My first 'socpe (which I still have) is a little 15MHz Solartron (double beam, I have the optioanl delayed timebase for it). It, too, uses SO239 sockets for all the inputs. I think some older Teks did too. but it was a common mod to replace them with BNCs (wheter this improved the performance I doubt, but it made it easier to use 'normal' probes). One advantage for DC/audio work is that a 4mm banana plug will push into the middle of the SO239 socket. The Solartron I mentioned has 4mm holes dripped into the heads of the plug-in module fixing bolts to use as a ground connection. Tektronix did that too... One word of warning -- these delay lines are _hell_ to set up properly (I head from a chap who worked at Tek that some people could do it almost instantly, others could never get it right). Don't tweak them unless you really know it's essential, and only then if you have all the recomended test gear (in the case of a Tek 500 series, you need a special Y plug-in module containing a mercury relay to put a fast rise step into the Y amplifier). Now's the time to learn. It's actually not that hard to repair, much harder to design, such things. I've never had any real problems keeping my valved instruments going. -tony