Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive? What does this
do?

Thanks in advance!

"Cowboy67" wrote:
My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive? What does this
do?

The S-20R is a receiver. If it were used in conjuction with a transmitter,
the toggle switch would be switched to "send" when transmitting to mute the
receiver.

If you were using separate antennas for transmitting and receiving, the
routine would be :

- Switch the receiver to "send"
- Activate the transmitter
- At the end of transmission, switch the transmitter to "standby"
- Switch the receiver to "receive"

Some receivers have an input on the rear panel to mute the receiver. An
extra switch on the antenna changeover relay is wired to that input to mute
the receiver during transmissions.

Art, N2AH

"Cowboy67" wrote in message ...
Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive? What does this
do?

Thanks in advance!

The S-20R receiver was often used by hams along with a separate
transmitter.
The toggle switch was used to disable the receiver when transmitting
so that
it would not overload, feedback, or burn out something. I do not have
a schematic handy, but I think the switch turns off the B+. When using
the S-20 as a receiver only, just leave thge switch on receive and
forget it.

"Cowboy67" wrote in message
...
Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am
trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive?


The switch is would be used if the radio was used as a ham receiver. The
operator would switch the radio to "send" to silence the receiver. It's a
confusing label, and the later radios used "standby" rather than "send".


What does this
do?

By modern standards, the S-20R is a poor ham radio, so the switch is almost
useless. Actually, it's worse than useless, because flipping the switch
puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in
my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped
the switch back to "receive". I guess that was OK back in the days when
rectifier tubes could be bought with pocket change. Now you should just
leave it on receive all the time.

By the way, the S-20R is still a good radio for shortwave listening. But
the original paper capacitors and probably the electrolytic capacitors are
bad. The paper capacitors are little more than a roll of foil and paper,
and the paper holds up about as well as any other cheap paper from around
1940 would. When these capacitors fail completely, they can also ruin tubes
and hard to find transformers. Modern plastic film capacitors are cheap and
last indefinately.


Thanks in advance!



You're welcome!

Frank Dresser

On Mon, 3 May 2004 23:59:10 -0500, "Cowboy67"
wrote:

Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive? What does this
do?

Thanks in advance!

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 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.

Frank Dresser wrote:

"Cowboy67" wrote:

Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am
trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive?

The switch is would be used if the radio was used as a ham receiver. The
operator would switch the radio to "send" to silence the receiver. It's a
confusing label, and the later radios used "standby" rather than "send".

What does this
do?

By modern standards, the S-20R is a poor ham radio, so the switch is almost
useless. Actually, it's worse than useless, because flipping the switch
puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in
my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped
the switch back to "receive". I guess that was OK back in the days when
rectifier tubes could be bought with pocket change. Now you should just
leave it on receive all the time.

By the way, the S-20R is still a good radio for shortwave listening. But
the original paper capacitors and probably the electrolytic capacitors are
bad. The paper capacitors are little more than a roll of foil and paper,
and the paper holds up about as well as any other cheap paper from around
1940 would. When these capacitors fail completely, they can also ruin tubes
and hard to find transformers. Modern plastic film capacitors are cheap and
last indefinately.


Thanks in advance!



You're welcome!

I have a soft spot for the S-20R because it was my first 'real'
shortwave receiver more than 30-years ago. I bought it for $15 when I
was in college. I replaced all the paper caps' and did a complete
alignment. Over the years the radio became a test bed for trying
different kinds of circuits. I learned a lot about shortwave receiver
design from that radio. I also built a digital frequency display for it.
The parts cost about $100 in the mid' 70's! Today you can buy a
shortwave portable with a display and much better performance for that
amount. I wouldn't recommend using the S-20R as your primary receiver
but you can still have fun with it for casual listening.


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starman ) writes:
Frank Dresser wrote:

"Cowboy67" wrote:

Howdy all,

My brother-in-law just gave me a Hallicrafter S-20R receiver. I am
trying
to gather some info regarding it. My question to you is...If this is a
receiver, then why the toggle switch for send and receive?

