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Returning to the accordion amplifier, the primary is in series with both output tubes and has no other connection to the power supply other than through the output tubes. Thus, if the chain (the electrical circuit) is broken at any point, the current ceases to flow. For example, imagine removing one output tube while the amplifier is in use. Furthermore, since there is but one current path, all three elements (both output tubes and the output transformer primary) must see an equal current flow. This makes sense, but why can't this net current vary with opposing signals applied to the output tubes?
If the bottom tube in the accordion amplifier sees a +20 volt pulse at it grid and the top tube sees a -20 volt pulse at its grid, the bottom tube will pull down as it tries to conduct more and the top tube will let go as it tries to conduct less. If we look at only the voltage swing that the bottom triode's plate undergoes, we might be fooled into believing that the amplifier is still delivering power into the load; it isn't. The transformer's primary still sees the same voltage across it as it did at idle.
What is happing is that as the bottom tube's cathode-to-plate voltage drops, its cathode-to-grid voltage is increasing. And the inverse situation is taking place at the top triode; as its cathode-to-plate voltage increases, its cathode-to-grid is decreasing by a ratio equal to the triode's mu, a constant current flow. It's almost as if the top triode had been replaced by an active constant current source. Why doesn't the same principle apply to the conventional single-ended amplifier? It does, but to a much smaller degree. The plate's movement subtracts from the triode's gm, which means that the triode undergoes a smaller current swing than it would if the plate had attached to the B+.
Notice the inverse relationship between the parallel and the accordion: the parallel amplifier sees its output tubes undergoing large anti-phase current swings (yet constant cathode-to-plate voltages), but the accordion amplifier sees large anti-phase cathode-to-plate voltages swings (yet at a constant current draw).
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If we return the accordion amplifier's inputs to their correct phasing, we see how the amplifier can only work when its single current path undergoes a variation in current draw, which can only happen when both output tubes conduct in unison. Furthermore, because the accordion amplifier works in a strictly single-ended mode, this amplifier will not cancel even-order harmonics or common noise signals. Nor can this amplifier be used in any other class than true, pure Class-A. Nor will it allow the direct use of a non-airgapped output transformer, as the primary still sees an unidirectional current conduction. So if the accordion amplifier still functions entirely as a single-ended amplifier, why bother? The answer is the same as the one for why we bother with the parallel single-ended amplifier: flexibility.
Flexibility
Let's say you need twice the miserly 8 watts that a single 300B yields: paralleling two 300Bs will give you that twofold increase. Or let's say you have inherited a single-ended output transformer with a primary impedance of 800 ohms: using three 2A3s in parallel will match this impedance nicely. Both of these examples illustrate how we can achieve either the wattage or the impedance match we desire by paralleling output tubes. But let us now imagine different scenarios. Let's say you have a regulated power supply that puts out 560 VDC and you have your heart set on using 2A3s. What can you do? Paralleling will not limit the plate voltage, as each 2A3 will still see the full 500 volts. But placing the two output tubes in series will halve the cathode-to-plate voltage that each tube will see, as the two tubes in series define a voltage divider. Or let's say you that you inherited a single-ended output transformer with a primary impedance of 5000 ohms, far too high for a single 2A3. But with the two tubes arranged in the accordion topology, the transformer's primary impedance is effectively halved, or rather, the 2A3's rp is effectively doubled, making an excellent match.
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