Communications Receiver Audio

A number of years ago, I came into the position of a large, 1930'ish speaker enclosure, bearing the Kolster name, but containing a late 1940's Altec 604 15-inch  "Duplex" speaker.  These were designed to be studio monitors, and cost a good deal of money.  This sat in my living room for awhile.

Then I decided I really should use the thing.  The top was just the right size for a communications receiver, so I put a restored SX-28A on top of it.  These are reputed to have good audio from push-pull 6V6's and complicated tone controls.  The sound was good, but not great.

Next I tried the Hammarlund SP-110.  It sounded a lot better.  This radio was designed as power pentodes were gaining favor for audio output, mostly because they are easy to drive to high output with just a couple of low level tubes, i.e. they are economical.  It appears the Hammarlund guys listened to the pentodes, and decided to spend money instead.  The Super-Pro was perhaps the best general coverage receiver in existence at the time, and an important part of the market was for broadcast monitors, where a modicum of audio power was needed.  They opted for push-pull triode connected pentodes, 6F6's in the second generation SP-100,  these were driven by a third triode-connected 6F6 via a surely-expensive driver transformer.  Negative feedback was poorly understood at this point.  Triodes, with out feedback, were the answer for clean audio.  (There is some basis to all that audio-tomfoolery.)

SP-100 audio stage

I related my results to a fellow collector, and good friend.  His suggestion was try my AR-88 in the same environment.  My response: "Aw Pete, it only has a single ended 6K6, how good can it be?"   He said "Just try it."  The silly radio sounded noticeably better than the Super-Pro.  How could this be, when the highly regardes SX-28 sounde so much worset?  The answere. feedback.  By 1941, negtive feedback had become better understood, and the crew at RCA , Camden were the 700-pound gorilla of receiver design.  Here's what they did:

Notice that the feedback come from the output side of T2, and extends back to the cathode of the 1st Audio amplifier.  

Later, I applied this technique to improve the audio in the Hammarlund SP-600.    SP-600 Audio Mod.