The switch is would be used if the radio was used as a ham receiver. The
operator would switch the radio to "send" to silence the receiver. It's a
confusing label, and the later radios used "standby" rather than "send".

What does this
do?

By modern standards, the S-20R is a poor ham radio, so the switch is almost
useless. Actually, it's worse than useless, because flipping the switch
puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in
my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped
the switch back to "receive". I guess that was OK back in the days when
rectifier tubes could be bought with pocket change. Now you should just
leave it on receive all the time.

By the way, the S-20R is still a good radio for shortwave listening. But
the original paper capacitors and probably the electrolytic capacitors are
bad. The paper capacitors are little more than a roll of foil and paper,
and the paper holds up about as well as any other cheap paper from around
1940 would. When these capacitors fail completely, they can also ruin tubes
and hard to find transformers. Modern plastic film capacitors are cheap and
last indefinately.


Thanks in advance!



You're welcome!

I have a soft spot for the S-20R because it was my first 'real'
shortwave receiver more than 30-years ago. I bought it for $15 when I
was in college. I replaced all the paper caps' and did a complete
alignment. Over the years the radio became a test bed for trying
different kinds of circuits. I learned a lot about shortwave receiver
design from that radio. I also built a digital frequency display for it.
The parts cost about $100 in the mid' 70's! Today you can buy a
shortwave portable with a display and much better performance for that
amount. I wouldn't recommend using the S-20R as your primary receiver
but you can still have fun with it for casual listening.

That's interesting.

I went to the Rochester hamfest in 1973 or 74, and bought the ICs
(and LED readouts) for about thirty dollars. They fit in a nice
small box, a tad smaller than my fist, which was useful so I could
dismiss it's value at the border.

Michael

"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

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.

"matt weber" wrote in message
...
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.

Yes there really is a large surge current when hot switching vacuum tube
rectifiers! In fact, they went to the trouble to develop a spec for the
maximum hot-switching transient plate current. The RCA tube manual says
it's 2.5 amps for the 80/5Y3. That's with the tube manual's recommended
input filter cap value of 20uFd. This radio uses an input cap something
like 40 uFd.

I'd rather play it safe, and not hot switch the rectifier.

The tube
itself is a very effective surge limiter that gets better with age as
the thorium on the cathodes is evaporated off.

The hot switching problem gets worse as the tube ages. As the emissions go
down, the voltage drop goes up. Hot switching old, high voltage drop tubes
can cause internal arcing.

Hot switching vacuum tube rectifiers needlessly reduces their useful life.

By the way, there's no thorium in the 80/5Y3. These tubes use oxide coated
filiments. I'm not aware of any thoriated tungsten filament tubes used in
normal consumer electronics since the days of the '01A.


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 what happens when tubes are operated out of their ratings. The RCA
tube manual says the maximum plate voltate allowed for the GZ34/5AR4 is 475V
rms, with a capacitor input filter. That's a peak voltage of 672V. The
recommended rms voltage with a choke input filter is 600V, but the output
voltage from the choke input filter will be lower than the rms voltage
input.

The recommended maximum hot-switching transient plate current for the
GZ34/5AR4 is 3.7 amps.

I'm sure some people exceed these ratings, but at the cost of life of the
tube.


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.

I'm sure such rectifiers also had a much higher hot-switching spec than
either the 80 or the GZ34. Although it doesn't matter much. Mercury vapor
rectifiers were almost always used with a choke input filter to avoid
letting the thing turn into a relaxation oscillator. Mercury vapor
rectifiers aren't often found in consumer equipment, either.


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.

Exceeding the maximum hot-switching transient plate current will reduce the
life of the rectifier tube. I can't think of any good reason to hot switch
the rectifier in a radio, anyway.



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.

Is that supposed to be a question? I read books, including tube manuals. I
work on radios. I also know how to write!


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.


