Choosing an SDR Front-End for the Hammarlund SP-600: Variants, Architectures, and Companions
The Hammarlund SP-600 Super-Pro is the R-390A’s older, more stylish, slightly less disciplined cousin. Released in 1951 and produced in more than forty distinct variants through 1972, it was the receiver of choice for signal intercept stations, CIA and FBI listening posts, NBC network timing, diversity reception sites, and commercial long-haul circuits. It was never quite the precision instrument the R-390A is — it drifts more, its crystal IF filters have different skirts than the R-390A’s mechanical ones, and its dial calibration is a sporting proposition — but its signature smooth tuning feel, 20-tube double-conversion architecture, and robust professional construction give it a character the R-390A can’t match.
Pairing an SDR with an SP-600 raises a question that doesn’t arise with the R-390A: which SP-600? The JX-17 diversity receiver, the JX-21A with its SSB product detector, the JLX longwave variants, the VLF-31/38, and the run-of-the-mill JX-1 through JX-26 all have meaningfully different characteristics. Some of those differences change the SDR integration calculus significantly.
The Variant Question
The SP-600 family’s naming convention is consistent once you know the code:
- J — all components meet JAN (Joint Army Navy) specifications. Military build quality.
- X — crystal-controlled fixed-frequency oscillator with six crystal positions for spot frequencies.
- L — longwave coverage substituted for the mediumwave broadcast band (for NDB / aeronautical use).
- VLF — very low frequency variants, covering 10–540 kHz only, not the full HF range.
- Number suffix — sequential engineering revision, roughly chronological.
A representative subset of variants, chosen for their relevance to SDR pairing decisions:
| Variant | Year | Frequency Range | Distinguishing Feature | SDR Implications |
|---|---|---|---|---|
SP-600-JX-1 |
1951 |
540 kHz – 54 MHz |
Base military variant; R-274A/FRR designation. 20 tubes, 455 kHz / 3.955 MHz dual-IF. |
Standard IF-tap approach applies. Reference variant for most discussion. |
SP-600-JX-6 |
1952 |
540 kHz – 54 MHz |
R-274B/FRR. Early production refinements. |
As JX-1. |
SP-600-JX-17 |
1952 |
540 kHz – 54 MHz |
Diversity reception variant, Air Material Command. Red metal knobs. Two additional PL-259 connectors for external 2nd VFO inputs. Most common surviving variant. |
Unique opportunity: the external 2nd VFO inputs offer a path for SDR-driven frequency synthesis, not just IF tapping. |
SP-600-JLX-2 / 15 / 23 |
1951–54 |
100–400 kHz + 1.35–29.7 MHz |
Longwave + HF, skipping the broadcast band. J and X both present. |
The split coverage means a single wideband SDR on the antenna covers both bands simultaneously — an advantage. |
SP-600-JLX-27 |
1957 |
200–400 kHz + 540 kHz–29.7 MHz |
Later longwave-inclusive variant with broadcast band retained. |
As JLX-2 but with broadcast coverage intact. |
SP-600-JX-21 |
1953 |
540 kHz – 54 MHz |
Crystal frequency control, BFO ±10 kHz. Navy R-274B/FRR. |
As JX-1. Wider BFO range useful for SSB if no product detector modification. |
SP-600-JX-21A |
1961–72 |
540 kHz – 54 MHz |
22 tubes, built-in SSB product detector. Last production variant. Knobs without metal skirts; engraved panel markings. |
Native SSB means no need for SDR-side product detection. Otherwise standard IF-tap approach. |
SP-600-VLF-31 / VLF-38 |
1957 / 1963 |
10–540 kHz |
VLF-only specialty variants. |
Completely different SDR calculus: most consumer SDRs have poor VLF coverage or performance. RSPdx and RX-888 are the viable parallel-RX options; IF-tap still works normally. |
R-450/FRR-28 (Northern Radio) |
1957+ |
540 kHz – 54 MHz |
SP-600 JX modified by Northern Radio for FSK work. AN/URA-8b converter-comparator. X-tal position usually removed. |
Often already has auxiliary IF/FSK outputs — check before modifying. |
If you don’t know which variant you have, the typeplate is on the cover of the RF unit, and Les Locklear’s designations list is the authoritative reference.
What the SP-600 and R-390A Don’t Share
Four technical differences between the SP-600 and the R-390A meaningfully change which SDR choices make sense:
1. Dual conversion above 7.4 MHz
Below 7.4 MHz the SP-600 is a single-conversion receiver with a 455 kHz IF, exactly like the R-390A. Above 7.4 MHz a crystal-controlled first oscillator converts down to a 3.955 MHz first IF, which is then heterodyned to the 455 kHz second IF. This gives you two possible tap points on the upper HF bands: the conventional 455 kHz tap, or a 3.955 MHz first-IF tap with potentially much wider instantaneous bandwidth. No R-390A tap has this option.
