Web-888 SDR Integration Series
National HRO-60 + Web-888 SDR Integration
First Published SDR Integration Guide for the Iconic National Boatanchor
The HRO-60 has no factory IF output and no existing SDR integration documentation — this guide provides the complete procedure for adding a high-impedance IF tap at 455 kHz and connecting the Web-888 SDR for panadapter and waterfall display
The National HRO-60 (1952–1964) is the final and most capable vacuum-tube receiver in the legendary HRO series that began in 1935. It is a common boatanchor restoration project — sought after for its mechanical precision (the famous epicyclic micrometer dial), plug-in coil architecture, and excellent sensitivity. The HRO-60 introduced dual conversion on the higher-frequency coil sets (A and B bands, above 7 MHz), regulated heater current for the HF oscillator and mixer, and an internal power supply — all improvements over the HRO-50-1 it replaced.[1]
The HRO-60’s 455 kHz IF chain is a straightforward two-stage amplifier with all interstage coupling via tuned IF transformers:[2]
Mixer output → Crystal Filter → T3 → 1st IF Amplifier (6BA6) → T4 → 2nd IF Amplifier (6BA6) → T5 → Detector/AVC/1st Audio (double-diode triode)
The crystal filter is positioned between the mixer output and the first IF transformer (T3). When the crystal filter is bypassed (SELECTIVITY switch position 0), signals pass directly from the mixer to T3 with only the IF transformer bandwidth limiting the passband. When the crystal filter is engaged (positions 1–4), progressively narrower selectivity is applied, down to approximately 200 Hz at the sharpest setting. The PHASING control provides a tuneable rejection notch for eliminating interfering carriers.[2]
AVC voltage is applied to both 6BA6 IF amplifier stages and the RF amplifier, controlling the receiver’s gain in response to signal strength. The AVC can be switched off for CW reception where manual RF gain control is preferred.
Unlike the Collins 51J-4 (which has a factory 50Ω IF output), the HRO-60 has no provision for external IF connection. A high-impedance IF tap must be added. There are two recommended tap locations, each with different trade-offs:
| Tap Location | Signal Path Position | Bandwidth Visible | Pros | Cons |
|---|---|---|---|---|
Option A: Plate of 2nd IF (before T5) |
After crystal filter, after both IF amps, before detector |
Limited by crystal filter setting (200 Hz to ~8 kHz) |
Strong signal; AVC-controlled level; lowest risk of loading |
Narrow bandwidth when crystal filter engaged |
Option B: Plate of mixer (before crystal filter) |
Before crystal filter, before IF amps |
Full IF transformer bandwidth (~8–15 kHz) |
Widest panadapter display; sees signals outside crystal filter passband |
Lower signal level; not AVC-controlled; higher risk of loading mixer |
Recommended: Option A (plate of 2nd IF amplifier). This location provides the strongest, cleanest, most stable signal with the least risk of degrading receiver performance. The signal is AGC-controlled, so the Web-888 sees a level-managed output that won’t overload the SDR’s ADC. The bandwidth limitation is acceptable for most applications — with the crystal filter bypassed (position 0), the IF transformer passband provides several kHz of visible bandwidth on the waterfall, sufficient for signal identification and tuning aid use.
For advanced users only: Option B (pre-crystal-filter tap at the mixer plate) provides a wider panadapter display but requires careful implementation to avoid loading the mixer circuit, which would degrade sensitivity and alter the crystal filter’s source impedance. A JFET source-follower buffer (2N5457 or MPF102) with very high input impedance is essential at this location. Option A can use the simpler capacitive coupling approach described below.
⚠ HIGH VOLTAGE WARNING: The HRO-60 chassis carries lethal voltages (250V+ B+). Disconnect power, discharge filter capacitors, and verify zero volts with a meter before working inside the receiver. The HRO-60’s internal power supply means B+ is always present when the receiver is powered — there is no separate power supply to disconnect.
High-Impedance Buffer Alternative: For a more robust installation that completely eliminates any risk of loading the IF chain, install a G4HUP-design PAT (Panoramic Adapter Tap) board or equivalent JFET buffer between the tap point and the coaxial output. The PAT board presents approximately 1 MΩ input impedance to the IF circuit and provides a 50Ω output to drive the coax. Power for the buffer (8–13.8V DC) can be taken from a switched B+ line inside the receiver that is only active during receive. This is the recommended approach for permanent installations.[3]
| Parameter | Setting | Notes |
|---|---|---|
Centre Frequency | 455.000 kHz | Adjust ±500 Hz to centre the display on the actual IF |
Span | ±5 kHz (10 kHz total) | Shows full IF transformer passband with crystal filter off |
RF Gain | Start minimum, increase | Prevent ADC overload from strong signals |
FFT Size | 4096 or 8192 | Higher gives better frequency resolution at 455 kHz |
Waterfall Speed | Medium | Adjust to taste — faster for CW, slower for SSB/AM |
With the HRO-60 tuned to a busy amateur band and the crystal filter bypassed (selectivity at position 0), the Web-888 waterfall will display the full IF transformer passband — typically 8–15 kHz wide depending on the alignment of T3, T4, and T5. Multiple signals within this passband will be visible simultaneously, creating a miniature panadapter effect centred on the receiver’s tuned frequency.
