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The KP-81 is pretty rare and I was lucky to find mine. I got it at the Peoria hamfest about 4 or 5 years ago. There was an older ham struggling to get a Johnson transmitter out of his pickup and while helping him I spotted a KP-81. When I asked him how much his response was you have to buy all of them. So I ended up with a stock Pierson KP-81, a second unit modified with miniature tubes in the front end, and a third parts unit along with two complete power supply/speaker units.
The KP-81 is the only "non-National" receiver I am aware of that uses a sliding coil catacomb for band switching. The Pierson unit is much larger than that used by National. The receiver proper weighs around 70 pounds and the matching speaker weighs another 45-50 pounds. The speaker console contains the power supply and push/pull audio output stage. The frequency is pretty easy to read as long as you are viewing it straight on. The dial markers are projected by a lamp and lens assembly from behind the dial. Tuning is very smooth and the variable capacitors have ball bearing assemblies at both ends, there is an additional bearing assembly where the tuning shaft goes through the front panel. A moderate spin of either tuning knob will send it to the end of the range. Turning the bandspread cap just beyond the marked set point grounds the antenna input and turns on the calibrator. It is a single conversion design with the IF spread across two plug in chassis on either side of the receiver. The RF section has two RF amplifiers and sits in the middle of the receiver. The crystal filter can be used in either series or parallel (notch) mode and it has a selectable bandpass audio filter in addition to adjustable high and low pass audio filters. There are separate noise limiters for AM and CW and a squelch system that works very well on AM.
The IF sections are very easy to work on; all you do is loosen 4 bolts and they unplug. But the RF section was a major pain to recap. You have to remove the sliding coil catacomb, unsolder around 40 leads from the tuning capacitors, unbolt the coil contact strip and finally the section can be removed. Then the real fun begins since Karl Pierson designed it to keep the RF leads as short as possible. The front end compartment consists of two interlocking U shaped shells with tube sockets and terminal strips riveted to one half and the contacts for the coil catacomb on the other half. In order to keep the leads short, everything was wired and then the sockets and contacts were riveted. I spent about 2 hours with a long tipped soldering iron in order to remove enough leads to separate the two halves. I then replaced every passive component inside the RF section since I hope to never go through that process again! The only other issue I ran into was dirty contacts on some of the loctal tube sockets, except for the two rectifier tubes all tubes are loctal.
Performance wise, the receiver is very good. It uses ten IF transformers so even without the crystal filter tuning is critical. The audio section sounds very good and the standard speaker is a 10" Jensen unit. It has enough BFO injection that it works OK on SSB but it does require reducing the RF gain a bit. Both mechanical and electrical stability are excellent. I use mine with my Viking 500 and it sits on the Johnson Desk under a SX-88. The SX-88 is slightly better under extreme QRM because the sharp skirted 50 Khz. Hallicrafters IF allows choice of either sideband on AM, something the KP-81 cannot quite manage with its 455 Khz. IF. Otherwise the KP-81 does extremely well.
The retail price for the KP-81 was exactly double that of the SX-28A when it was introduced and is probably the reason so few were sold during its less than one year availability. Even at this price I imagine the company lost money on every receiver they sold. The tuning caps have the thickest invar plates I have run across and are beautifully constructed. They have counter balance springs to prevent the weight of the plates from rotating the tuning drive and the springs take time to set up properly. The entire tuning mechanism appears to be very expensive to design/build. The KP-81 is a good example of what you get when an engineer runs his own company, the product is wonderfully built and a pleasure to use but it never made money. All three of my receivers came with their manuals, all marked preliminary with a note that an updated manual would be mailed out later. I believe the company failed before this manual was completed. I scanned the material I have and it is on the Bama site including a transcribed letter from Karl Pierson providing the only alignment instructions available for the receiver. Under a different company, Karl designed the KE-93 mobile receiver which was described as providing KP-81 performance in a small package. I have one and it is a nice receiver but doesn't compare to the big one. I haven't decided whether to convert the front end in my other complete KP-81 back to loctals or restore it with the replacement tubes in place. They are neatly installed on metal plates so I will probably leave them in. Hopefully the metal plates will provide easier access to the front end components.
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