[Boatanchors] Why 455?

Tue Jan 29 14:59:22 EST 2008

I believe that 455 became a standard for 2 main reasons.

1. To provide a standard that made purchasing cheaper. The set manufacturers 
bought a catalog transformer and had their part # stenciled on it if at all. 
Naturally there were custom made exceptions.

2. The set manufacturers discovered that if the LO section of the tuning 
variable was made smaller it saved them the cost of a padder trimmer as well 
as alignment time. In order to save money again a standardized LO section 
became the norm.

This all took place in the mid-late 30's as near as I can tell by looking at 
several home type radios I have here.

Dual conversion was available in the early 30's for some high end console 
receivers that covered several SW bands.

National went their own way as they manufactured their own variables and IF 
transformers well into the 50's & even the 60's for some models. The 
original HRO established the 465 Kc wheras the earlier FB-7 was 500. (the 
51J4 mechanical filters do wonders in that radio)

The cheap sets (AC-DC, etc) and eventually most others used RCC variables as 
did most other companies and those used 455 Kc IF's.

Carl
KM1H
National Radio 1963-69

----- Original Message ----- 
From: "Bob Macklin" <macklinbob at msn.com>
To: "Al Klase" <al at ar88.net>; "Boatanchors" <boatanchors at theporch.com>; 
"Boatanchors List" <boatanchors at mailman.qth.net>; "New Jersey Antique Radio 
Club" <njarc at mailman.qth.net>; "Delaware Valley Historic Radio Club" 
<dvhrc at mailman.qth.net>
Sent: Tuesday, January 29, 2008 1:58 PM
Subject: Re: [Boatanchors] Why 455?

> Not all IFs are 455. But 455 was the most common.
>
> There were some that were 465. These were not just retuned 455s. They were
> manufactured as 465.
>
> There were also 265KC IFs used in auto radios.
>
> But I believe that the choice of 455KC was because Miller produced them in
> very high quantities for a very low price. Up to the late 70's you could
> still by the Miller transformers over the counter at most 
> Radio/Electronics
> supply dealers.
>
> Also the BCB variable capacitors were made in large quantity to use with a
> 455KC IF.
>
> Bob Macklin
> K5MYJ
> Seattle, Wa,
> "Real Radios Glow in the Dark"
>
>
> ----- Original Message -----
> From: "Al Klase" <al at ar88.net>
> To: "Boatanchors" <boatanchors at theporch.com>; "Boatanchors List"
> <boatanchors at mailman.qth.net>; "New Jersey Antique Radio Club"
> <njarc at mailman.qth.net>; "Delaware Valley Historic Radio Club"
> <dvhrc at mailman.qth.net>
> Sent: Tuesday, January 29, 2008 10:44 AM
> Subject: [Boatanchors] Why 455?
>
>
>> Every so often the question comes up: Why are all the IF’s 455 KHz? I’d
>> like to get an article together that solves this riddle while the people
>> who know are still with us. I know parts of the story, but I need help
>> with a couple of issues.
>>
>> There are two major consideration is the choice of the intermediate
>> frequency used in a superheterodyne receiver. The lower the frequency,
>> the easier it is to attain high selectivity. Also, in the early days,
>> before tetrode and pentode tubes, it was easier to achieve a high degree
>> of amplification at lower frequencies. Conversely, a higher IF frequency
>> results in better image rejection.
>>
>> Early superhets had the IF at 100KHz or lower in order to get adequate
>> gain from the available triode tubes. They suffer severely from
>> “two-spot tuning” (images). By the early 1930’s, broadcast set had
>> settled in at 175KHz, and automobile receivers would later adopt 262KHz
>> as a standard.
>>
>> The advent of the short-wave craze, and multi-band broadcast receivers
>> dictated a higher IF frequency to achieve adequate image suppression on
>> the short-wave bands. The broadcast band occupied 550-1500KHz at this
>> time, and the designer encounters sever problems if his radio tunes
>> across it’s own IF. Some shortwave sets used 1600-1700KHz for better
>> image rejection, but one couldn’t go higher if the 160-meter ham band
>> (1800-2000KHZ) was to be covered. Most multi-band receiver settled in
>> near 450KHz, a comfortable distance from the first broadcast channel at
>> 550KHz.
>>
>> Questions:
>>
>> Odd multiples of 5KHz, 455, 465, etc., were usually chosen so that the
>> image of the carrier of a broadcast-band station could be zero-beat with
>> the carrier of the station being tuned to achieve minimal interference.
>> (This assumes 10KHz channel spacing. Did the Europeans (9KHz) do
>> something else?)
>>
>> The Radiotron Designers Handbook, Third Edition, p. 159, states “A
>> frequency of 455 Kc/s is receiving universal acceptance as a standard
>> frequency, and efforts are being made to maintain this frequency free
>> from radio interference.”
>>
>> (1) Do FCC and international frequency allocations reflect this?
>>
>> (2) I’ve heard the term “Clear-Channel IF.” Can anyone cite references?
>>
>> (3) At lease one news group posting claims that broadcast frequencies in
>> a particular market are assigned to prevent strong inter-modulation
>> products from falling near 455KHz. Is this factual? Need reference.”
>>
>> (4) Was this (3) at least part of the reason for “Radio Moving Day” in
>> 1941? See:
> http://www.dcmemories.com/RadioMovingDay/032341WINXFreqChange.jpg
>>
>> (5) Many National Radio sets used a 456KHz IF’s and I think I remember a
>> 437 somewhere. Why? Are there different considerations for short-wave CW
>> operation?
>>
>> Further input, corrections, and elaborations are greatly appreciated.
>> Scolarly reference will be looked upon with great favor.
>>
>> Regards,
>> Al
>>
>> --
>> Al Klase - N3FRQ
>> Flemington, NJ
>> http://www.skywaves.ar88.net/
>>
>> _______________________________________________
>>
>
> _______________________________________________
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