As the crystal filter is progressively engaged (positions 1 through 4), the visible passband narrows on the Web-888 waterfall. At position 4 (maximum selectivity, ~200 Hz bandwidth), only a single CW signal will be visible. The rejection notch created by the PHASING control will appear as a dark band on the waterfall — you can watch it move in real time as the PHASING knob is rotated. This provides a vivid visual confirmation that the crystal filter and phasing circuit are functioning correctly.
The Web-888 waterfall also reveals the HRO-60’s IF passband shape — any asymmetry, passband tilt, or spurious responses visible on the waterfall indicate IF alignment issues that may not be apparent from audio quality alone. This makes the SDR integration a powerful alignment verification tool.
The HRO-60 chassis is the signal ground for the entire receiver. The Web-888 introduces a second ground path through the coaxial cable shield and the Ethernet cable. This can create a ground loop that injects digital noise from the Web-888’s processor and Ethernet interface into the HRO-60’s IF chain — manifesting as a raised noise floor or spurious birdies on the waterfall.
Mitigation: Insert a galvanic isolator (1:1 RF transformer or common-mode choke) in the coaxial cable between the HRO-60 IF output and the Web-888 input. Mini-Circuits T1-1T or similar broadband RF transformers work well at 455 kHz. Alternatively, wind 10–15 turns of the RG-174 coax through a Fair-Rite #43 or #31 ferrite toroid to create a common-mode choke. Power the Web-888 from a linear supply if switching supply noise is detected.
The HRO-60 is a collectible receiver, and some owners are reluctant to modify it. The Option A installation described above is fully reversible — removing the coupling capacitor, coax cable, and rear-panel connector returns the receiver to stock condition with no permanent changes. The coupling capacitor connects to a tube pin, not to a printed circuit board trace, so removal leaves no physical evidence of the modification.
If even a single solder joint on the original chassis is unacceptable, the IF tap can be made externally using a probe clip on the 2nd IF tube’s plate pin cap (if the tube has one) or by constructing a plug-in adapter socket that fits between the 6BA6 and its socket, with the IF tap taken from the plate pin of the adapter. This requires fabrication of a tube socket adapter but leaves the receiver completely unmodified.
- Wikipedia. National HRO. Complete HRO series history from the 1935 original through HRO-60 (1952–1964): dual conversion on bands A and B (above 7 MHz), heater current regulation, internal power supply, plug-in coil architecture, military and amateur use. Wikipedia — National HRO
- Electronics Notes. National HRO Communications Radio. HRO circuit architecture: 455 kHz IF, two 6BA6 IF amplifier stages, crystal filter with 5-position selectivity and phasing control, BFO, AVC system, detector/1st audio double-diode triode, plug-in coil pack system. electronics-notes.com — National HRO
- KD2C Radio Products. PAT (Panoramic Adapter Tap) — Hi-Z Tap Board. G4HUP-designed high-impedance JFET buffer for IF tap applications: ~1 MΩ input impedance, 50Ω output, 8–13.8V DC supply, suitable for 455 kHz IF systems. kd2c.com — PAT Boards
- New England Wireless & Steam Museum. National NC HRO “Sixty” Receiver. HRO-60 history, production dates (1952–1964), pricing ($483 in 1952), restoration resources including Emmitt’s Fix It Shop. NEWSM — HRO-60
- Radiomuseum.org. National HRO-60 — Technical Specifications. Coverage 50 kHz – 30 MHz in 9 coil sets, dual conversion above 7 MHz, AM/CW/SSB (BFO), optional crystal calibrator XCU-60-2, optional NFM adapter NFM-83-50. radiomuseum.org — HRO-60
- Radiomuseum.org. The Evolution of the National HRO. HRO-60 as final vacuum-tube HRO: improvements over HRO-50-1, coil compatibility notes, 4H-4C heater regulator tube (6V6 substitute per National Service Bulletin), production through 1964. radiomuseum.org — HRO Evolution
Dave Powis, G4HUP (SK) — For the PAT (Panoramic Adapter Tap) high-impedance JFET buffer circuit design that makes non-invasive IF tapping practical on vintage receivers without factory IF outputs.
National Radio Company — For producing the HRO series from 1935 to 1964 — nearly three decades of continuous evolution that culminated in the HRO-60, a receiver whose mechanical precision and plug-in coil architecture remain unsurpassed in the boatanchor world.
Emmitt’s Fix It Shop — For the detailed HRO-60 restoration documentation that has guided countless HRO-60 restorations and makes this SDR integration guide possible by establishing the baseline of a properly functioning receiver.