[ARC5] Way OT -- SX-28

Fri Aug 6 16:01:16 EDT 2021

    One reason for eliminating push-pull audio is a saving in 
space and heat. Fairly high power (10 to 20 watts) push pull amps 
were found in many deluxe receivers in the days when broadcast 
reception was an important function. Hallicrafters made several 
receivers with push-pull amps. The push-pull feature has a couple 
of effects: first, it eliminates DC from the core of the output 
transformer, resulting in much better bass response. While a 
larger core can be used to compensate for the DC it results in a 
very much larger, heavier, and hotter transformer. The second 
advantage is the cancellation of even harmonic distortion. While 
even harmonics are not as ugly sounding as odd harmonics getting 
rid of them is still an advantage. I think in general push-pull 
amps have less intermodulation distortion. This can be important 
in the effect of noise on the desired signal. While CW is a 
single frequency CW signals sound much cleaner on a receiver with 
a P-P amp.  However, the amp requires one more output tube and, 
usually, a phase splitter. In the Super-Pro, which I am quite 
familiar with, the output uses a driver transformer with an 
additional power tube to drive it. The output stage on a 
Super-Pro is AB2, meaning it draws grid current and needs a 
driver stage capable of delivering some power.  I think 
Hallicrafters used either AB1 or Class-A, basically cleaner.
     Most communications receivers use very simple single pentode 
output amps. Even very high quality receivers with this type of 
output have poor audio quality. For instance the Collins 51J 
series is spec'd at 1.5 watts out at 15% THD. That's fifteen 
percent. They get away with it because the average power needed 
is usually a fraction of a watt and beside the 51J was really 
intended to drive an RTTY converter via its IF output.
     In many receivers hooking an external high quality amplifier 
to the diode load will very noticeably improve its audio quality.

On 8/5/2021 6:52 PM, Todd, KA1KAQ wrote:
> On Tue, Aug 3, 2021 at 8:12 PM MICHAEL BITTNER <mmab at cox.net 
> <mailto:mmab at cox.net>> wrote:
>
> <snip>
>
>      Of course those are asking prices and we don't what they
>     will sell for if at all. i see one presently has 17 lookers
>     and I'm one of em. Mike W6MAB
>
>
> Precisely. If you use the Advanced feature next to the Search 
> function and search for completed/sold items only, you'll see 
> that all of the SX-28s sold recently have sold for *under* 
> $500. Only one sold over $400, the rest under and most under 
> $300. Searching under Completed items shows that the high 
> priced examples have been listed numerous times without sale. I 
> suspect someone decides to list one for a crazy amount to test 
> the waters (or, in many cases, because they're clueless) and 
> others then jump in, resulting in several 'extraordinarily 
> priced' sets.
>
> As the owner of a later SX-28A I can say that it is one of my 
> favorite receivers - definitely top 10 and probably top 5 - due 
> to a number of things. As others have mentioned, for it's time 
> it was overall a really nice receiver. Not the best by any 
> means, I'd agree that the early Super Pros (SP-10, -100, -200) 
> take that honor followed closely by the HRO. Like the Super 
> Pro, the SX-28 shares a wonderful 'shove-yank' audio output 
> feature that, combined with a decent speaker, can fill a room 
> or even a home with audio of excellent fidelity. This from a 
> 1930s design, long before 'Hi-Fi' became a thing. It also 
> offers all the mod-cons of the day including weighted flywheel 
> tuning, bandspread, and a large, easy to read meter.
>
> But again to echo Richard, the SX-28 is, in the opinion of 
> many, easily the best looking receiver of the day, maybe ever. 
> The Art Deco influence, overall dimensions, and excellent 
> symmetry all work together perfectly. Those steering wheel 
> tuning knobs don't hurt, either. Sure, it has its shortcoming, 
> but overall it worked/works pretty well for when it was 
> designed and for what. I do think that the AR-88 story has 
> gotten mixed in somewhere, because SX-28s are not at all rare 
> or even scarce. At one time I knew production figures but want 
> to say something like 20-40K were made. Some were sold here 
> prior to WWII and many 28As were sold after. In fact, the 
> military-tagged examples (GRR-2?) are harder to find. IMO a 
> clean, complete, restored or unmolested working example is a 
> $500 radio all day. If you have to fix it or (gulp) re-cap it, 
> the price drops.
>
> I've always thought that the later SX-115 bears a striking 
> resemblance dimensionally to the SX-28. The cube look along 
> with the two large knobs and so on.
>
> A point of interest: many(most?) FCC models utilized only 
> single-ended audio output, apparently as a cost cutting measure.
>
> As to the SX-88, well....clearly driven by the limited 
> numbers/collector market. Apparently the performance didn't 
> match the price tag, or they certainly would've sold more.
> They are waaaay down from their $10K high water mark some years 
> back. Have seen $2K offerings languish for months without 
> selling and know of several selling in recent years for 'mere 
> hundreds'.
>
> ~ Todd,  KA1KAQ/4
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL