Why do I need this resistor

[tiltit]

Hi everyone,

I am currently playing around with nixie tubes and using this transistor configuration to turn on the anodes.
What I do not understand is why do I need R4. The base of U$2 should be pulled low by U$1 regardless of it's presents. 

Thanks

[neildarlow]

R4 applies negative feedback to limit U$1 collector current. R3 will only have U$2 Vbe across it and without R4 the current flow through the two transistors would be excessive and with U$1's relatively high Vce lead to excessive power dissipation.

[tiltit]

Thanks.
So if I understand correctly without R4 I am basically shorting out the emitter of U$2 through it's base? I have tested this circuit many times and it definitely does not work at all without R4.

[neildarlow]

Without R4 in place U$1 saturates when driven and its Vice will be near ground. U$1 collector current will only be limited by its hFE and limiting in the high voltage supply. U$2 will act as an emitters follower and pull the high voltage supply down. This is potentially destructive and R4 is a required component for correct operation of this circuit.