Hallicrafters S-40 Performance

[maxhifi]

I recently asked some questions about shortwave radio, and received some
very helpful response.

I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.

It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.

I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!

I've been thinking about it, and so far I've come up with

-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

[Unrevealed Source]

You are experiencing what is inherent in all radios, and is called the
"noise floor". That's the classic test - unhook the antenna and turn the
volume all the way up. That will tell you how noisy the radio itself is, as
opposed to externally-generated noise picked up through the antenna. All
radios have some noise.

Rather than screw around with your radio, the answer is to ut up an antenna
that will bring those weaker stations into the radio at a higher level than
the noise floor. And, you need to shield the input of that antenna from
externally-generated noise like computrs, flourescent lights, furnace
motors, etc. That's why you use shielded coax to bring the antenna in,
starting a good distance from the house (at least 20-30 feet, but 40-50 feet
is better). And an impedance-matching balun to connect the antenna to the
coax.

A good outside antenna solves a myriad of radio performance problems and is
the single most important factor. An average radio with a great antenna
beats the pants off a high-end rig with a substandard antenna every time.

Jeff

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

[maxhifi]

Post by Unrevealed Source
You are experiencing what is inherent in all radios, and is called the
"noise floor". That's the classic test - unhook the antenna and turn the
volume all the way up. That will tell you how noisy the radio itself is, as
opposed to externally-generated noise picked up through the antenna. All
radios have some noise.
Rather than screw around with your radio, the answer is to ut up an antenna
that will bring those weaker stations into the radio at a higher level than
the noise floor. And, you need to shield the input of that antenna from
externally-generated noise like computrs, flourescent lights, furnace
motors, etc. That's why you use shielded coax to bring the antenna in,
starting a good distance from the house (at least 20-30 feet, but 40-50 feet
is better). And an impedance-matching balun to connect the antenna to the
coax.
A good outside antenna solves a myriad of radio performance problems and is
the single most important factor. An average radio with a great antenna
beats the pants off a high-end rig with a substandard antenna every time.
Jeff

I was considering this, however, my S-40 has only two screw terminals as the
antenna input. Would I need a balun at both ends?

As is, I've got about 15feet of insulated, stranded wire (looks like #20 or so)
going straight down through the air from my long wire antenna, through the
window, and maybe about 5' inside which goes right to the back of my radio.

I think I should leave the antenna as-is for now, until I get the noise problem
at the receiver end under control - mostly based on the fact that my property
can't accomodate a significantly larger or higher antenna.

[John Doty]

Post by maxhifi
...

Post by Unrevealed Source
A good outside antenna solves a myriad of radio performance problems and is
the single most important factor. An average radio with a great antenna
beats the pants off a high-end rig with a substandard antenna every time.
Jeff

I was considering this, however, my S-40 has only two screw terminals as the
antenna input. Would I need a balun at both ends?

The manual shows direct connection from coax to the terminals. On the
other hand, reading a little between the lines suggests an input Z of
~400 ohms, so a 1:9 transformer there might be helpful.

[Carter-k8vt]

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Not practical...

Post by maxhifi
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.

Maybe. Do an e-place search for 'Ameco preamp*'. The Ameco 'PCL'
nuvistor series was quite popular.

Post by maxhifi
-Forget about it, and get a better radio??

This -may- be your best option, depending on your ultimate goal.

If your goal is 'nostalgia' or casual short wave listening, the S-40
will be fine (which, by the way, was only a middle-to-low end receiver
in its day -- and its day was 50 years ago).

If you want to do some really serious short wave listening, go with new.
Believe it or not, receiver technology -has- improved in the last 50
years! ;-)

Good luck!

[Martin Crossley]

Post by Carter-k8vt

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Not practical...

ISTR a design for a plug-in replacement for the 6SG7 RF amp using an ECC85
(I think, plus a resistor or two) in cascode.

Post by Carter-k8vt

Post by maxhifi
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.

Maybe. Do an e-place search for 'Ameco preamp*'. The Ameco 'PCL'
nuvistor series was quite popular.

The old (1950s-60s) RSGB and ARRL handbooks had designs for preselectors
which should do the job.

Post by Carter-k8vt

Post by maxhifi
-Forget about it, and get a better radio??

This -may- be your best option, depending on your ultimate goal.
If your goal is 'nostalgia' or casual short wave listening, the S-40
will be fine (which, by the way, was only a middle-to-low end receiver
in its day -- and its day was 50 years ago).
If you want to do some really serious short wave listening, go with
new. Believe it or not, receiver technology -has- improved in the
last 50 years! ;-)
Good luck!

