Swapping tube rectifiers

[Sonny ReVerb]

Trying to understand tube rectifiers a bit better.

My friend has an original 5G9 Tremolux. It's been modified for "adjustable" fixed bias. It currently has a 5Y3 rectifier in it, which he likes. He put a 5U4 (which the 5G9 was originally designed to use) in it briefly and said the amp was louder, but the trem was minimal. (I would think re-biasing would bring back the trem?)

For a non-adjustable fixed bias amp:
   Swapping to a higher voltage drop rectifier (say 5U4 -> 5Y3) results in lower plate voltage and leaves the bias voltage relatively unchanged? This would cause colder bias?
   Conversely, swapping to a lower voltage drop rectifier (5Y3 -> 5U4) results in higher plate voltage and leaves the bias voltage relatively unchanged, causing hotter bias? Probably much less safe if the amp isn't designed for it (cap ratings, etc.)?

I also wonder about tube rectifier recommendations for the tweed Tremolux here: "Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use 5AR4, GZ34, 5V4 or 5Y3."

Any idea why they would say a 5R4 was acceptable, but not a 5Y3 (similar voltage drop)? Actually, the 5E9-A (cathode bias) schematic shows a 5Y3.

I know there are other considerations such as 5V filament current (2 vs 3 amp) and the size of the first filter cap in a pi filter (the 40uF in the 5G9 does exceed the 5Y3 recommended max). What else am I missing?

[shooter]

What else am I missing?

experimentation ... with notes   

[Sonny ReVerb]

Granted, but I hate to experiment on a vintage tweed. I can probably tinker on one of my homebrews...

I found an old post of Steve's with some good rectifier info.