In article ,
] writes:

why were the transmitters of "twin"
receiver/transmitter combos, such as the R-4/T-4 pair, designed with
separate supplies?

First off, some receiver/transmitter pairs put the power supply in side the
transmitter as well as the receiver. This was more common before about 1960
than after. In fact, before WW2 it was common for receivers to have separate
supplies!

Several reasons:

Size/weight/minaturization: The power supply for a 100-200 watt transmitter
like the T-4 is much larger and heavier than the power supply for the matching
R-4. In order to keep the tx and rx to the same size and about the same weight,
the transmitter power supply was made external. As rigs got smaller, this
became more and more of an issue because most power supply parts could not be
made much smaller.

Heat: The heat produced in tube rigs is always a concern, and transmitters
produce lots from the final tubes to begin with. Making the power supply
external eliminated having to deal with the heat from it.

Magnetic fields: The power transformer and filter choke(s) have magnetic fileds
around them which can induce hum in various components and wiring. The fields
are much stronger in a transmitter like the T-4X because the power level is
much higher than in a receiver.

Cost/manufacturing considerations: IIRC, the same power supply that was used
with the TR-4 was used with the T-4. This saved the Drake folks the cost of
designing a power supply into the T-4. It also reduces the *apparent* cost of
the T-4X. (Same sort of marketing gimmick that has an item priced at $599.99
instead of $600 - they think we're so math-impaired that we'll be more inclined
to buy the "less expensive" item.)

Homebrewing/existing supply/upgrading: Some hams might want to save money by
building their own power supplies (they aren't complicated). Others might have
a TR-4/AC-4 setup, and then decide to get the R-4/T-4 compbo for the shack and
use the TR-4 mobile. Such a move would require buying a DC supply for the car
and just the R-4/T-4X for the shack, since the AC supply was already on hand.
Brand loyalty is/was a big thing with manufacturers, so being able to offer
inducements like that was a selling point. (And the TR-4 could still be used in
the shack).

73 de Jim, N2EY