The Collins R-390A is arguably the finest tube-era military HF receiver ever built — but it was designed in the early 1950s for AM and CW reception. As amateur and military communications transitioned to single-sideband (SSB), several aftermarket solutions emerged to bring SSB capability to this legendary receiver. This article compares the three principal approaches documented in the R-390A community, drawing on the work of Bob Thomas (VE3TOU), Dr. Dallas Lankford, and Sherwood Engineering.

1. Treetop Circuits SB-390

The SB-390 was the product that most defined Treetop Circuits’ reputation in the boatanchor community. It mounts externally on the receiver’s back terminals, is powered from the receiver itself, and leaves AM operation completely unaffected. The adapter uses a single-balanced product detector architecture with wide dynamic range, paired with a dual-time-constant AGC system designed to resist blocking on impulse noise while providing the fast attack and smooth recovery that makes SSB copy comfortable over long operating sessions.

Pros

• Purpose-built specifically for the R-390 and R-390A receiver
• Mounts externally — no internal modifications required
• AM operation completely unaffected
• Powered from the receiver (no external power supply needed)
• Dual-time-constant AGC with impulse noise resistance for comfortable long-session SSB listening
• Refined through five documented revisions over its production run
• Full designs released as public domain — schematics, PCB layouts (ExpressPCB format), alignment procedures, and test rig instructions freely available for self-build
• Over 700 units shipped across all Treetop models; widely regarded as the finest commercially available SSB adapter for vintage military HF receivers

Cons

• No longer commercially available — must be sourced used or self-built from public domain plans
• External mounting adds a box/enclosure to the rear of the receiver
• S-meter behaviour is affected in SSB mode — the meter does not read as accurately as in AM
• Succession of production (Perry Sandeen, KM6FQV acquired the business) proved challenging

2. Lankford 2-Diode SSB AGC Modification

Lankford’s most famous contribution to the R-390A community, the 2-diode SSB AGC modification adds two small-signal diodes in parallel with resistors in the AGC circuit of the IF deck. The modification is so widely adopted that references to “the Lankford mod” on the R-390 mailing list require no further explanation. The Y2K-R3 reference manual and the R-390A FAQ at r-390a.net both document it extensively.

“It is so simple and reversible, and makes such a huge difference in SSB reception, I can’t think of a good reason not to do it.”

— R-390 MAILING LIST CONTRIBUTOR, DESCRIBING THE LANKFORD 2-DIODE SSB AGC MOD

Pros

• Extremely simple — two diodes (1N270 germanium, or 1N4148/1N914 silicon) and optionally a 47 pF capacitor
• Completely reversible — no parts removed, no holes drilled
• Entirely internal; no external box or enclosure
• Virtually free (a few cents in parts)
• Makes the stock AGC function properly on SSB — operators can listen at full RF gain without riding the gain control
• The most universally adopted R390A SSB modification in the community
• Documented in the Y2K-R3 reference manual and the R-390A FAQ
• Lankford also recommended increasing BFO injection (47 pF across existing 12 pF C535) to prevent audio distortion during SSB reception

Cons

• AGC modification only — does not add a product detector
• SSB audio quality is limited by the receiver’s stock AM envelope detector
• Requires opening the IF deck and soldering (though minimally invasive)
• Requires technical comfort with schematics — Lankford’s documentation is rigorous but demanding
• Does not address the fundamental limitation that the R-390A’s detector was designed for AM, not SSB demodulation

3. Sherwood Engineering SE-3

The Sherwood SE-3 is a premium outboard product detector and synchronous detector that connects to the receiver’s IF output. It is not designed specifically for the R-390A but works well with it and with many other vintage receivers. Clemens Stubbe Østergaard, a longtime R-390A community member, has described it as something he values as much today as when he first acquired it.

Pros

• Superior SSB audio quality — true high-quality product detector
• Also provides synchronous AM detection, significantly improving AM reception quality
• Works with multiple receivers — a versatile investment, not limited to the R-390A
• No internal modifications to the receiver required
• Highly valued by serious DXers and collectors worldwide

Cons

• Expensive — a premium product with premium pricing
• Requires its own separate power supply
• Adds to the operating footprint (external unit)
• Not R-390A-specific — integration is not as seamless as the purpose-built SB-390
• Very limited availability on the used market

Comparison Table

Recommendation

For most R-390A operators, the practical approach is straightforward: install the Lankford 2-diode AGC mod first. It costs virtually nothing, is fully reversible, and is universally recommended as the essential starting point for any R-390A used on SSB. From there, pursue an SB-390 — either built from the public domain plans or found on the used market — if you want dedicated product detector capability without compromising the receiver’s internals. The Sherwood SE-3 represents the premium path for operators who demand the best possible audio quality across all modes and who operate multiple vintage receivers.

Sources & Further Reading

• Bob Thomas, VE3TOU — Boatanchor Legends Tribute (VK6ADA)
• Dr. Dallas Lankford — Boatanchor Legends Tribute (VK6ADA)
• Clemens Stubbe Østergaard — Boatanchor Legends Tribute (VK6ADA)
• R-390 Series Frequently Asked Questions (FAQ) — r-390a.net
• Hollow State Newsletter Archive (VK6ADA)
• Dallas Lankford Collection of Modifications and Projects — kongsfjord.no

Mike Peace VK6ADA / r-390a.net Administrator