path for the triode. Ideally, a servo-loop could be implemented that would allow removing the 10µF coupling capacitor. (A FET and IC and few resistors and capacitors is all that would be needed.)

If using a complete solid-state amplifier seems daunting, or if you prefer single ended amplifiers over push pull amplifiers, then any of the following three amplifiers would prove interesting.

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