As many as 40 (!) different SP-600 versions have been manufactured. It started with the SP-600 JX-1 in 1951. Most versions have a frequency range as stated above, but there were some exceptions. For instance the SP-600 VLF only covers 10-540 kHz and the SP-600-JLX-2 (1951) covers 100-400 kHz and 1.35-29.7 MHz. The last SP-600 was the SP-600 JX-21A which was in production until 1972. This was also the only SP-600 with a SSB product detector.
From the outside, most versions look quite similar, but circuit modifications where made throughout its lifetime. Many models were custom ordered for military or naval duties. It is not believed that the SP-600 was used on board ships, only on shore stations.
For reference, here is a list of all the versions I could find: SP-600-JX-1, SP-600-JLX-2, SP-600-J-3, SP-600-J-4, SP-600-JX-6, SP-600-JX-7, SP-600-JX-8, SP-600-JL-9, SP-600-JX-10, SP-600-J-11, SP-600-JX-12, SP-600-J-13, SP-600-JX-14, SP-600-JLX-15, SP-600-JL-16, SP-600-JX-17, SP-600 JX-18, SP-600-J-19, SP-600-J-20, SP-600-JX-21, SP-600-J-22, SP-600-JLX-23, SP-60O-JL-24, SP-600-J-25, SP-600-JX-26, SP-600-JLX-27, SP-600-JX-28, SP-600-JX-29, SP-600-JX-30, SP-600-VLF-31, SP-600-JX-32, SP-600-JLX-33, SP-600-JL-34, SP-600-JX-35, SP-600-JX-36, SP-600-JL-24, SP-600-JX-37, SP-600-VLF-38, SP-600-JX-39 and SP-600-JX-21A.
The SP-600 requires an external loadspeaker.
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It's a great receiver that compares with the best of today at least as far as sensitivity and selectivity are concerned and it's miles ahead of modern rigs in many ways. Dial read out of course is not on par with today's digital receivers but it is quiet, sensitive and selective and dial read out is very good for a 60 year old receiver. When I bought it half the RF coils had been blasted out, probably transmitted into but I had it restored about 7 or 8 years ago and since then it has been a solid reliable well performing rig, in fact I'm listening to it now. It's a great DX machine, SW and BCB, it easily picks up the European BCB splits and is selective enough with it's 6 selectivity positions to hear them. The audio is excellent, very clear. I compare it to my restored R-390A and go back and forth between them, it's that good to operate. You can also listen to SSB with it by turning on the BFO and backing off on the RF gain. Once it warms up (10 minutes) it's very stable, usually if you read negative reports of this radio it's because it probably hasn't been restored, they need to have the paper caps replaced which is not a fun job but once it's done you'll never have to do it again. It weighs a ton but heats up the shack in the winter and overall is a very pleasurable receiver to own and use, a good restored SP-600 with outlive you.