2. Crystal IF filters, not mechanical filters
The SP-600’s selectivity is shaped by crystal IF filters, not Collins mechanical filters. Skirt shape is different, and the filter bandwidth positions feel different under a waterfall display. Pre-filter taps are still the right approach, but the tap-point IF envelope is shaped differently than an R-390A’s — typically 40 to 80 kHz before the crystal filter rather than the R-390A’s broader 80 to 100 kHz envelope.
3. Frequency drift
SP-600 drift specification is 0.001 to 0.01 percent after warm-up — 10 to 100 ppm. At 14 MHz that’s 140 Hz to 1.4 kHz of drift over a listening session. The R-390A’s crystal-stabilised PTO is considerably better. This matters for panadapter work because the SDR’s centre frequency is referenced to the SP-600’s LO chain; when the SP-600 drifts, the waterfall drifts with it unless you periodically recalibrate or rely on the SDR’s AFC.
4. No factory IF output connector
The R-390A has a rear-panel IF OUTPUT (J116 on most variants). Standard SP-600 variants do not. Every SP-600 panadapter installation is a modification by definition — a buffered tap must be added. This isn’t a showstopper, but it means budget for a small metalwork job or accept a more adventurous internal tap.
Architecture Recap
The same two architectures discussed in the R-390A article apply here, with SP-600-specific nuances:
IF-tap panadapter: Buffered tap on V7 plate (or V6 plate for >7.4 MHz bands only), SDR tuned to 455 kHz (or 3.955 MHz). The SP-600’s wider crystal IF bandwidth and the optional dual-conversion first-IF tap give you somewhat more flexibility than the R-390A in choosing how much spectrum to display. The drift penalty is the cost.
Parallel direct-sampling receiver: Split the antenna between the SP-600 and a wideband SDR. Same considerations as the R-390A. For the VLF-31/38 variants this architecture is actually harder to achieve with consumer SDRs, since most have poor performance below 500 kHz.
The Comparison Matrix — SP-600 Context
Same six SDRs, ratings adjusted for the SP-600 use case specifically. Ratings that differ meaningfully from the R-390A article are noted in the discussion that follows.
| Criterion | QCom | Web-888 / KiwiSDR | Hermes Lite 2 | RX-888 MkII | SDRplay RSPdx | Airspy HF+ Disco |
|---|---|---|---|---|---|---|
455 kHz IF-tap suitability |
||||||
3.955 MHz first-IF tap suitability |
— tune to 3.955 MHz |
— wide capture |
||||
Drift tolerance |
||||||
Parallel HF RX suitability |
||||||
VLF coverage (for VLF-31/38) |
||||||
JX-17 external-VFO integration |
||||||
Physical / aesthetic fit |
||||||
Approx. price (USD) |
$349 |
$300 |
$325 assembled |
$300 |
$220 |
$169 |
Deep Dive — SP-600-Specific Considerations
Quantum SDR QCom
The QCom remains the clean “no-PC” answer for IF-tap panadapter duty and it suits the SP-600’s vintage aesthetic as well as it does the R-390A’s. The caveat with the SP-600 is drift management. The QCom’s IF input mode expects a reasonably stable centre frequency; an SP-600 that’s been warming up for less than 20 minutes will walk the panadapter display across the screen enough to be distracting. In practice this means budgeting for periodic re-centring from the touchscreen, or waiting for the SP-600’s VFO to settle before trusting the waterfall as anything but a relative indicator. On a fully warmed-up SP-600-JX-21A or late JX variant with modern replacement tuning capacitor dielectric, drift is manageable; on a fresh-from-storage unrestored JX-1, less so.
Web-888 / KiwiSDR
The parallel-receiver role that Web-888 fills so well alongside the R-390A applies equally to the SP-600. Where it becomes more interesting is with the JLX longwave variants: the Web-888’s 0–62 MHz coverage includes the 100–400 kHz band that the JLX-2/15/23/27 specifically targets, so a single Web-888 on the antenna gives you remote visibility into exactly the spectrum the SP-600 is set up for. For the VLF-31/38 variants below 540 kHz, the Web-888 is serviceable but not ideal — its strength is HF, not VLF.
Hermes Lite 2
The HL2’s interesting SP-600-specific role is with the JX-17 diversity variant. That variant has two PL-259 connectors on the rear panel intended for external second-VFO inputs — a feature Hammarlund provided so that a pair of JX-17s could share a common frequency reference for true diversity reception. A Hermes Lite 2’s TX chain, attenuated and buffered appropriately, can drive those inputs and effectively provide SDR-controlled tuning of the SP-600’s second conversion. This is a non-trivial modification and not for the faint of heart, but it’s the only path in this comparison that offers anything like genuine frequency control integration on an SP-600. On non-JX-17 variants the HL2’s role reverts to straightforward parallel-receiver / QRP-transmitter duty.
