The increased flexibility afforded by the accordion amplifier is similar to the increased flexibility afforded by the isobarik (constant pressure) loudspeaker enclosure, wherein two bass drivers are used, one in back of the other, effectively halving the V_as, while maintaining the same radiating surface (F_s and Q). Prior to this loudspeaker design, the only other arrangement was placing two loudspeaker drivers side by side on the front panel, doubling the radiating surface, but also doubling the V_as. 
       
Z_o, g_m, r_p, and mu
       The details should never be forgotten. Isn't God to be found in the details? What is the output impedance of an accordion amplifier? And while we are at it, what is the output impedance of a parallel single-ended amplifier?
       Surprisingly, in all three topologies--the conventional, the parallel, and the accordion--the output impedance remains constant, providing that all use the same output tube and are optimally matched to the load impedance. How can this be? Doesn't using twice the number of output tubes in a parallel single-ended amplifier halve the output impedance? It might, if we retain the same output transformer winding ratio, which would reflect the halved rp of the output stage to the secondary. But this seldom is the chosen path, as retaining the same output transformer means that we do not double the output power. By doubling output tubes, we effectively double the output transformer's primary impedance, which although it serves to lower the distortion, also moves us away from the optimal power transfer into the loudspeaker.   In order to return to optimal use, we must also halve the transformer's primary impedance.

    If we had been loading a single 2A3 with 2500 ohms, the maximum plate voltage swing might be 100 volts. But using two 2A3s in parallel might only increase the maximum plate voltage swing to, let's say, 110 volts, a value insufficient to double the power output. However, using an output transformer with a primary impedance of 1250 ohms will retain the 100-volt plate voltage swing, but at twice the current swing, which then yields the desired power doubling, but retaining the same output impedance. The lower impedance transformer has a concomitantly lower winding ratio. This ratio squared gives us the transformer's impedance ratio. In other words, we have halved both the rp and the impedance ratio by using two output tubes in parallel with an output transformer with half the primary impedance; the result is unity. For example, the 2A3's rp equals 800 ohms, which when divided by the 2500 ohms output transformer's impedance ratio of 312.5, equals 2.5 ohms as the output impedance. Using two 2A3s in parallel with a 1250-ohm output transformer yields an effective rp of 400 ohms, which when divided by this transformer's impedance ratio of 156, equals the same 2.5 ohms output impedance. So what do we gain by paralleling two output tubes? We can either double the power output or halve the output impedance, but not both.     
     Men do not like to give up power; no man wishes he were shorter. So giving up potential watts is painful, but maybe it's the better choice. Maybe a 2A3-based single-ended amplifier (sporting three output tubes and delivering just 3.5 watts, but with a one-ohm output impedance and the ability to drive ugly reactive loads) would sound much better than a 10-watt version.

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