Note
ARRL Handbook, 25th edition (1948, copyright 1947), advertising section p. 47. Prototype. (After the 1947 Handbook ad, the SP-600 was not advertised again in QST until August 1952.)
QST, Aug 1952, page 77.
Allied Radio catalog 1953 (copyright 1952), p. 37.
CQ, Apr 1963, p. 103.
QST, Jan 1964, p. 175.
QST, Nov 1961, p. 133. ($1140 price)
QST, May 1969, p. 4. (some details here)
QST, Nov 1969, p. 4
The Hammarlund SP-600, introduced in 1951, is unequivocally the zenith of Hammarlund's "Super-Pro" line of communications receivers and arguably one of the most iconic and successful vacuum tube receivers ever produced. Its exceptional performance, legendary reliability, and adaptability to a vast array of demanding applications solidified its position as an industry standard for over two decades. From military intelligence and diplomatic communications to government monitoring, commercial broadcasting, and scientific research, the SP-600 was the workhorse of professional shortwave reception, earning it a revered status among operators and collectors alike.
While many sources (including general overviews and some collector sites) often cite 1951 as the introduction year for the Hammarlund SP-600, more detailed historical research, particularly from dedicated Hammarlund historians like those at the Western Historic Radio Museum, reveals a slightly more complex timeline:
1947/1948: Some sources, including Wikipedia, state that the SP-600 was "introduced" in 1947 (or 1948 as an advertisement). This almost certainly refers to announcements, initial advertisements, or perhaps very early pre-production units or even specific contract work, as Hammarlund was known for getting ahead of itself and advertising products before they were actually available. See, for example, ARRL Handbook, 25th edition (1948, copyright 1947), advertising section p. 47.
1950 (First Contracts): The first actual production contracts for the SP-600 receiver were issued in late 1950.
September 1951 (First Deliveries): It was September 1951 before the first Hammarlund SP-600 receivers were actually ready to be delivered on those contracts.
So, while the concept or announcement might have been as early as 1947-1948, the true mass production and delivery of the SP-600 series began in late 1951.
A New Standard of Excellence:
Building on the robust foundation of its predecessors, specifically the SP-200 and SP-400, the SP-600 was designed from the ground up for unparalleled stability, selectivity, and continuous duty.
Key design philosophies and advancements of the SP-600 include:
Conversion scheme: Single Conversion (Below 7.4 MHz): On its lower frequency bands (up to 7.4 MHz), the SP-600 operated as a single-conversion superheterodyne receiver with a 455 kHz IF. This was the same architecture as the SP-10, SP-100, and SP-200. This design was sufficient at these lower frequencies where image rejection was less of a concern. Double Conversion (Above 7.4 MHz): For the higher frequency bands (from 7.4 MHz to 54 MHz), the SP-600 automatically switched to a double-conversion superheterodyne design.
First IF: The incoming signal was first converted to a higher, fixed first IF of 3.955 MHz. This high first IF provided excellent image rejection on the high bands, a common weakness of single-conversion receivers operating at those frequencies. Second IF: The 3.955 MHz signal was then converted a second time to the receiver's main second IF of 455 kHz, where the receiver's powerful IF amplifiers and crystal filters took over to provide the legendary selectivity.
This sophisticated, dual-conversion design was a major leap forward and a key reason for the SP-600's superior performance, particularly at the upper end of its tuning range. It allowed the receiver to maintain a consistently high level of image rejection across its entire wide frequency range.
Stability: The SP-600 was engineered for exceptional frequency stability, a crucial requirement for professional monitoring applications. This was achieved through meticulous mechanical construction, temperature-compensated components, and highly regulated power supplies. Its dial calibration was remarkably accurate, often within a few kilohertz across its entire range.
Ruggedized Construction: Even more so than previous Super-Pros, the SP-600 was built like a battleship. Its heavy-gauge steel chassis, robust tuning mechanisms, and high-quality components ensured reliable operation under the most demanding conditions, including military field deployments and continuous 24/7 operation in fixed stations. Many units still function perfectly decades after production.
Comprehensive Frequency Coverage: The SP-600's primary tuning range typically covered from 0.54 MHz to 54 MHz (or sometimes 0.54 to 30 MHz for earlier versions), often divided into six or seven bands. This wide coverage spanned the broadcast band, all major shortwave bands, and extended into the lower VHF spectrum, making it incredibly versatile. Specialized VLF versions (e.g., SP-600-VLF) were also produced, covering 10 kHz to 540 kHz, showcasing its adaptability.
