[Boatanchors] 5U4 vs 5Y3

[L L bahr]

I can't imagine not having a spare 5U4 if you are dealing with boat anchors.  Also, not knowing what application you are talking about limits the amount of useful advice one might acquire.  However, if I were substituting a 5U4 and only had a 5Y3, I would opt for using a couple 1N4007 diodes.  Yes, the voltage will be higher but you have no worry of filament current or tube current or voltage rating.  At 5 cents each or less, that is what I would use with no spare 5U4 and not knowing other parameters or limitations.
Lee, w0vt