a real challange?

[Electronic M]

I'm not surprised it passes color video passband of monochrome sets was originally 4.5MHz in late 53 they mixed color into monochrome video 3.58MHz and up. And 55 and later models started being designed to limit response over 3.5MHz. Some pre-54 sets actually had color wheel NTSC adapters connected to them with no RF/IF alignment change.

The soild state 5V4 damper may fail in your application... that part was designed for 60Hz guitar amp power supplies its diodes may not switch fast enough for horizontal rate and damper service is at the edge of voltage ratings of the tube...the solid state part may or may not be rated to handle a voltage spike that would harmlessly arc the tube.... I'd stick with a tube unless you are fine blowing the SS part to learn if it works.... Alternatively you could take the base off a dud octal tube, and spec out a diode that will handle what the circuit can supply and make your own.
The current tube in it is likely just fine. Blue glow is usually fine (stray electron bombardment of the glass), but purple/Violet is usually indicative of gas leakage.