1. If it is a 20 Mhz scope then you can be sure the tube is NOT a post deflection acceleration type. It will then have a 1 to 2.5 kV acceleration supply. Given the relative modernity of the CRO. I expect the HV to be more than 2Kv.
2. I have several CRO circuits using transistor amplifiers, and these are usually using a CRO tube 150 BTB. This is a rectangular screen.
3. I think in every case, the HV supply is by means of a separate HF supply. I think the last time I saw a 50 Hz HV supply was with a homemade CRO using a 5BP1.
4. You say there are 6 secondaries, and my guess is that one of them is the 6.3 volt supply for the CRT heater. My TEK 564 has 10 secondaries. One is dedicated to the CRT heater and is biased to -3300 volt.
5. All the circuits I have for CRO’s, are based on transistors and they all include a switched mode power supply for generating the HV supply voltages at least.
6. I’ve posted a Philips Pm3200 type, using a D10-16 CRT. It has a 1500 volt HV supply. It uses AC187 transistors to switch the SMPS, so you can see it is an old design. A round CRT face I think.
7. Posted is the power supply for a BWD 820 CRO. BWD was an aussie company that tried to be a TEK. The 820 uses the 130BTB CRT.
8. Posted also a Hitachi V202 power supply. Again, the CRT heater is biased to the HV supply. The CRT is a 140BMB31R
9. Protek P3502 power supply is on 2 sheets. The CRT is either 150BTB31 or 150DEB31. The Protek uses a SMPS for the HV supply and the CRT heater.