Selectivity (IF Bandwidth): The hallmark of the Super-Pro line, the continuously variable IF bandwidth control, was perfected in the SP-600. It offered bandwidths from a razor-sharp 200 Hz (perfect for CW) up to a broad 13 kHz (for high-fidelity AM broadcasting), achieved through adjustable coupling of its high-Q IF transformers. Specifically: 13, 8, 3, 1.3, 0.5, and 0.2 kHz (-6dB)
Dual Crystal Filters: Unlike earlier Super-Pros that had one crystal filter, the SP-600 incorporated two. One was for general narrow-band reception (around 3 kHz), and a much sharper second filter (often 200 Hz) was used for extreme selectivity, particularly for CW and RTTY signals. This dual-filter system provided unparalleled interference rejection.
Highly Sensitive Front End: The SP-600 employed an exceptionally sensitive front end, featuring two stages of RF amplification with a low noise figure. This, combined with its exceptional gain, allowed it to pull in very weak signals from across the globe.
Powerful and Clean Audio: The audio section of the SP-600 was designed for clarity and power. It typically used a push-pull 6L6 or 6V6 audio output stage, capable of delivering up to 2.5 watts of clean audio into an external loudspeaker. This allowed for comfortable long-term listening and monitoring. Note that previous versions of the Super Pro pumped out 14 watts of audio.
Precision Tuning and Bandspread: The large, smoothly geared main tuning dial, coupled with a separate, precise bandspread dial, allowed for very accurate and repeatable tuning.
S-Meter and Other Controls: A large, illuminated S-meter provided accurate signal strength indication. Other comprehensive controls included RF Gain, BFO with pitch control, noise limiter, AVC on/off, and tone control.
Separate Power Supply: As with all Super-Pros, the SP-600 utilized a heavy, separate power supply unit. This critical design choice minimized heat, hum, and electromagnetic interference within the sensitive receiver chassis, contributing significantly to its outstanding performance and reliability.
Vast Array of Variations (J-Series Suffixes):
One of the most remarkable aspects of the SP-600 is its sheer number of variations, estimated to be over 40 different models produced throughout its long lifespan. These variations were usually designated by suffixes following "SP-600," most commonly beginning with "J" for JAN (Joint Army/Navy) specifications, indicating military-grade components and construction. Subsequent numbers (e.g., JX-1, JX-6, JX-17, JX-21) typically denoted specific contracts, minor circuit revisions, or tailored features.
Some notable variations include:
SP-600-JX-1: An early, general-purpose military contract version (R-274A/FRR).
SP-600-JX-6: A common Navy version (R-274B/FRR), featuring a crystal trimmer and wider BFO range.
SP-600-JX-17: A unique model designed for diversity reception. These receivers had additional front panel controls for diversity mode and are easily identified by their three distinctive red knobs (RF gain, IF gain, and BFO). They were used in pairs for improved signal reception under fading conditions.
SP-600-JX-21: A very widely produced version for various agencies.
SP-600-JX-21A: The "holy grail" for many collectors, produced in the late 1960s to early 1970s. This was the last, most advanced version, featuring 22 tubes (more than other models), including a dedicated product detector circuit. This made it far superior for receiving modern single sideband communications. It had a redesigned front panel with different knobs.
SP-600-VLF: Specialized versions for very low-frequency reception, vital for navigation and submarine communications.
Other numerical suffixes: Many unique model numbers existed for specific government agency contracts (e.g., CIA, FBI) or specialized commercial applications.
Legacy and Collectibility:
The Hammarlund SP-600's production spanned over 20 years, a remarkable feat in the rapidly evolving electronics industry. Its eventual discontinuation in the early 1970s marked the end of an era for mass-produced, high-performance vacuum tube communications receivers. The advent of solid-state technology made tube-based designs obsolete for new commercial and military procurement.
Today, the SP-600 is a highly prized receiver. Its quality build, iconic appearance, and still-impressive performance make it a favorite among vintage radio collectors, amateur radio operators, and shortwave listeners. Many thousands of these units were built, ensuring their continued availability, yet their enduring quality means countless examples are still operational, serving as tangible links to a golden age of professional radio communications. The SP-600 is more than just a receiver; it's a testament to engineering excellence and a symbol of a bygone era in telecommunications.
(2) 6BA6 (RF amp)
6AC7 (FFO)
6C4 VFO,
(2) 6BE6 (mixer)
6BA6 (gate)
6C4 (FFO)
(2) 6BA6 (IF amp)
6BA6 (driver)
6BA6 (buffer)
6C4 (BFO)
6AL5 (detector/AVC)
6AL5 (limiter/meter)
12AU7 (IF out/AF amp)
6V6GT (output)
0A2 (regulator)
5R4GY (rectifier)
6AL5
Power: 120/240 VAC, 130 W, 50/60 Hz
References
Brief description. Electronics World, Jun 1970, p. 74. (SP-600 JX21A)
