[Sideband]

It might be a simple Bush....maybe too simple! They've probably cut the circuit to the bone in order to reduce costs so it may have been marginal in the first place. The idea of separating the screen grids is a good one and using separate decoupling on both. You could also try a grid stopper between 47k and 100k in the grid of the EL41. Recently I had a Murphy set that howled like a Banshee above a certain volume.....it was the volume control itself...just something else worth considering.