RX-888 MkII
The RX-888 MkII’s headline advantage over the rest of the pack — full 32 MHz of instantaneous capture at 16 bits — is particularly interesting for the SP-600 in one specific scenario: 3.955 MHz first-IF tapping on the upper HF bands. The 3.955 MHz first IF has a much wider pre-mixer bandpass than the 455 kHz second IF, and an RX-888 tuned to 3.955 MHz can capture all of it and then some. You get a genuinely wide panadapter view — on the order of several hundred kHz — that none of the narrower SDRs can match. The trade-off is that this only works on bands above 7.4 MHz; below that, you’re back to the 455 kHz tap where the RX-888 is overkill.
SDRplay RSPdx
The RSPdx earns a special mention for the VLF-31 and VLF-38 variants. Its coverage extends down to 1 kHz with a dedicated HDR (high dynamic range) mode below 2 MHz, which is the best VLF performance in this comparison for a reasonable price. For a VLF-31 owner wanting a parallel wideband receiver that actually sees the same spectrum the SP-600 does, the RSPdx is the obvious choice. As an IF-tap panadapter for standard HF variants it’s as capable here as on the R-390A — SDRuno handles 455 kHz or 3.955 MHz centre frequencies cleanly.
Airspy HF+ Discovery
Everything said about the HF+ Discovery in the R-390A article applies here, with one SP-600-specific plus: its excellent PC-side AFC implementation in SDR# handles the SP-600’s drift considerably better than the QCom’s standalone IF mode does. For the technically-correct IF-tap panadapter build on an SP-600 — the one where you care about spectrum quality more than aesthetics — the HF+ Discovery paired with SDR# and a properly buffered V7 plate tap is hard to beat.
Recommendations by Use Case and Variant
Standard JX-1 through JX-26 / JX-21A (HF, no special considerations)
Best standalone panadapter: QCom, accepting that drift will require occasional manual re-centring.
Best PC-based panadapter: Airspy HF+ Discovery with SDR# for good AFC behaviour.
Best parallel HF receiver: RX-888 MkII for local work; Web-888 for remote access.
SP-600-JX-17 (diversity variant with external VFO inputs)
Unique option: Hermes Lite 2 driving the rear-panel second-VFO inputs for SDR-controlled tuning. This is an advanced modification but unlocks capability no other combination offers.
Conventional path: Same recommendations as standard variants — nothing stops you from ignoring the diversity inputs and using the JX-17 as a straightforward receiver.
SP-600-JLX-2 / 15 / 23 / 27 (longwave + HF variants)
Best parallel coverage match: Web-888, whose 0–62 MHz range includes both the longwave segment (100–400 kHz or 200–400 kHz depending on variant) and the full HF range that the SP-600 covers.
For IF-tap: Same recommendations as standard variants.
SP-600-VLF-31 / VLF-38 (10–540 kHz)
Best parallel receiver: SDRplay RSPdx in HDR mode, with coverage down to 1 kHz. Second choice: Airspy HF+ Discovery, down to 0.5 kHz but narrower instantaneous bandwidth.
IF-tap panadapter: The VLF variants still use the 455 kHz IF, so conventional tap approaches work unchanged — any of the standard IF-tap recommendations applies.
Avoid: QCom (no meaningful VLF coverage for parallel-RX duty).
Northern Radio R-450/FRR-28 and similar modified variants
Check before modifying: these often already have auxiliary IF or FSK outputs installed by Northern Radio. The wheel may already be invented on your specific unit. Any of the SDRs in this list can then connect to existing outputs without further SP-600 modification.
Closing Thoughts
The SP-600 doesn’t demand the same reverent treatment the R-390A sometimes attracts — it’s a working receiver, usually modified several times over its sixty-plus years of service, and most of what you can do to integrate an SDR has probably already been tried by someone. The variant question is where the genuine intellectual interest lives. A JX-17 is not a JX-21A is not a VLF-31, and the right SDR companion differs accordingly.
For most owners of standard HF-range JX variants, the recommendations collapse to the same short list as the R-390A article: Airspy HF+ Discovery for PC-based panadapter work, QCom for standalone operation, RX-888 MkII for wideband parallel reception, Web-888 for remote access. The SP-600’s main gifts to this comparison are the dual-conversion first-IF tap option above 7.4 MHz (which rewards wideband SDRs uniquely) and the JX-17’s external-VFO inputs (which open a door no other receiver in this class offers).
If you’re pairing with a JLX longwave variant or one of the VLF-31/38 specialty sets, the SDR choice shifts noticeably — the RSPdx in particular moves from “solid generalist” to “best-in-class for this variant”. Pay attention to which SP-600 you have before buying the SDR.