No worries, Gene, happy to share the details. I paid $80 for the oscilloscope and $50 for the Sencore. When he told me $50 I was floored as these are going for $400-$500 online. I asked if he was sure and he said yes, that he was happy his equipment was going to get some use again. He then threw in a B&K frequency Counter, which I told him I didn't need, but he said just take it.
20MHz is well enough and is about the lowest they go these days. You might be able to find an old analog that is even lower but would still be sufficient for an amp.
Key features should be able to read 500 VDC and maybe 1000VAC but you can buy probes with 10x and maybe 20x that will allow you to see high voltages scaled back by 10x or 20x.
If It were me I would also want a FFT function. That will help you determine if you have an oscillation that is out of the hearing range but will definitely reduce life of the tubes or OT.
It would also be nice if it had a spectrum analyzer to determine the odd v even harmonic output, more odd harmonics fatigue the ears fast, you'll have ringing in your ears at lower volumes. But it is the by product of heavy distortion. I don't know if you can get an Oscope with a spectrum analyzer built into it but it would be nice.