[Boatanchors] Best 1940's Receiver

[Al Klase]

Hi Keith,

Well, I'm a big fan of the AR-88. (See my domain name.)  While I don't 
have first-hand knowledge of the SP-400 and RBB, I do have some valid 
thoughts about the audio circuits in each.  Let's start with a blurb I 
posted to the RCA list a while back:

[QUOTE]
AR-88 Audio
Posted to rca at mailman.qth.net 7 July 2018
The recent posts re:  AR88 transformers prompts me to relate an AR-88 story.

A long time ago, I obtained a large Kolster radio cabinet that had been 
stripped out, and now contained a 15-inch Altec Duplex speaker.  These 
speakers were the standard for recording-studio monitors for many years, 
and sound really nice.  They also cost about 150 bucks in 1950.  Think 
about that.

This cabinet sat in my living room for a long time, as I had no place 
else to put it.  One day I sat my Hallicrafters SX-28 on top of it, 
plenty of room, and hooked it up.  The SX-28 uses push-pull 6V6's and 
has sophisticated tone controls, so radio collectors just knew they 
sounded great.  I listened to this combination for a week or so.  It 
sounded good, but I wasn't exactly blown away.

So I replaced the Halli with my Hammarlund SP-110 Super-Pro.  The 
difference in audio quality was immediately obvious.  It was simply much 
cleaner.   I'd always been curious about the Super-Pro audio circuits.  
The design was laid down circa 1934-35, and used the same push-pull 
power pentodes, big-pin  42's migrating to octal 6F6's, found in all the 
high-end broadcast sets of that era.  The rated output was 14 Watts.  
However, the tubes were connected as triodes in the Super-Pro.  This 
cost them a third 6F6, as a driver, plus a probably-expensive driver 
transformer, that weren't necessary in a pentode amplifier.  I just 
wrote this off to primitive design, but it was now obvious that these 
guys knew exactly what they were doing.  (Triodes are better than 
pentodes, unless one uses negative feedback, which was unheard of at the 
time.)  The 'Pro had a variable-bandwidth IF section that made it pretty 
close to hi-fi when opened up,  but more important is the fact that high 
inter-modulation distortion in the audio stages will make it harder to 
pick out weak signals in background noise.

I related this story to a communications-receiver-collecting friend of 
mine.  He says "Now try your AR-88."   I reply "But Pete, it's only a 
single ended 6K6!"  Pete says "Just try it." So, down came the 70-pound 
Super-Pro and up went the 100-pound RCA.  Again, the improvement in 
audio quality was obvious to even the most casual observer.

Clearly, the RCA engineers appreciated the importance of clean audio in 
a communication receiver.  But now, in 1940, they had the powerful tool 
of negative feedback.   They designed an amplifier using "global" 
feedback.  The feedback signal is picked off the output transformer 
secondary, and applied to the cathode of the triode first-audio 
amplifier.  This sort of circuit forces the amplifier output to look 
almost exactly like the input, flattening frequency response, and 
correcting nonlinearities in the components inside the loop.   It also 
materially reduces the output impedance of the amplifier which improve 
speaker damping. (Contrary to popular opinion, a perfect 1:1 match isn't 
really what you want here.  See: "Damping Factor.")

So, the AR-88 family are probably equal to the best tube-based AM 
receivers ever built.  The only real shortcoming is the inability to put 
them precisely on frequency, without using an external standard.  That 
gets solved after WWII by the Collins 51J's and R-390's. "If you want to 
find them, use a Collins.  If you want to listen to them, use an AR-88."

[END QUOTE]

Now, the SP-400 uses the same triode-connected push-pull 6F6 audio as 
the SP-200's.  The audio in the RCA-designed RBB is very similar to the 
AR88.  And, I suspect, there's a lot of AR-88 thinking in the rest of 
the radio.   Keep in mind you need a RBB and an RBC for complete HF 
coverage.

My two cents,
Al


On 4/6/2023 11:05 AM, Keith Densmore wrote:
> Greetings,
> Over the years I have had the privilege of owning 4 AR88 (and variants)
> receivers. The best of these was the CR-91A variant which had a built in S
> meter and crystal phasing.
> I have always considered the AR88 to be the pinnacle of receiver design
> from that era. But now I am wondering if I have missed something. I am
> looking at the specs for a Hammurlund SP-400 and a navy RBB, two receivers
> I have never owned.
> Would like an opinion as to what is best from those who have owned all.
> Sensitivity, selectivity, stability. Ease of tuning and audio quality.
> Thanks, 73,
> Keith ve3ts
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-- 
ARK Sig Block Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/