"matt weber" wrote in message
...

The send puts the receiver in standby, filaments remain on but that is
about all. I suspect it disconnects the B+, or at least mutes the
audio.

The switch is between the center tap of the high voltage secondary and
ground. Opening the switch disables the B+ circuit. Closing the switch
with the tubes warmed up forces a large surge current through the rectifier
as it charges the filter capacitors. It's a poor circuit design which was
commonly used back then.


The idea is that when you are transmitting on the same antenna
even with a T-R switch, you really don't want the receiver active. It
isn't good for the receiver, or you ears.

Disabling the B+ dosen't protect the radio in any way. It might protect the
speaker, but no more than turning the volume control all the way down. The
antenna coils are the first parts to be damaged by excessive power through
the antenna terminals, and they are just as vunerable with the B+ on or off.
Not that the antenna coils are easy to damage or anything, but I fixed up a
once nice radio which was had a few goofy "ham mods". It was also one of
the few radios with burned up antenna coils.

Using the "send - receive" switch also reduces the radio's frequency
stability. The converter tube and oscillator coils run a little warmer when
they're carrying their normal current. The tube and coils cool a bit in the
send position, and rewarm up in the receive position. The frequency shifts
as the temperature shifts.

Frank Dresser