Here is a simpler version of the non transformer, non isolated cold cathode indicator tube tube PSU.
Rather than dissipating the power in an array of MOSFETs, the power is dissipated in two high power resistors which can operate at a much higher temperature than semiconductors and thus will be adequately cooled by a smaller heat sink.
The two power NBJTs dissipate 23W worst case so they will also need a heat sink which they can share as only one of the NBJTs will be dissipating power at any one time. This is also the case for the power resistors, which are the bolt-down type as shown in the data sheet links.
ERRATA
(1) Increase C1 to 220uF 400V or larger.
(2) Fit 100uF 250V capacitor across the respective tube supply rails for every 25 tubes
(3) C4 should read 100uF 250V not 400V
(4) Fit 1N4148 diode anode to Q6 emitter and cathode to D7 anode (upper). This is for reverses VBE protection on mains power switch off.
(5) Fit 1N4148 diode cathode to Q6 emitter and anode to D3 cathode (lower). This is for reverse VBE protection on mains power switch off.
NOTES
(1) with a 240V mains input, the voltage across C1 will be 340V peak. The function of the high power transistors, and associated components, is to generate a midpoint in the 340V rail to give two 170V supply rails, one for the upper bank of 50 tubes and a second for the lower bank of 50 tubes.
(2) With a 110V mains input, the voltage across C1 will be 153V peak. This will be the supply rail for all the tubes which will be connected in parallel by the mains selector switch.
(3) For 110V mains supply operation the high power transistors and resistors and associated circuit components are not required.
(3) The high power transistors and resistors will only dissipate worst case power when one bank of 50 tubes is conducting no current and the other bank of 50 tubes is conducting full current (600mA total)
(4) The two 220 Ohm high power resistors dissipate 74W worst case and can be mounted on the same heat sink as only one resistor will be dissipating power at any one time. This heat sink will get very hot under certain circuit conditions and needs good ventilation and safety measures to prevent human contact. Also the resistor heat sink should not be mounted in a position to heat other components and especially should not be able to heat any flammable materiel.
(5) The two high power NBJTs dissipate 23W worst case and should be mounted on a separate heat sink to the power resistors. But similarly, the two high power NBJTs can be mounted on the same heat sink.
DATA SHEETS AND SOURCES/COST
(1) MJW18020
https://www.onsemi.com/pub_link/Collateral/MJW18020-D.PDF
https://uk.rs-online.com/web/p/bipo...D4D4A573138303230267374613D4D4A57313830323026
https://www.digipart.com/part/MJW18020
https://www.mouser.co.uk/Search/ProductDetail.aspx?qs=HVbQlW5zcXX%2bSmQG5przbA==
(2) 2N6517
https://www.onsemi.com/pub_link/Collateral/2N6515-D.PDF
https://uk.rs-online.com/web/c/semi...67573743D324E36353137267374613D324E3635313726
(3) 2N6520
https://www.onsemi.com/pub_link/Collateral/2N6515-D.PDF
https://uk.rs-online.com/web/c/semi...67573743D324E36353230267374613D324E3635323026
(4) Power Resistor
**broken link removed**
https://cpc.farnell.com/1/1/4742-te-connectivity-cgs-hsc100220rj-resistor-ww-100w-5-220r.html