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From: rons@tv.tv.tek.com (Ron Selberg)
Subject: Rectifier tubes
Date: 22 Jun 1994 12:52:18 GMT

Reguarding rectifier tubes,
If you will take the time to look through your tube manual, you will notice
that many of the octal 5 series rectifier tubes are quite similar. Many are pin
compatible.
It typically follows that indirect heated rectifiers are low impedance while
the directly heated types are high impedance. Often times a spec is given that
indicates a voltage drop at a specified current.
It does not necessarily follow that the ones with cathodes have a higher
current rating.

Something I've been able to do when comming up with a supply for various amp
projects is to start with a "best guess" choice of rectifiers. Those of us
who roll our own know what it's like trying to get exactly the right pwr
transformer. Anyway, once the amp is running, it is quite easy to adjust the
B+ up or down depending on the type of rectifier used.
I sterted with a 5R4 in my last project but the supply came out about 20v too
low, so I substituted in a 5U4. Bingo! 
For grins I also tried a 5V4 and a 5AR4, with each giving succesively higher
voltage.

I need to mention that Henry is right about the load variation vs tube type
used. My amp was very low power so this was not an issue.

Also...cathode type rectifiers have a nice slow warm up time as opposed to the
direct heated types which come up quite fast. This could be a concern if your
filter caps aren't a high enough voltage.

Ron