[Carter-k8vt]

Post by Martin Crossley

Post by Carter-k8vt

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Not practical...

ISTR a design for a plug-in replacement for the 6SG7 RF amp using an ECC85
(I think, plus a resistor or two) in cascode.

Didn't say it -couldn't- be done, just not practical. This is a 50 year
old entry level radio and no matter how you slice it, it's -still- tough
to make a silk purse out of a sow's ear...

I vote for the OP to just enjoy it as it is and for what it is.

[Martin Crossley]

Post by Carter-k8vt

Post by Martin Crossley

Post by Carter-k8vt

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Not practical...

ISTR a design for a plug-in replacement for the 6SG7 RF amp using an
ECC85 (I think, plus a resistor or two) in cascode.

Didn't say it -couldn't- be done, just not practical.

Well, I'd consider that to be just about practical, along with increasing
the BFO injection voltage by simply increasing the coupling capacitor to
improve SSB performance and adding a Zener or 2 to stabilise the frequency
stability of the oscillators, but not rebuilding the mixer stage or anything
else drastic.

Post by Carter-k8vt
This is a 50
year old entry level radio and no matter how you slice it, it's
-still- tough to make a silk purse out of a sow's ear...

Yes, but it could be improved from '50s entry level to '60s entry level
reversibly and at a low enough cost and effort to be worth considering.

Post by Carter-k8vt
I vote for the OP to just enjoy it as it is and for what it is.

I'd equally agree with that, now it's been restored to work for another 50+
years.

73,
Martin.

[John Doty]

Post by Martin Crossley

Post by Carter-k8vt

Post by Martin Crossley

Post by Carter-k8vt

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Not practical...

ISTR a design for a plug-in replacement for the 6SG7 RF amp using an
ECC85 (I think, plus a resistor or two) in cascode.

Didn't say it -couldn't- be done, just not practical.

Well, I'd consider that to be just about practical, along with increasing
the BFO injection voltage by simply increasing the coupling capacitor to
improve SSB performance and adding a Zener or 2 to stabilise the frequency
stability of the oscillators, but not rebuilding the mixer stage or anything
else drastic.

Post by Carter-k8vt
This is a 50
year old entry level radio and no matter how you slice it, it's
-still- tough to make a silk purse out of a sow's ear...

Yes, but it could be improved from '50s entry level to '60s entry level
reversibly and at a low enough cost and effort to be worth considering.

50 years ago, the magazines frequently published articles on homebrew
improvements to this kind of receiver. It was "entry level", but for
many that meant a couple of weeks' wages: this stuff was expensive in
its day. So, many bought what they could afford and tried to tinker it
into something better. That was a normal part of the experience of
owning such a receiver.

Post by Martin Crossley

Post by Carter-k8vt
I vote for the OP to just enjoy it as it is and for what it is.

I'd equally agree with that, now it's been restored to work for another 50+
years.
73,
Martin.

[Michael Black]

Post by Martin Crossley

Post by Carter-k8vt
Didn't say it -couldn't- be done, just not practical.

Well, I'd consider that to be just about practical, along with increasing
the BFO injection voltage by simply increasing the coupling capacitor to
improve SSB performance and adding a Zener or 2 to stabilise the frequency
stability of the oscillators, but not rebuilding the mixer stage or anything
else drastic.

And all of those were the sorts of things you'd have done to the receiver
back then if you'd bought one. SOmeone didn't buy it back then as a
collector's item or to be retro, they bought it because they couldn't
afford something better. And since you had to live with it, you'd try
to improve it as much as possible.

Post by Martin Crossley

Post by Carter-k8vt
This is a 50
year old entry level radio and no matter how you slice it, it's
-still- tough to make a silk purse out of a sow's ear...

Yes, but it could be improved from '50s entry level to '60s entry level
reversibly and at a low enough cost and effort to be worth considering.

I don't know. I'm not sure the entry level receivers in the sixties
were really any improvement on the ones from the fifties. They did not
make any big leap in circuitry, they were still limited by the costs of
the day. The later receivers were mainly a repackaging of the same
basic arrangement. Or worse, a move to solid state; with little
experience and a desire for low cost, those early cheap solid state
shortwave receivers were pretty lousy.

