on a mark 1 spider valve, there was no tube scream effect, but there was a boost, which you turned simply on or off. there was no more settings to it than this. to turn on you held the tap button in and turned the amp drive knob past 12 0'clock for on and before 12 o'clock for off.
on the mark II this isn't the case. you need to go into the edit menu, but you should then be able to assign the tube screamer effect. check out the advanced manual here: http://line6.com/data/6/0a060b316ac34f05933ca8847/application/pdf/Spider%20Valve%20MkII%20Advanced%20Guide%20-%20English%20(%20Rev%20A%20).pdf
If your amp is a Spide Valve MKII then you need to update to FX Infusion 2.0:
Then you can select the "Screamer"effect by turning FX knob 1 and using the presets button to scroll to the screamer effect.
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