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".
>
>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".