Fascinating as always from you Glen.
Clever use of a PUT.
Expected BW ?
About 100kHz or so.
The amplifier board with the long-tail pairs is actually my second deflection amplifier design. The original board had a pair of single-ended amplifiers driving only one deflection plate of each pair as per the Philips TA115. That made this a 4-transistor 'scope. My original aim was to replicate the TA115 in solid state (3 transistors). However the CRT I choose, as it transpires, doesn't work well with single-ended deflection plate drive. The deflection plates act to a degree as electron beam accelerators and if the mean voltage between a deflection plate pair does not remain constant with ac signal swing (as it does with differential drive) the net electron beam acceleration potential will be modulated in accordance. Deflection sensitivity is inversely proportional to the acceleration potential, so the single-ended drive results in significant deflection non-linearity - I was getting about 25%. Imagine an AM envelope display of an unmodulated carrier. The vertical deflection sensitivity at the left-most extremity of the CRTs display was ~25% less than at the right most, so the displayed envelope became trapezoidal shaped. I liked the ultra simplicity, but I decided that I couldn't live with such horrible non-linearity, so I re-designed the amplifier board for differential drive and the total transistor count grew by two to six.
I also couldn't make do without the emitter-follower buffer for the sweep generator. The capacitor charging current must be kept below (by a safe margin) the PUTs valley current, otherwise the PUT, once triggered, won't turn off again. The input impedance of the BJT horizontal deflection amplifier could not be (practically) made high enough to make loading of the capacitor charging circuit acceptable. So I'm afraid that, as far as the active device count goes in this application, tubes still reign supreme.
Also, unfortunately, the PUT appears to have gone the way of the tunnel diode. AFAIK the last manufacturer of the 2N6027 was ON Semi and they've now stopped production. I got five examples from a Jaycar store which still had stock.