[bridkarl]

I grabbed the wrong tube and put in a 5U4 tube instead of the 5Z4 - I just had the rectifier in with no other tubes.
After 30 seconds I realised I was wrong and turned off the variac. No fuses blown, no issues, but I did get heater voltage since the pilot light came on.
No harm done, but I didn’t think these substiuted at all.

[GarySP]

I doubt anything cooked in 30 seconds because of the oops! These are very similar tubes, and nothing but transformer rectification was happening since the rest of the tubes were out of the set. Take care, - Gary

[bridkarl]

I saw no evidence of anything going wrong.
The transformer is fused already which is nice.
Interestingly the 5z4 did nothing. So I’ve ordered a NOS replacement. We’ll see how this goes.

[Vlad95]

5U4 and 5Z4 almost identical. I think nothing happened if you left 5U4 instedad 5Z4 long time.
Just heater current 5U4 is 3 Amp from 2 Amp with 5Z4. But it is not critical for not long time ( but maybe even permanently)

[Arran]

The pinout is the same but the 5U4 has a 3 amp filament, verses 2 amps or less on the 5Z4. I think that the pinout of a 5Z4 lines up with a 5Y3, but the latter is directly heated verses an indirectly heated cathode. It's hard on the rectifier tube heater winding ,a d arguably the power transformer primary, but it's not going to detroy anything instantly like a dead short. I'm not sure why they came out with the 5Z4 other then it was one of the original metal tube types, come to think of it I don't remember running across a "G" style 5Z4 at all.
Regards
Arran

[bridkarl]

Well this is a Hallicrafters SX/9 circa 1936.
No real issues so far although I dodged a real bullet with a cap that was hidden under the tuning deck. Managed to get it replaced without having to diassemble anything, but that was a close one

[Arran]

A pre war Hallicrafters, and an early one too, very nice! 1936 is pretty early for metal tubes too, which would explain why they used a 5Z4, I've mostly seen those in Canadian Westinghouse sets from that period, 1936-37, later ones used a 5Y4. Canadian Westinghouse owned their own tube factory so they went all in for metal tubes in many sets, I have another with a single ended 6L6, not G or GL, a metal one.
Regards
Arran

[MrFixr55]

I think that RCA and GE introduced metal tubes in the USA in 1936.  I don't know if they were being made by RCA, GE or both.  GE and Westinghouse (and Wireless Specialty) built RCA sets and tubes until 1930 when RCA bought Victor talking Machine  As a result of a Federal antitrust suit, GE and Westinghouse divested themselves of RCA, GE transferred their Harrison tube factory to RCA, and RCA built all GE and Westinghouse sets as well as their own until about late 1935.

RCA and GE pushed metal tubes in a big way.  My 1936 GE G-63 (designed and built by GE) had a sticker advertising metal tubes, although only the 6A8 and 6K7 tubes were metal.  The 6Q7 was glass and shielded.

All tubes in my 1936 RCA 5T7 are metal, including a 5W4 rectifier and 6F6 output tube.

Attached below is a copy of the 1937 RCA receiving Tube Manual, version RC13.
   

It is interesting to note that the 6L6 is illustrated in this manual as a metal tube.  ST style 6L6 tubes were seen at the same timeframe and both metal and ST profiles are shown in the RCA Receiving Tube Manual, Version RC14 (1940).  The 6L6 GC was introduced later in the early 1960s.  BTW, never substitute a 6L6 or 6L6G for a 6L6 GC, as the 6L6 GC can withstand higher plate and screen voltages than the earlier models.

+1 on other comments. I wouldn't leave a 5U4 in place of a 5Z4 for a long period of time but for a half hour, I wouldn't worry.

RE 5Y4s, I don't know who came up with that tube, but the pinouts are very different than other rectifiers such as the 5U4, 5V4, 5W4, 5Y3, and 5Z4. The pinouts of the 5X4 and 5Y4 make them incompatible with the other rectifiers without rewiring the socket.