[Hallicrafters] SX-73 VS: SP-600-JX Comparision question

[Richard Knoppow]

----- Original Message ----- 
From: "Greg Mijal" <bluebirdtele at embarqmail.com>
To: "Carl" <km1h at jeremy.mv.com>; <anchor at ec.rr.com>; 
"Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: <Hallicrafters at mailman.qth.net>
Sent: Wednesday, November 03, 2010 6:30 PM
Subject: Re: [Hallicrafters] SX-73 VS: SP-600-JX Comparision 
question


> Here's my two cents worth.
> I've had two of the SX 73's (R 274 version)
> It's more or less identical as a deluxe band cruiser.  The 
> biggest
> difference in performance is the sheer sound volume of the 
> SX 73.  It uses
> the big bulb version of the 6Y6 and has enough volume to 
> literally be heard
> across the street. Sound quality is the same for 
> Hammarlund and Halli.  The
> sx 73 also uses a ballast tube (R80) for filament 
> stabilization but will run
> without it just fine on HF.  The SX 73 is also lower in 
> the overall parts
> count.  One thing odd about the SX 73 is that if you use 
> it with a
> transmitter.  The AVC takes a long time to recover and you 
> will miss the
> other stations first three seconds or so of transmission. 
> Halli later added
> a relay to over come that and mounted it to the back 
> frame.  The SX 73 meter
> is also very scotch. SX 73 parts are very difficult to 
> come by these days.
> 73's
> Greg
> WA7LYO
> Kinston NC
   Thanks to all who answered. I've been curious about this 
"other" version of the R-274. From just the schematics it 
appears to be a very well thought out receiver. It would be 
interesting to know the history of its production because it 
was obviously made to meet the military spec for the SP-600.
    A couple of observations:
    The SP-600 has no band spread. It has a logging dial 
which can be calibrated for any band of interest but the 
tuning rate is a bit high for normal ham use. The tuning 
mechanism is perfectly smooth and has no play or backlash so 
its possible to tune it accurately for SSB if you have a 
delicate touch. The SX-73 appears to have the same 
arrangement.
     About 25,000 SP-600'ds of various versions were built. 
That is a hell of a lot of receivers. I think only the 
R-390A was built in greater quantity. That makes it 
relatively easy to find parts. I suspect that not many 
SX-73's were built.
     From the paper information I would not be surprized if 
the SX-73 was not even _more_ stable than the SP-600 but you 
can't really tell from that.
     Audio: In general the quality to be expected from 
single ended amplifiers, especially those employing pentodes 
or beam tetrodes, is not high. Most of them are very simple 
using an output transformer directly in the plate circuit. 
That means the transformer carries the plate current so will 
have poor low frequency response. Also, without feedback the 
distortion of such amplifiers is rather high. There are used 
in many receivers for reasons of economy but in the SP-600 I 
think there was a space and weight consideration. The 
previous Super-Pro receivers all had rather high quality 
push-pull amplifiers. They also had separate power supplies. 
The first announcement of the SP-600 indicated it had a 
push-pull audio amp but the illustration was art work and I 
wonder if even a mock-up or prototype was produced. I think 
there simply was not enough room on the chassis for both the 
power supply _and_ a push-pull amplifier. The SP-600 
chasssis is not a standard size, being deeper than usual to 
make room for everything on it.
    If you take the audio off the diode load at the back and 
feed it to a high quality amplifier you will be amazed at 
the quality. Of course, most broadcast stations now are so 
highly processed that you will only hear the distortion they 
like to think makes them sound loud. On a good souce the 
SP-600, and probably the SX-73 will be quite high fidelity.
     I think a lot of SP-600's are in poor condition, that 
is, they work, but not really up to capabilities. For 
instance, the frequency calibration is often off as is 
tracking because of the design of the tuning capacitor. 
There are eight stators each held in place by three clamp 
screws with fiber washers. I think these drift mechanically 
with age. The idea is that all of the stators must be 
exactly centered and exactly parallel to the rotor and the 
stators should also match at the top and bottom. I've seen 
at least one site recommending plate bending, DO NOT DO 
THIS, you will ruin the tuning cap.
     The capacitor in the SX-73 appears to be of a different 
construction and may be immune from this problem.
     A further note: While H built SX-28/A SX-32 for the 
military I don't think they were originaly intended for that 
purpose. Those built under contract probably used mil spec 
components in some places. I have a Navy version of the S-36 
and it definitely has changes from the civilian version so I 
would expect other originally civialian models to follow 
suit.
     Perhaps some day I will come by an SX-73 (or even an 
SX-28). I will say that the SP-600 and SX-28 are among the 
sexiest looking receivers ever built.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com