It was only later, when circuitry could be much fancier at the same
or less cost, that the low end receivers improved.

Michael

[Brenda Ann]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

You might find that if you replace all the old resistors in your S-40 that
your noise floor will come down somewhat. Also, much of the noise you hear,
even without an antenna, is likely locally generated noise (television sweep
sections, switch mode power supplies, routers, modems, and tons of other
stuff in modern homes).

[Tio Pedro]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I >
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

It is the nature of the beast. They had very noisy mixers and required
a bit of front-end gain to overcome the mixer noise, and the RF amp NF
isn't that great either. You could substitute or modify the set for a
quieter RF frontend, but those tubes will load the input tuned circuits
and upset the imagine rejection on the higher frequencies. More gain
will overload the mixer too. Perhaps modifying the set
(re-engineering?) to use a stronger, and quieter, mixer
(perhaps a Pullen using a 6J6?) and a better front-end RF might do some
good. Some guys have used two Nuvistors in a cascade configuration
for that purpose.

But, you're still looking at an entry level receiver that is single
conversion and has those limitations. Enjoy it for what it is. I can give
you a link to a thread where we were discussing RF tube noise figures
if you really want to dig into it and go that far.

Pete k1zjh

[Alex]

Post by Tio Pedro
Perhaps modifying the set
(re-engineering?) to use a stronger, and quieter, mixer
(perhaps a Pullen using a 6J6?) and a better front-end RF might do some
good. Some guys have used two Nuvistors in a cascade configuration
for that purpose.

1. What is "Pullen". I have not heard this term, though perhaps have come
across the circuit or principle. Explain please.

2. You obviously ment the nuvistors in the "cascode" connnection: common
cathode - common grid. TV tuners use this principle. 6ES8 tubes often are
used for this purpose.

Alex

[Tio Pedro]

Post by Alex

Post by Alex
1. What is "Pullen". I have not heard this term, though perhaps have

come
across the circuit or principle. Explain please.
2. You obviously ment the nuvistors in the "cascode" connnection: common
cathode - common grid. TV tuners use this principle. 6ES8 tubes often are
used for this purpose.
Alex

Yep, you are correct: cascode. Here is a Nuvista Plug for the SP-600.

http://www.mines.uidaho.edu/~glowbugs/SP600upgrd.pdf

for the Keats Pullen mixer info:

http://www.mines.uidaho.edu/~glowbugs/A%20Like-New%20Mixer%20Circuit.pdf

Pete
Pete

[maxhifi]

Post by Tio Pedro

Post by Alex

Post by Alex
1. What is "Pullen". I have not heard this term, though perhaps have

come
across the circuit or principle. Explain please.
2. You obviously ment the nuvistors in the "cascode" connnection: common
cathode - common grid. TV tuners use this principle. 6ES8 tubes often are
used for this purpose.
Alex

Yep, you are correct: cascode. Here is a Nuvista Plug for the SP-600.
http://www.mines.uidaho.edu/~glowbugs/SP600upgrd.pdf
http://www.mines.uidaho.edu/~glowbugs/A%20Like-New%20Mixer%20Circuit.pdf
Pete
Pete

The "pullen" using a 12AT7 is something I could easily build with parts on
hand. Thanks greatly for the info, I think I'll try this one, and report back
with results!

[Alex]

Post by maxhifi

Post by Tio Pedro

Post by Alex

Post by Alex
1. What is "Pullen". I have not heard this term, though perhaps have

come
across the circuit or principle. Explain please.
2. You obviously ment the nuvistors in the "cascode" connnection: common
cathode - common grid. TV tuners use this principle. 6ES8 tubes often are
used for this purpose.
Alex

Yep, you are correct: cascode. Here is a Nuvista Plug for the SP-600.
http://www.mines.uidaho.edu/~glowbugs/SP600upgrd.pdf
http://www.mines.uidaho.edu/~glowbugs/A%20Like-New%20Mixer%20Circuit.pdf
Pete
Pete

The "pullen" using a 12AT7 is something I could easily build with parts on
hand. Thanks greatly for the info, I think I'll try this one, and report back
with results!

Well, I read the article and not so impressed.

Firstly there are many errors in the article. The author claims that gain of
a pentagrid mixer is 5. Typical conversion transconductance of 6SA7 or 6BE6
is 450uA/V, and the impedance of the IF transformer at 455kHz is 100K easy.
So the gain of 45 is more realistic.

