[FERNSEH]

Hi John,
The CRT is the original Emiscope 3/4. The replacement is the TA10 as is fitted in my electrically similar Marconi VT50.
As for the 1804, there has indeed been some strange modifications done to it.
The original Z66 pentodes replaced by EF50s. If that wasn't bad enough the X81 triode-hexode frequency changer valve has been replaced by another EF50 which functions as the mixer and an additional valve an EC52 triode does the job as the local oscillator. The D43 vision detector diode replaced by an EB34, why bother doing that?
All these adventurous modifications must have worked OK so why not just leave it as it is rather than rebuild the set back to it's original specification?
The sound IF amplifier should be a KTW61, replaced by a Mazda SP61, now that's going too far, a Mazda valve in an EMI set!
So what was the reason for doing all these modifications? Perhaps to make the set perform better which might make sense because the set was found in a farm house in Cambridgeshire, way outside the range of the Alexandra Palace transmitter.
To sum up, the Z66 pentodes were replaced because the person who did the mods believed the EF50 was a better valve. The pentode frequency changer valve possibly had a lower noise factor than the X81 hexode.
The KTW61 sound IF amplifier replaced by the SP61 to improve amplification of that part of the set.
Even the single diode interlace filter circuit was replaced by a Pye type of circuit using an EB91 double-diode.

Why not just plug the set in and see what happens?

DFWB.