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It's interesting to see how the EL34s transition from clean to overdrive compares to 6L6s. The 6L6 transition is rather quick with little compression beforehand. OTOH, the EL34 starts to noticeably compress before it actually starts clipping.

…due to the EL-34's larger cathode surface area (relative to a 6L6), I suspect (no data to back this up though) that they're capable of higher peak plate current and a lower saturation plate to cathode voltage.
There's your tonal difference.

John (Tremo) Stokes