RCP 665 — Vol 3: Bench Role & Reference

3.1 Bench presence — when to reach for it

The 665 stays on the bench because (a) it still works after 80+ years, (b) the high-Z input impedance and the in-probe-tip RF rectifier topology are genuinely useful for vintage-radio voltage measurements that a modern DMM does worse, and (c) it is the instrument that first taught voltage measurement in the early learning years — the one that read the first transistor-stage base bias, the first ham-rig grid voltage, the first ohmmeter-checked resistor in a 1960s breadboard build. The bench-presence angle is honest: it stays because it earned the right to stay, not because it’s the best tool for any single modern measurement.

For modern day-to-day voltage measurement, a Fluke 87V or a Keysight U1272A handheld DMM is faster, more accurate, more flexible (current measurement, frequency counter, capacitance, diode test, thermocouple, true-RMS AC, etc.), and battery-powered. The 665 doesn’t replace those instruments — it complements them. It comes off the shelf when (a) a tube radio needs restoration, (b) the question is “is this 30-year-old paper capacitor leaking measurable current at its rated voltage,” or (c) the modern DMM is reading something suspicious on a high-Z node and a sanity check from a different topology is wanted. Three or four times a year. The rest of the time it sits as a monument to the discipline of measuring electricity the way it was measured when the engineers who built the first amateur HF stations were still in the field.

3.2 When to reach for the RCP 665

Table 1 — When to reach for the RCP 665

Use case	Instrument	Notes
Measure DC voltage on a tube radio’s grid circuit (1+ MΩ source impedance)	RCP 665	11 MΩ input + in-probe-tip rectifier topology. See Vol 1 (Why a VTVM matters).
Measure RF voltage at a tube radio’s plate circuit	RCP 665 with HV RF probe	The probe topology is the load-bearing advantage.
Test a paper capacitor for leakage (500 V applied)	RCP 665 megger range	The original mid-1940s use case.
Check a tube radio’s transformer winding insulation	RCP 665 megger range, 500 V	Verify HV supply integrity before trusting readings; see Vol 2 (Care & storage).

3.3 Resources

Manuals (in 02-inputs/manuals/):

• rcp-665/radiomuseum-radio-city-products-vacuum-tube-voltmeter-665-3081418.pdf and ...3081419.pdf — Radiomuseum schematic + service notes for the RCP 665
• rcp-665/RCP_422_665.jpg — original October 1944 Service magazine advertisement for the 665 and 422 Supertester

Vendor / authoritative web references:

• Radio City Products Company: company defunct since approximately 1970. The authoritative reference is Radiomuseum.org’s entry for the 665 at https://www.radiomuseum.org/r/radio_city_vacuum_tube_volt_ohm_665.html (free read; schematics and service notes behind a one-time membership at ~$25 USD). The BAMA mirror at https://www.bama.edebris.com may carry a free copy of the schematic; verify availability when needed.