The triode section of a 6K8 will normally draw about 3 or 4 ma. All of that
current flows through the oscillator coil. The mixer section draws 2 or 3
mils through the plate and maybe 6 mils through the screen. So, in a normal
Hartley hookup, at least 3ma flows through the top section of the oscillator
coil, and and at least 11 ma flows through the bottom section of the coil.
I know that's still not much DC current, but is much higher than your
estimate. But wait! There's more!! The oscillator has a parallel resonant
circuit, and as all good EEs know, a parallel resonant circuit has a very
large circulating current, which is limited by the Q of the circuit. Higher
Q means more current. The oscillator's feedback ratio will also have a
large effect on the current of the coil. More feedback means more coil
current. All that extra current warms the oscillator coil a bit. Not
finger burnin' hot, just a bit. I don't know how many orders of magnitude
the AC current is higher than the DC current, nor do I know how many degrees
the temperature of the oscillator coil changes as it warms up. I don't
care. The effect is that the frequency drifts. The radio has better
frequency stability if the B+ isn't interrupted.

I have
never seen the coils in a receiver get even slightly warm from I^2 R
heating.

It doesn't take a huge temperature rise of the oscillator coil to cause a
few hundred parts per million drift at 10MHz. I'll get about 100 Hz just
from the air conditioner cycling on and off in the same room. Very
annoyiing on SSB. But these aren't very good radios for that sort of work.

No, it's not the voltage shift, either. My SX-62 drifts with temperature,
and that one has a voltage regulated oscillator.


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.


In most receivers, the tubes are above the chassis and the coils are below
the chassis.

If you're saying that there's more to the observed frequency drift as the B+
is switched on and off, you have a point. But most radios, including the
S-20R, have the coils under the chassis and the tubes above the chassis.
However, there's some under chassis power resistors which will also
contribute to oscillator coil heating and frequency drift.


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.


Well, let's run the numbers. The typical AA5 uses 150ma tubes in a series
string rated at 120V. That's 18Watts. If the input power is 40 watts, the
total percentage consumed by the filament string is 45%, not 90+%. But AA5s
use a higher percentage of their power in the heaters because the audio
output tube has a high power heater to optimize it for lower plate voltge
use. For example, the 50L6 uses a 7.5 watt heater, while the 6F6 uses a 4.4
watt heater. Hallicrafters substituted a 6K6 for the 6F6 in their later mid
level radios. The 6K6 had an even more economical heater, at 2.5 watts.

Anyway, these tubes are above the chassis, the coils are below.


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%)....

Radiated from the above chassis tubes to the below chassis oscillator coil?
Ignoring the actual heating effects in the oscillator coil itself, I have to
figure the under chassis B+ dropping resistors have a far larger effect on
the temperature of the oscillator coil than the above chassis tubes do.
Anyway, the frequency drift after switching the B+ starts right away, and
that implies the source of the drift is right in the oscillator circuit.


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.

But the under chassis temperature will rise relatively slowly after the B+
is switched. The frequency drift after switching starts immediately.


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,

Oh. How many S-20R radios have been designed with regulated voltage for 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.

Temperature compensation might work over a small range of frequencies. As
the tuning capacitor is closed, the reletive effect of the temperature
compensation will be reduced. These radios use a bimetal sort of temp
compensating capacitor a couple of inches away from the oscillator coil.
It's nowhere near the above chassis tuning cap, which is subject to the heat
from the tubes. The temperature compensation doesn't work very well. The
compensation capacitor is also microphonic.

As long as we're on radio design, a good frequency stability technique would
be use of low expansion coefficent coil forms such as some of the ceramics.
The forms on this radio are bakelite. Bakelite isn't as good as low
expansion ceramic, but it better than cardboard.

This radio wasn't designed for a high level of frequency stability. It was
designed to be a good value for the money. From that point it was a
success.


Once warmed up and in steady state, these things were
often stable to a few PPM.


The S-20R is stable to a few PPM? That is the most incredible statement
yet!!!

Frank Dresser