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The comment about thyratron dimmers made me think of the old teletype machines once common in use that used a thyratron PSU, was this an issue?
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Nothing wrong with thyratron control itself, just that the Strand dimmer only used the positive half cycle from each of the three phases. A full bridge using six thyratrons would have been fine although extravagant; a more conventional solution would have been a back-to-back pair running from one phase, as universally adopted with thyristors. Any phase-angle control can distort the mains waveform and increase the neutral current due to the presence of harmonics, but at least it should do so symmetrically.
Where multiple thermionic or vapour devices are used as a rectifier or control bridge, there is an inbuilt tendency to favour common-cathode arrangements. Solid state, however, is ambidextrous - take a pair of diodes with a common cathode, another pair with a common anode, connect their AC terminals together and you have a bridge rectifier. The common-cathode pair is easy to do with valves too, but the common-anode pair is a pain. Unless they are indirectly heated with cathodes isolated for the full peak-peak voltage, each needs a separately isolated heater supply (hence three for a bridge, one at HT+ potential and one attached to each end of the HT secondary.) If they are controlled devices, the same is true for bias supplies, so it all gets a bit complicated.
Arc rectifers are easy to build with as many anodes as you like (6 was common, 12 was available) but only the one cathode. Each anode draws from the transformer secondary a pulsating DC current just like the Strand 'electronic' dimmer, however, the transformer is integral to the rectifier plant and wound specially to accommodate the DC in its secondary. Not having a transformer, the Strand dimmer passed the DC directly into the mains.