[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