Preamp valve going?

[thinman]

I suspect the preamp valve in my H&K is going - after switch-on there's a period when the amp behaves like someone is turning the gain up and down - the volume increases and decreases smoothly before eventually stabilizing.

Has anyone experienced this - could it be something like the valve heaters failing?

I read about valves going microphonic or noisy but not this. I've ordered a replacement anyway.