• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

TRIO 15Hz Oscilloscope CS-1560A

Should be just fine for tube audio, 15MHz bandwidth is more than enough for very low speed digital as well, although in this instance one channel is a handicap, but you can build a simple multiplexer if you need more channels.. (for PIC micro-controllers, etc.)
Just buy a decent probe for it on eBay and you'll be all set.

More advanced scopes offer multiple channels, more bandwidth and delayed sweep - none of which you probably need. I have a very good Tektronix analog scope at home with a microprocessor controlled interface with lots of bandwidth, delayed sweep and 4 channels. I hardly ever use more than one channel, and I can't even remember the last time I used delayed sweep.