I seriosly doubt the figures given by the author. The author claims that the
gain of the Pullen circuit reaches a 100, which requires Sc (conversion
transconductance) of 2mA/V for a 50K IF load. Since the Sc (conversion
transconductance) is about 1/4 of the peak transconductance, that would
require a peak S of 8mA/V. It is not realistic since the S of 12AT7 is at
best 5.5mA/V, and the two tubes are cathode coupled, so S is half of this
value. I do not expect a significant gain improvement compared to a heptode.

Surely, the equivalent noise resistance will be much smaller -- about
15...20KOhm compared to 100...150KOhm for a pentagrid. This is indeed one
and only advantaage of the circuit.

A big disadvantage of the Pullen circuit is low output impedance. Ndeed, if
combined S is about 2...3mA/V, and mu=60, Rout = mu/S = 30K (actually it
will be slightly larger because of cathode coupling, but I am not going into
details). Such low output impedance will reduce Q and selectivity of the IF
transformer.

Another problem of the circuit is that a cathode follower capacitively
loaded (by cathode-to-heater capacitance) is naturally produces negative
impedance. (It resembles a Kotpitz circuit on parasitic grid-to-cathode and
cathode-to-heater capacitors). It might cause oscillation of the RF tank at
the high frequency ends of bands, especially on the MW band.

In the end, the Pullen circuit does not give as good LO/RF isolation.
indeed, on the common cathode LO component is about 1/2 of the injection
level. On the same cathode node RF voltage is about 1/2 of what comes fron
the RF tank. Now if LO leaks into antenna, then it will be pulling the LO
while AGC is working or even if you tune the antenna trimmer. Also, tuning
the RF tank will affect the LO, and the tracking alignment will be
difficult.

So be careful implementing the circuit. And please share your experience if
and when you try it.

Regards,
Alex

[Tio Pedro]

"Alex" <***@bigpond.com> wrote in message news:NJxwj.19087$***@news-server.bigpond.net.au...
Alex, do a Google search for Pullen Mixer and you will many hits. I can't
vouch for veracity of any information posted on the Internet. However the
Pullen mixer has been used with some success by others. You'll have to
separate the wheat from the chaff. There is also some concerns about LO
pulling mentioned in some articles.

Pete

[maxhifi]

Post by Tio Pedro
But, you're still looking at an entry level receiver that is single
conversion and has those limitations. Enjoy it for what it is. I can give
you a link to a thread where we were discussing RF tube noise figures
if you really want to dig into it and go that far.

How is the 6AC7 in this regard? I notice "TerryJ" used it as the first RF
stage, and it's good info for me, as I've got a bunch of them surplus from a
former NORAD station.

[TerryJ]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

Try replacing the 6SA7 mixer with a 6SB7-Y. Double the conversion
transcondance. It is a noisy radio and a good antenna is a must.

[John Doty]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!

This technology is noisy, but note that natural and manmade noise at
medium and shortwave frequencies is also high. With a good antenna, you
should not be limited by receiver noise, at least on the S-40's three
lower bands. The classic way to tell is just to tune between stations
and disconnect the antenna: if the noise declines, noise coming in from
the antenna is the limiting factor.

Post by maxhifi
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

Better antenna. You want an antenna that delivers enough signal power to
overcome the receiver's noise, and also one that prevents locally
generated "conducted" noise from all your modern gadgets from getting
into the receiver. There's plenty of antenna information on the web by
shortwave hobbyists including some things I wrote:

http://www.hard-core-dx.com/nordicdx/antenna/feed/feed1.html
http://www.naswa.net/badx/ice.htm

[Alex]

Post by John Doty
Better antenna. You want an antenna that delivers enough signal power to
overcome the receiver's noise, and also one that prevents locally
generated "conducted" noise from all your modern gadgets from getting
into the receiver. There's plenty of antenna information on the web by
http://www.hard-core-dx.com/nordicdx/antenna/feed/feed1.html
http://www.naswa.net/badx/ice.htm

Would you recommend ferrite cores or clamps on the coax cable to reduce
common mode currents travelling on the outside surface of the coax from the
chassis of your receiver to the earth stud in the yard?

If yes, is it better to place them at the receiver side or at the earthing
side (in the yard)?

