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View Full Version : Re: Adapting an older design? (was Re: Tube AM Tuner...)


Robert Casey
July 10th 03, 09:29 PM
Jon Noring wrote:

>
>Since my interest is in building a tube-based AM tuner using modern
>components, adapting the circuit design of a highly reputed older
>radio is certainly intriguing. Just remove the final audio amplifier
>section (and tweak the line-out as necessary to conform it with
>today's standards), and maybe replace a few other "autonomous"
>sections with modern solid-state equivalents if there is any advantage
>to do so (e.g., the DC power supplies.)
>
>Of course, it continues to be suggested to just find an older radio
>and cannibalize it to my heart's content. In my case, by the time I've
>cannibalized it and rebuilt the chassis, the mechanicals, replaced
>all wiring, resistors, caps, etc., etc., it probably would have been
>better to just build the damn thing from scratch per a schematic and
>general layout. Thus the idea is to come up with a general schematic
>and layout design, and build from that.
>
>
There is the issue that building an AM radio from scratch is more
difficult then you might
first think. The problem is taming undesired feedback loops in say the
IF amp for example.
You will likely end up with howls and whistles as the stage self
oscillates. Stray capacitences
from input to output, also couplings thru the B+ supply, etc. Modifying
an existing radio
is easier in that you are starting where some tube radio engineer left
off in creating a stable
design. Gaining enough experience to create from scratch a good tuner
takes many
iterations of homebrew designs. The thing to remember is that, while
AM radios were
cheaply made, a lot of design skill was used to create a design that
could yield decent
performance using low cost parts with loose tolerances and layouts that
don't cause
problems. This knowledge usually doesn't show in textbooks, but was
essentially
"passed down" from seasoned engineers to younger ones in the same company in
the tube era.

If you do want to design and build something from scratch, realize that
you may need to
modify and iterate several times before you "get it right".

Jeff Goldsmith
July 11th 03, 12:31 AM
The magic brain tuner from an RCA 9-T and add the variable selectivity
IF of the Philco 37-116 would be a good radio.


Check out the military BC-314 receiver; 2 RF stages, 2 IF stages at
85KHz with BFO. 4 bands from 150 to 1500KC. I can hear the stations in
between the US stations with it. A local station on 1030KC does not
stop me from listening to Des Moines on 1020KC.
The broadcast range ARC-5 receiver has variable selectivity IF and
along with the BC-314 one of the best dial drive mechanisms available.

Me thinks you are going to want two seperate IF channels, one for the
DX receiver and the other for the high-fidelity section. The RF and
coverter part are probably good for either purpose, but the same IF
design is not going to be good for both DX and quality sound without
some trick IF transformers like those in the Philco 37-116. If
complexity is not a significant issue, you might consider using a second
crystal-controlled converter to change the 455KC IF for the fidelity
channel into 100, 85, or 50KC for the DX section.

If you want everything you are seeking from one receiver design, I
suggest the Hammarlund SP-600.


Jeff Goldsmith

Jeffrey D Angus
July 11th 03, 02:43 AM
Jon Noring wrote:


> I appreciate the feedback, and yes, designing it from scratch is not
> a good idea except for those who want and cherish the challenge.
>
> As rereading my full message indicates, I am focusing on adapting an
> established, older design. Thus my call for candidates from the
> "vintage" era.

Hence my comments about using something like the Kenwood KT-313 or
KT-5100 (Or is that 5500) chassis as a good starting point. All the
physical cabinet stuff, dial etc. is already there. And there's
enough room inside to put a chassis, tubes, and other electronica
inside.

Jeff



--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"

Patrick Turner
July 11th 03, 04:42 PM
Jon Noring wrote:

> Mark Oppat wrote:
>

snipeth some interesting considerations,

>
> Of course, it continues to be suggested to just find an older radio
> and cannibalize it to my heart's content. In my case, by the time I've
> cannibalized it and rebuilt the chassis, the mechanicals, replaced
> all wiring, resistors, caps, etc., etc., it probably would have been
> better to just build the damn thing from scratch per a schematic and
> general layout. Thus the idea is to come up with a general schematic
> and layout design, and build from that.

Well, then off to the workshop with thee,
Pray, do not tarry by thy square framed flickering lantern,
permitting all confabulation of betwixing figures,
but to your forge process, and produce fine chassis,
and musical equipments to adorn thy courts of pleasure.

Ye will need to spende much evening hours at vexing toils,
and refrain from such course wordage, which will do
no effect to hasten your productions.

When you have achieved such sweet tones of music, conveyed across the
ether,
as if sent by teams of angels, hasten here to tell us your
igenuities, and dazel us with glowing wonders.


> And once a group of enthusiasts have come up with a working design......

Deteteth many more conditions and fascinations.....
in the concern of sweet brevities....

And within this worldly Web, there resides enomous talent,
and much resolve is mused by minds considering, seated in plush easy
chairs.

The real vexations of possiblities anew,
were solved by my grandfathers,
and humbly you may try, to take their brighter view.
Their carries on, and propels our new inventions,
and so it is with patience,
we await your new constructions.

Shakespeare, 1504,
After considering the possibilities of
"casting broad the song and mirthe of playes throughout the lande" ....

Uncle Peter
July 13th 03, 06:47 PM
"Jon Noring" > wrote in message >
> >
> Of course, it continues to be suggested to just find an older radio
> and cannibalize it to my heart's content. In my case, by the time I've
> cannibalized it and rebuilt the chassis, the mechanicals, replaced
> all wiring, resistors, caps, etc., etc., it probably would have been
> better to just build the damn thing from scratch per a schematic and
> general layout.

But, a good design would give you the mechanical layout, the RF
section would include the tuning cap and matched coils for tracking,
the IF transformers would be there and mounted.. Even if you gutted
90% percent of the existing electronics, just having the RF and IF
sections available for modification represents a majority of the time
consuming work. Never mind the cost of trying to dupicate a dial
drive and tuning system.

Pete