[35Z5]

Tom,
If you need more of a drop use a 5R4. Has a 2a filament but it's the same size as a 5U4G or there is a mil spec version. [Image: https://proxy.duckduckgo.com/iu/?u=https...nt.jpg&f=1]

The 5R4 has 67v drop at 250ma, meaning in real world, it drops approx half voltage of the 5Y3 that's rated 60v drop at 125ma. In said amp, that draws roughly 80ma, the 5R4 & 5V4 gave similar B+. A 5V4 is rated 25v drop at 175 ma.

As far as heat, will be similar in any 5v, 2A filmant that handling same amount of current.

The 84 has a 20v drop at 60 ma. Being a big pin and 6.3v tube vs 5v, are less choices for substitutes. Almost any workable choice will increase voltage.