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I wouldn't recommend substituting 5U4 in a circuit designed for 5AR4 as the filament current draw of 5U4 is 3 amps vs. 1.9 amps for 5AR4...this may overheat the power transformer.
I believe that 5AR4, even though it's smaller, has the same or higher plate current ratings as 5U4 so it should be able to replace it...however the tube resistance is less as Blue lateral says so the output voltage will be higher.
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