An AZ4 built with the inverted cup getter, the early construction that collectors of rectifiers pick out from the rest. The code etched into the base carries Philips's own type symbol for the AZ4, so the valve states what it is even with the printing long gone.
Tested very strong on a Tesla BM-215A, reading well above minimum with both rectifier sections perfectly balanced against one another.
AZ4 = WE56
Similar, different base: AZ12. Similar, different base and heater: 1861.
The EZ81, GZ32 and GZ34 succeeded the AZ4 in the catalogue but none of them is a drop-in substitute: base, heater and rectifier arrangement all differ, so a circuit and transformer check comes first. The filament here is directly heated at 4 volts drawing 2.3 amps, which needs its own winding.
Philips brought the AZ4 out in May 1938 as the heavy full-wave rectifier for its side contact range, and it stayed in service long enough to turn up in receivers dated well into the 1960s. The filament is directly heated, four volts at 2.3 amps, which is a serious slice of any mains transformer and the reason sets built around it give the rectifier its own secondary winding. The eight contacts wrap around the skirt of the base rather than projecting from the bottom, a Continental arrangement that predates the octal entirely.
Two things about this particular valve. The getter is the inverted cup, a form found on early production and one that collectors of rectifiers look for specifically. And the code etched into the base is worth understanding: Philips marked most of its valves with a group of symbols encoding the type, the place of manufacture and the date, and the published code list assigns F+ as the type identifier for the AZ4. That is precisely why the code exists. As the original Philips documentation puts it, the group still provides useful type identification once the commercial markings have become illegible, which is exactly the situation here.
Inverted cup getter, both halves in step, and the code on the base to prove what it is.
SKU: 225