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A More Detailed Answer

Posted by FlaCharlie on April 3, 2021 at 06:18:53:
When the builder said it was OK to use either the 5Y3 or the 5U4 I'm sure they were thinking in terms of how much B+ voltage the circuit would see as a result. Both of them drop quite a bit of voltage and the resultant B+ is in the range the builder suggests.
The exact voltage drop will vary depending on the circuit but the data I've seen indicates a 60v drop for the 5Y3 and 44v for the 5U4. So the 5Y3 will result in slightly lower B+, but the builder said either one is fine.
Of the two, the 5Y3 will stress the amp less because its heater only draws 2A, while the 5U4 draws 3A.
The other factor is fast vs slow startup. Both the 5Y3 and the 5U4 are directly heated which means fast startup. If you want to minimize stress, slow startup is preferable. Some versions of the 5Y3, such as the 6087 and others listed earlier, are indirectly heated versions which will give you a slow startup.
So the least stressful would be one of these indirectly heated versions of the 5Y3.
There are other indirectly heated 5v octal rectifiers, most notably the 5AR4, but they drop considerably less voltage and the resultant B+ would be higher than the builder suggests, so it should not be used.