"Keith G" wrote

Ex poster Pinkerton used to claim it was internal reverb. caused by the
Miller Effect or somesuch:

The "Miller effect" is the limitation of the slew-rate of anode
current caused by the grid-to-anode capacitance of the valve. Whilst it is
significant at RF frequencies, at audio frequencies it's effect is
negligable. No way does it cause "internal reverb".

David.