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I've now had a chance to look at the Vox Cooltron pedals.... there is no trick to it.It is a grounded cathode 12AU7 with the triodes paralleled and an opamp servo providing a voltage to the grids for bias.
(The ideal thing would be to design a smaller more efficient heater-cathode system, but nobody is designing new tubes today.)
That'a what I call "pending"