Regards,
Alex

[John Doty]

Post by Alex

Post by John Doty
Better antenna. You want an antenna that delivers enough signal power to
overcome the receiver's noise, and also one that prevents locally
generated "conducted" noise from all your modern gadgets from getting
into the receiver. There's plenty of antenna information on the web by
http://www.hard-core-dx.com/nordicdx/antenna/feed/feed1.html
http://www.naswa.net/badx/ice.htm

Would you recommend ferrite cores or clamps on the coax cable to reduce
common mode currents travelling on the outside surface of the coax from the
chassis of your receiver to the earth stud in the yard?

Generally no. The concept here is to allow the common mode current to
flow harmlessly on the coax shield from the receiver to the ground
point. If you block it with a choke, it may flow somewhere else that's
more troublesome. However, there are many variables here and a choke
might help in some setups: it's a matter for experiment.

[TerryJ]

See ABPR for schematic of mods I made to an S40 a few years back.

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

[Tio Pedro]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
-Substituting low noise tubes (but where to get them?)
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.
-Forget about it, and get a better radio??

Ed Engleken designed a twin triode detector for the S40.
For CW and SSB it works as a product detector, and as
a regular detector for AM reception. Pretty simple and
easy mod.

Pete

[Frank Dresser]

Post by maxhifi
I recently asked some questions about shortwave radio, and received some
very helpful response.
I tuned up my S-40, including touching up the IF transformers (they were
close, but I got a few dB), and modifying the headphone jack to make it
useful (normally, it comes off the 6SQ7 plate, which is useless for
modern 'phones.
It looks as though most of the capacitors were replaced in the 60s or
70s, so no problems here (for now), and the receiver receives very well,
except for one problem.
I've noticed that with no antenna, when you turn up the RF gain and the
volume, there is a loud rushing noise. My S-77 does the same thing, so I
don't think my S-40 is defective, however, I think this is tube noise,
and it is masking some of the quieter stations. If I could get rid of
it somehow, I could pull in weaker stations than I currently am able to.
I think this might be attributable to the design of the set, however, it
would be nice to do something about it!
I've been thinking about it, and so far I've come up with
-Substituting low noise tubes (but where to get them?)

Nearly all of the noise is coming from one tube, the 6SA7 converter tube.
Try pulling it and notice how quiet the radio gets. Pulling the RF amp tube
won't make nearly as much difference.

The 6SA7 was better than nearly all the other economical frequency tubes of
the era due to it's superior isolation between it's oscillator and input.
Good isolation keeps the local oscillator from trying to sync itself to the
recieved signal. The pulling problem gets worse as the relative spread
between the local oscillator and received signal decreases. The 6SA7 has
good frequency stability as AVC voltage changes but AVC induced frequency
instability gets worse with increasing frequency and the S-40 works up to 44
Mc or so.

The 6SA7 also had a high plate impedance for a converter tube which helped
selectivity.

However, the 6SA7 is really noisy. Most 6SA7s were used with one IF
amplifier but the S-40 has a second which amplifies the noise quite well.
The usual 6SA7 converter circuit was AVC controlled, which reduced the noise
on stronger signals, but I'm sure Hallicrafters ran the converter in the
S-40 without AVC to enhance frequency stability at SW frequencies.

The RF amp stage on the S-40 isn't much of a RF amp, it's more of an AVC'd
gain throttle to keep the full-gain, full-hiss converter stage from
overloading.

Post by maxhifi
-Installing a pre-amplifier, and then using the S-40 at a low enough
gain setting its own noise is not objectionable.

That would help some, but the S-40's RF amp would partially fight the preamp
by decreasing gain with increasing signal strength.

You might fight the noise problem more directly by lifting the cold end of
the converter input side from ground to the AVC line. This is how most
radios with 6SA7s are wired and noise will go down with increasing negative
voltage on the signal grid. I have a S-40A, and as I remember this looked
like a very easy mod, almost as if Hallicrafters intended this as an option.
Sorry to say, I didn't actually try it out. I was worried about frequency
stability and I started looking into pentode mixer -- triode/FET oscillator
schemes. Needless to say, I never tried those out either.

I did try the 6SB7Y converter tube, but I wasn't really impressed. Although
the signal to noise ratio may have been a bit better, the backround hiss was
still there and the radio pulled more than I liked with changing signal
strengths on SW.

Post by maxhifi
-Forget about it, and get a better radio??

I was annoyed by the backround hiss on my S-40A but I didn't put much effort
in trying to fix it. I found the hiss most annoying on the medium strong
signals which are quiet on most radios. I don't think the noise really
hurts potential sensitivity because all radios of that era ran their
converter tubes at maximum gain-noise on weak signals.

