Now beware these are my opinions, I think
they are right but others
may disagree.
A Variac is useful especially if it has meters
showing the current
being drawn. A volt meter is also useful but
less so.
The main purpose of bringing up a unit on a
variable transformer is
to catch sudden shorts and be able to turn it
off quickly. The idea of
starting at a low voltage and bringing it up
over a long period of time
is really misleading. The idea is to catch
anything that is going to
short quickly and turn it off. A filter
capacitor that is going to short
usually does so quickly. Otherwise,
electrolytic caps that have not been
used for a long time may need to "reform" but
will do this in a few
minutes at full rated voltage. Just letting
them run for a while will
usually reform them if they are going to
reform. Lots of mis-information
about this.
Watch the ampmeter while bringing up the
voltage. Usually, vacuum
tube rectifiers begin to conduct pretty well
at quite low voltages so
any shorts will show up fairly quickly.
Sensitivity: for the most part a receiver like
the SX-42 has pretty
good sensitivity. We must differentiate
between sensitivity and noise
level. The biggest difference in modern
receivers is noise level. Most
vacuum tube receivers with typical phone
bandwidth will have a gain
sufficient to produce about half a watt or a
watt of audio power from an
input signal of around a micro-volt. Now,
signal to noise is another
thing. A decent receiver with a vacuum tube RF
and mixer will have a SNR
of around 2.5 uV for something like 6 to 10 db
noise ratio. Some modern
receivers, tube or solid state may be ten
times better than this. The
requirement is that the noise contribution of
the receiver be small
compared to the noise from the atmosphere. Up
to maybe 10Mhz even rather
noisy tubes will do. For higher frequencies
the quality of the tubes but
also the quality of the input circuits will
make a large difference. The
later tubes, like 6SG7 or 6BA6, will be
reasonably good to around 50Mhz
earlier tubes like the 6K7 or 6SK7, may become
a bit deaf above about 20
Mhz. Some miniature tubes have considerably
better signal to noise
ratio. One measure is the transconductance or
Gm. The higher it is the
quieter the tube.
The tubes used in the S-20R are fairly old.
The noise level is
higher than in later receivers like the SX-42,
which was designed to
work at FM frequencies above 50Mhz. My S-20R
has good sensitivity to
perhaps 15Mhz but gets a bit deaf above that.
The quality of the coils,
etc, makes a difference so that a receiver
like the RCA AR-88, which has
very low loss coils, has excellent sensitivity
and low noise right up to
30Mhz.
There two ways of specifying "sensitivity" An
old method, which one
finds in the specs of early receivers, is
really a measure of gain. It
states that for an input of some given level,
often 1 microvolt, the
audio output will be of some value, often 1
Watt. This is sometimes
useful to determine if a set is working
correctly but is not really much
of a measure of performance. Another method,
which has more valididity,
is a statement of signal to noise ratio for a
given input level. In both
cases the input is modulated, usually 30% at
some medium frequency,
typically 400Hz. The second kind of
measurement is stated as some
sensitivity for a signal to noise ratio of,
say 10db, but often some
other value. Typical sensitivity for older
receivers is around 2
microvolts. Neither of these measurements can
be used to compare designs
because both depend on the bandwidth. For a CW
signal using a narrow
bandwidth, the sensitivity may be much lower
than for AM with a, say
6Khz bandwidth.
There is a better method but its usually used
for VHF or microwave.
That is a measure of equivalent noise input,
also stated as noise figure
or noise factor. This is measured by applying
a calibrated noise
generator to the receiver (or amplifier) and
measuring the level
compared to the calculated noise from thermal
agitation, i.e. the output
of a perfect, noise free, receiver. It is
usually stated in db. A
typical high quality communication receiver
has a noise factor of around
6 to 10 db, much less for low noise microwave
amplifiers. The advantage
of this method of measurement is that it is
independent of bandwidth and
band shape of the set. That allows comparisons
of different designs. It
is not a difficult measurement to make but is
not often stated for HF
communications receivers. Again, this depends
on the transconductance of
the amplifier and mixer stages and the quality
and loss factors of the
tuned circuits.
There are other qualities or properties that
depend on the quality
of the components, such as the bandwidth of
the RF stages and their
ability to reject images and the gain possible
from the RF amplfiers
which depends on the Q and losses of (mostly)
the coils.
Older text books on receiver design go into
these factors fairly
thoroughly but are usually simple enough to be
understandable to the
non-specialist. The higher the frequency the
more complicated all this
gets, but up to communications frequencies
(say 30 to 100 Mhz) its not
too bad.
I am giving you a long answer to your
question. In fact, one can
expect pretty good performance from even older
receivers if they are of
good quality.
I should also point out that up to around 15
or 20 Mhz the
atmospheric noise dominates things, far
outweighing the electronic noise
of the receiver circuits. This outside noise
diminishes generally to
where receiver noise becomes of considerable
importance above about
20Mhz and dominates in the FM band above about
50Mhx. All this is
discussed in general text books. I find the
older ones (say 1950s) are
easier to understand than later ones. They
also have the advantage of
being available at no cost via the web. See
<tubebooks.org> for instance.
Enough already, I will shut up now.
On 5/15/2024 8:09 PM, Thomas Latimer wrote:
(I know this is a little off topic
but I hope someone out there can answer)
I have had two Panasonic RF-2200 radios for
a lot of years and want to
get them back on-line again,? so I have a
couple of questions for those
of you who are knowledgeable:
1.?? Should I start them gently on a variac
or similar equipment (do you
run the same risk with the capacitors)
2.?? If they need repair, is there anyone
that works on these
3.?? How does this model compare in
sensitivity and selectability to
something like one of the main-line
Hallicrafters stuff? (I have an
S-20R and three SX-42's)
Thanks in advance, any and all advice is
appreciated
Tom Latimer
Okemos, Michigan
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
