In article , k3hvg writes:

I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?

I've always used the rule-of-thumb that if you can't keep your hand on a power
transformer for more than a few seconds, it's running too hot. But in the case
of old gear, the transformer may be near the end of its useful life, rather
than overloaded. It's also possible that there is some sort of overload
elsewhere in the receiver, causing excessive B+ current draw that will destroy
components. For example, a cathode bypass capacitor in the output stage that is
shorting out will reduce the bias on the stage, causing excessive plate
current. A leaky coupling capacitor will do the same thing by putting positive
bias on the tube grid(s) (and it doesn't take a lot of leakge!). Filter caps
that are leaking will do the same thing. National tended to use power supplies
that were just adequate for the receiver, so any overload can cause excessive
heat.

Try this: Pull out the 5U4 and run the receiver without it for an hour or so.
The transformer load will then be just the filaments. See how hot the
transformer gets under those conditions. Also measure the B+ voltage and
current under normal operation and see what's going on.

73 de Jim, N2EY