I think the S-40(A) is a pretty good radio for it's purpose. It isn't
supposed to be a nice sounding home entertainment radio with some SW
capability, nor is it a high end ham radio. It was competent for either use
and Hallicrafters sold alot of them. There are some design compromises in
this radio, but it seems like most of the quick fixes involve giving up
something in order to reduce the hiss.

I do use a Heathkit Q-multiplier with my S-40A. It can reduce the high
frequency part of the hiss by reducing the bandwidth.

Also, don't use much coax at the antenna input. This radio has a high input
impedance which works best with end-fed wires. The capacitance of coax will
reduce the signal strength with increasing frequency.

Frank Dresser

[maxhifi]

The response to my inital message has provoked an almost overwhelming amount
of useful information! I will try to summarize my thoughts so as to not let
this thread lose direction too much.

My S-40 is not a mint condition example, so I don't feel bad about doing
modifications which may not have been possible at the time it was made. I
really like the look and feel of it, and also it sounds good. I also really
like the two step tone control, which isn't present on my other SW
receivers.

My plan is as follows, based on a summary of all who have posted here.

1) Replace the 6SA7 with the "pullen" circuit, using a 12AT7, realign
receiver, and assess results

2) Possibly replace 6SG7 RF amplifier with a 6AC7 (if consensus is this will
be a useful mod, however, I have the tubes on hand, so cost and therefore
risk will be almost zero)

3) If above mods prove satisfactory, leave well enough alone, otherwise,
search eBay for an appropriate preamplifier.

[maxhifi]

The response to my inital message has provoked an almost overwhelming amount
of useful information! I will try to summarize my thoughts so as to not let
this thread lose direction too much.

My S-40 is not a mint condition example, so I don't feel bad about doing
modifications which may not have been possible at the time it was made. I
really like the look and feel of it, and also it sounds good. I also really
like the two step tone control, which isn't present on my other SW
receivers.

My plan is as follows, based on a summary of all who have posted here.

1) Replace the 6SA7 with the "pullen" circuit, using a 12AT7, realign
receiver, and assess results

2) Possibly replace 6SG7 RF amplifier with a 6AC7 (if consensus is this will
be a useful mod, however, I have the tubes on hand, so cost and therefore
risk will be almost zero)

3) If above mods prove satisfactory, leave well enough alone, otherwise,
search eBay for an appropriate preamplifier.

[Alex]

Post by maxhifi
The response to my inital message has provoked an almost overwhelming amount
of useful information! I will try to summarize my thoughts so as to not let
this thread lose direction too much.
My S-40 is not a mint condition example, so I don't feel bad about doing
modifications which may not have been possible at the time it was made. I
really like the look and feel of it, and also it sounds good. I also really
like the two step tone control, which isn't present on my other SW
receivers.
My plan is as follows, based on a summary of all who have posted here.
1) Replace the 6SA7 with the "pullen" circuit, using a 12AT7, realign
receiver, and assess results

You will definitely get less noise, but most likely will loose IF
selectivity and LO pull will become worse. Besides, do you really want to
install a 9-pin socket?

Post by maxhifi
2) Possibly replace 6SG7 RF amplifier with a 6AC7 (if consensus is this will
be a useful mod, however, I have the tubes on hand, so cost and therefore
risk will be almost zero)

I would not recommend doing it. 6AC7 is a sharp cut-off pentode and has
higher transconductance S=9mA/V. You will get more gain, overload your
mixer, get more intermodulation, possibly parasitic oscillations.

On the contrary, I would suggest changing 6SG7 tubes in IF to 6SK7 (connect
pins 3 & 5) to reduce excessive IF gain and subjectively reduce audible
noise.

Post by maxhifi
3) If above mods prove satisfactory, leave well enough alone, otherwise,
search eBay for an appropriate preamplifier.

Not a good idea since it might overload your receiver.

Your task is not enviable.
Unfortunately, it is impossible to make something in all aspects better than
a humble heptode mixer, and yet simple, without resorting to transistors,
JFETs, etc. Consider that: if the "Pullen" circut were really much better,
probably all commercial receivers would have been using it. However, they
predominantly use pentagrids and triode-heptodes...

Regards,
Alex

[John Doty]

(snipped discussion of possible S-40 mods
Your task is not enviable.

On the contrary, it should be fun and educational. That's what this sort
of receiver is for: tinkering, optimizing, learning...