[ARC5] Some info on the S-38
K5MYJ
macklinbob at gmail.com
Fri Nov 20 02:02:39 EST 2015
In 1957 I had an ORIGINAL S-38 with the REAL BFO. Other than that it had all
the other problems of classic S-38s. No RF stage and only one IF stage. No
calibrated bandspread.
In 1960 I bought a National NC-109. It was a much better receiver. I likes
the TWO SLIDERULE dial better the halfmoon dials on the S-38.
About 15 years ago I bought a Hallicrafters SX-110. Much better than a S-38.
It has a RF stage and TWO 455KC IF stages. It also has a crystal filter.
What it is lacking is crystal calibrator.
The SX-110 has a classic Hallicrafters ROTARY dial for the main tuning and a
SLIDERULE dial for the bandspread. The SX-99 and SX-110 are the same except
for the bandspread dial.
There are other Hallicrafters receivers the same as these but without the
crystal filter.
About 10 years ago I was able to buy another NC-109. It's much better than
any of the low end Hallicrafters receivers. But it also does not have a
built in crystal calibrator. But it does have an accessory socket on the
back and the manual has the schematic for National's crystal calibrator. I
built a copy of the National crystal calibrator and that solved that one
problem. The NC-109 has a good crystal filter and both PRODUCT detector for
SSB/CW operation and a classic diode detector for AM operation. I like the
adjustable BFO for setting CW pitch and use as a SSB clarifier.
About five years ago I got a Hallicrafters SX-111. It's close to a HAM BAND
only version of the SX-100. The SX-101 is the HAM BAND only version of the
SX-100.
I never liked the complexity of the dial on the SX-101 is why I never bought
one. The SX-111 has a nice clean SLIDERULE dial.
The SX-111 has the same HF front end as the SX-100 and SX-101. 1700KC(?)
first IF. Crystal converted to the 50KC second IF. But the SX-111 has one
less 50KC stage. And the SX-100 has a better audio amplifier stage.
But I like the operation of the SX-111.
There are a few better Hallicrafters receivers but I have never used one.
They are rarer and more expensive. The SX-88 is OVER RATED!
I have had ARC-5/SCR-274 receivers and either a BC-312 or a BC-342. But that
was over 50 years ago. They were POOR performers on the ham bands. But fine
for typical military operation of the time.
My main station today is a Heathkit SB-301/SB-401 pair. I've had them
operating for 12+ years.
I currently have one other good HF receiver. A Heathkit HR-1680. The
replacement for the HR-10. It might be nice to get the matching HX-1681
transmitter but the way the bands have been that would probably be a waste
of money.
A few years ago I accumulated the parts to make a copy of the Millen 50W
6L6/807 exciter. I often think about building it since I do have all the
major parts. I might be nice to use with the SX-111 on CW.
----- Original Message -----
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <arc5 at mailman.qth.net>
Sent: Thursday, November 19, 2015 10:02 PM
Subject: Re: [ARC5] Some info on the S-38
> A note on "sensitivity". The specs given are really for gain, i.e.
> audio output for a given RF input. The input signal should also have a
> spec for modulation frequency and percentage. Such specs are sometimes
> useful for trouble shooting but are not a measure of the sets ability to
> recover weak signals. A better, although not really satisfactory measure
> is the signal to noise ratio. This is a very common spec on all sorts of
> better quality receivers. It is the level of a modulated signal plus
> noise over unmodulated signal plus noise for a stated ratio. The ratio is
> sometimes 10db (i.e. 3 to 1 voltage) and sometimes for 6db (2 to 1
> voltage). A receiver like the S-38B, with a noisy converter working
> directly from an antenna, that is, without an RF stage, will have anything
> from around 10uV to 100uV to obtain the 10db ratio. It will vary with the
> tube characteristics and the Q of the RF coils. Typical 1940s
> communication receivers will have something like 4 to 10uV again depending
> on the tubes and RF coils but will usually be better than the cheap
> receiver because of having two or more IF stages and narrower bandwidth.
