The fundamental issue here, for Tim, is that with a LTP arrangment there is no degenerative feedback in the cathodes which means there is no DC feedback to stabalize the operating point of the individual valves. This could mean that the DC balance between the two halves of the pair could drift out of sink much more seriously than with self bias and without some way of knowing this and compensating the core could approach saturation.
Shoog
Shoog
But if you use individual BJT sinks for each cathode, cascodes, for instance, with a shared voltage reference and matched as close as you like current setting resistors, then there is absolutely nothing which can become unbalanced as far as the DC conditions are concerned. AC coupling of the cathodes with a single non polarized cap can produce a similar bass extension as happens in parafeed topology though this will only occur to a significant extent with low mu valves for sensible cap values.
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