AZ33

AZ33 is octal verion of AZ3, but AZ3 has indirect heating. Is really AZ33 indirect heated or there is a mistake in datasheet?

The different 1st digit and same 2nd digit isn't always the same internal design

In 1951 Mullard claims AZ33 is obsolete and IW4/350 is replacement but different base.

They also claim IW4/350 replaces AZ3

The IW4/350 is shown as indirect but cathode and filament connected!

However the recommended full wave in 1951 are AZ31 (direct), GZ34 (indirect), FW4/500 (direct), PZ30 (indirect)  in this class of full wave rectifier..

The 1934 doesn't list AZ3 or AZ33. But IW4 (Indirect) and DW4 (direct) series.

1946 also lists IW4 & DW4. Adds AZ1 and AZ31 as direct heated FW.

The IW4/350 is listed as replacement for AZ3 and AZ33 with note "no circuit change"

Implies that it's either Direct or Cathode is internally connected to Filamant/Heater.

I find a Mullard Red Valves advert for AZ3 but no data on it or AZ33.

However schematics using AZ3 I looked at connect cathode to filament.

So though AZ33 is listed as Mullard, no data in 1934, 1946 or 1951 books and where mentioned the IW4/350 is given as replacement. Perhaps the Heater / Cathode insulation is very poor on AZ3 and this is why they claim a part with cathode internally connected to heater requires no circuit change.

Frank Pocnet net doesn't list AZ33 at all except on equivalents spreadsheet, where it's listed as Philips with no equivalent.

Nor can I find anything that uses an AZ33. I think in practice it's not significant if Indirect or not (I suspect it is like Octal version of 1W4/350 so if indirect the cathode is connected to heater internally, or needs to be externally.