I have its 'big brother' the 555 alongside me at the moment. That has double _everything_ -- double beam (so double Y-amplifiers and plug-ins), double timebase (also on plug-in modules, but the only time you remove them is for ervicing), double _boxes (a separate PSU that lives on the bottom of the scopemobile) and so on. I also have an assortment of plug-ins. Nothing too exotic, but I do have the 1S1 sampler unit with the special cathode-follower probe (this is probe about 5/8" in diameter containing a tiny tiode valve). And a 3rd party specturm analyser plug-in. And ahe type M 4-trace unit (I'd prefer a 1A4, but it's what I could find). I even have the test plug-in. It contains a mercury-wetted relay running off an AC supply that applies fast-rise pulses to the input of the main Y amplifer for setting up the various trimmers. The 555 even has regulated heater supplies. The heater transformer is fed through a saturable reactor. The current in the control winding of that comes from a pentode valve in the PSU unit, the grid voltage of that is controlled by a bright-emitter diode running off one of the heater lines. So the thing actually gets the RMS value of the heater supplies correct (which is what you want, of course). -tony