> More modern receivers, post WW-2, with higher Gm miniature tubes, will
> have something on the order of 2uV for 10db SNR. Of course the SNR
> depends on the bandwidth and to some degree on noise from spurious
> responses which add to the overall noise.
> A much better measure is Noise Factor or Noise Figure but it is seldom
> encountered in lower frequency equipment although common for VHF through
> microwave gear. NF is the ratio of the noise power output from the
> receiver to the theoretical noise power output from a "perfect" i.e.
> noise-free system. It is measured by measuring the output of the receiver
> from a calibrated noise source and comparing that to the output with no
> input but with the input terminated in a "perfect" resistor of the design
> value for the antenna. Since the test signal is subject to the same
> bandwidth limits and spurious responses as an actual signal it is fairly
> valid measure of the noise performance of the receiver or amplifier being
> tested. Typical "modern" vacuum tube receivers have NF on the order of
> perhaps 4 to 10db in the HF band. Because atmospheric noise is high in
> this region this is quite good performance. However at VHF and higher
> frequencies the noise of the antenna system is mostly resistive (i.e.
> Johson noise) with perhaps some galictical noise so much better NF is
> desired, hense the use of super-cooled devices at the input.
> It is common to state the performance of an IF filter as its bandwidth
> ratio from 6db down to the bandwidth at 60db down (1000 to 1). For a
> single tuned 455 Khz IF transformer a ratio of perhaps 10 to 1 is common
> but can be a little more if the coils are high Q or if the frequency is
> lower. The skirt width is pretty much a function of the type. The
> selectivity is also a function of the mutual coupling of the two coils.
> When more IF stages are added the selectivity becomes greater (meaning
> sharper) and the combination of critically tuned and over coupled stages
> can yield good skirt selectivity. An example is the Hammarlund HQ-120-X
> and 129-X. Complex filters using crystals or high-Q magnetic resonators
> (mechanical filters of the Collins type) can have much smaller 6 to 60db
> ratios. Around 2 to 1 is typical for the 6 or 8 pole filters found in
> today's equipment. Obtaining greater ratios can be done but the problem
> with eliminating ripple across the pass band and obtaining reasonably good
> phase and time delay response become difficult and expensive. Bell System
> used crystal lattice filters up to 20 poles (maybe more) in its carrier
> and microwave systems but one does not usually encounter more than 8 poles
> in ham or commercial communications equipment.
> The S-38B does OK for a simple receiver.
>
> On 11/19/2015 9:29 PM, K5MYJ wrote:
>> Here's what a dB is:
>> https://en.wikipedia.org/wiki/Decibel
>>
>> Bob Macklin
>> K5MYJ
>> Seattle, Wa.
>> "Real Radios Glow In The Dark"
>> ----- Original Message ----- From: "Leslie Smith" <vk2bcu at operamail.com>
>> To: <arc5 at mailman.qth.net>
>> Sent: Thursday, November 19, 2015 7:22 PM
>> Subject: Re: [ARC5] Some info on the S-38
>>
>>
>>> OMG! OMG! OMG! How I hate it when I'm wrong!
>>> Worse still - how I hate it when I was wrong and I should have known I
>>> was wrong!
>>> Worse than that - how I hate it when EVERYONE sees the mistake and
>>> thinks "YOU WERE WRONG!"
>>>
>>> Sob, sob, sob. I'm off to eat spaghetti! (Better than eating worms!)
>>> Tnx everyone. I feel better now!
>>>
>>> 73 de Les Smith
>>> vk2bcu at operamail.com
>>>
>>> On Fri, Nov 20, 2015, at 11:26, Leslie Smith wrote:
>>>> Correction: I wrote that 60dB "down" amounts to one-millionth of the
>>>> signal (or something like that).
>>>>
>>>> I should have written 60dB "down" amounts to an attenuation of the
>>>> signal by one-million. The signal is attenuated 10x log(one-million).
>>>> Anyway, I figure that every-one on this list knows that, and made the
>>>> correction for me already. - Les
>>>>
>>>> --
>>>> http://www.fastmail.com - Access your email from home and the web
>>>>
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>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
> ______________________________________________________________
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