On Wed, 05 May 2004 13:40:55 GMT, "Frank Dresser"
wrote:


"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.
Not really. The vacuum tube rectifiers have very high internal
resistance, it is why you can safely use cap input filtering. The tube
itself is a very effective surge limiter that gets better with age as
the thorium on the cathodes is evaporated off.

If you did it with a solid state rectifier, the rectifiers burn up
unless you protect them. Some of the high voltage/high vacuum
rectifiers like the GZ34 had trouble delivering 250ma with 800 volts
on the plate. That's one of the reasons really big, vintage power
supplies user Mecury Vapor rectifiers. They have much lower
resistance, and you haven't seen a rectifier until you have seen a big
3 phase 800 amp mercury pool rectifier.

It's a poor circuit design which was
commonly used back then.
I disagree. Unlike a solid state rectifier, the vaccum tube rectifier
provided surge protection. That is just the way they work.


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.
Where did you learn electrical engineering.

In a Vacuum tube system, the current in the oscillator coil is maybe a
milliamp or two
So unless R is a big number, I^2 is on the order of .000001. I have
never seen the coils in a receiver get even slightly warm from I^2 R
heating. They are heated far more by radiated and convection energy
from the filaments, rectifier, and Audio output tube heat dissipation.
In most receivers, the filament power dwarfs everything else. If you
have a reciver that is rated 40 watts, and has an audio output of 1-2
watts, the power isn't in the B+. In an All America 5 design, 90+% of
the power dissipated is in the filaments. Radiated heat goes up at
T^4, so a reduction in power input of 10% results in a change in
temperature that is tiny (on the order of 1.7%)....


That is often measured in tens of wattts. What is dissipated in the
coils is microwatts to milliwatts. Ambient temperature inside the
cabinet had far more to do with coil temperatures then the current in
the coil.
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.
Not it if was well designed. Designer did two things. They used
regulated voltage on the oscillator, and NPO caps, negative
temperature coefficient, so the temperature of the coils would drive
the inductance one way, the caps went the other way, cancelling the
changes out. Once warmed up and in steady state, these things were
often stable to a few PPM.