Collins 32S-3 PA Neutralisation — Step by Step
The Procedure Most Likely to Be Skipped — And Most Consequential When Wrong
Definitive guide to 6146 PA neutralisation in the 32S-3/3A, KWM-2/2A, and 32S-1 — covering both the ceramic trimmer and air-variable circuits, the 6146B compatibility issue, oscilloscope technique, and the driver shield problem that kills finals
PA neutralisation is the most consequential alignment procedure in the Collins S-Line transmitter family — and the one most frequently skipped. The 6146 final amplifier tubes in the 32S-3 operate in a push-pull circuit where internal plate-to-grid capacitance provides a feedback path that can sustain oscillation, particularly at VHF frequencies well above the operating band. The neutralisation capacitor (C184) introduces a counter-phase signal that cancels this internal feedback path. When C184 is correctly adjusted, the PA is stable; when it is not, the PA can break into parasitic oscillation that is invisible on an HF wattmeter but devastating to the tubes and surrounding components.[1]
An un-neutralised or poorly neutralised Collins transmitter will exhibit one or more of the following: plate current dip and maximum output occurring at different PA TUNING settings; unexplained high plate current; erratic ALC behaviour; glowing 6146 plates at idle; excessive grid current; and in severe cases, destruction of the neutralising capacitor, the coupling capacitor, and the 6146 tubes themselves.[2]
⚠ The 32S-3 has a marginal neutralising circuit from the factory. Collins originally used poorly chosen component values that were later revised via circuit updates. If you are running 6146B or 6146W tubes in an early-circuit 32S-3 or KWM-2, the original ceramic trimmer neutralisation circuit may not have sufficient range to accommodate the higher plate-to-grid capacitance of these later tube variants. The air-variable upgrade is strongly recommended.[1]
Collins produced the 32S-3 and KWM-2 with two distinctly different neutralisation circuits over the production run. Identifying which circuit your transmitter has is the essential first step before attempting neutralisation.[3]
| Feature | Early Circuit (Ceramic Trimmer) | Late Circuit (Air Variable) |
|---|---|---|
C184 Type | Erie ceramic trimmer, 1.8–8.7 pF | Johnson air variable, ~8.1 pF max |
Series Capacitor | 2 kV rated | 6 kV rated (0.001 µF) |
Series Resistor | 1 watt | 2 watt (470 kΩ) |
6146B/W Compatible | Marginal — may not have sufficient range | Yes — adequate range for all 6146 variants |
Adjustment Tool | Non-metallic alignment tool (ceramic trimmer) | Non-metallic tool from below chassis |
Identification | Ceramic disc visible on phenolic board in PA | Small air variable mounted through phenolic board |
Applies To | Early 32S-3, early KWM-2 | Late 32S-3A, 9th ed. KWM-2/2A |
Upgrade Recommendation: The air-variable neutralisation upgrade is available as a kit (W7KSG and others have produced them) and is strongly recommended for all early-circuit units, particularly if 6146B or 6146W tubes are being used or may be used in the future. The upgrade replaces the ceramic trimmer, series capacitor, and series resistor with higher-rated components that provide more stable, wider-range neutralisation adjustment.[4]
The original 6146 tube was designed for HF service and has relatively low plate-to-grid capacitance. The later 6146B and 6146W variants, designed by RCA for improved performance, have slightly different internal geometry that results in higher stray capacitance — particularly from the larger anode structure coupling to external elements. While these tubes work perfectly in most transmitters, Collins’ original ceramic trimmer neutralisation circuit in early 32S and KWM-2 units had insufficient adjustment range to compensate for the increased capacitance, leading to the widespread reports of PA instability and “hot” 6146B operation that circulate in the Collins community.[3]
The 6146B and 6146W are fully compatible with the Collins transmitters when the neutralisation circuit has been upgraded to the air-variable configuration. With the upgraded circuit, any member of the 6146 family can be used without concern.[5]
This procedure applies to the 32S-3/3A transmitter, the KWM-2/2A transceiver, and with minor differences to the 32S-1. The principle is the same for both circuit variants: remove plate and screen voltage from the 6146 PA tubes, apply drive signal, and adjust C184 for minimum RF feedthrough from the grid circuit to the plate circuit. A properly neutralised PA will show zero (or near-zero) RF output with the plates dead.[6]
⚠ HIGH VOLTAGE WARNING: The PA plate supply carries approximately 800V DC in the 32S-3 (higher in the KWM-2 with some power supplies). Always discharge the PA before touching anything inside the PA compartment. Short the plate supply to chassis ground through the plate caps before removing tubes. Verify with a voltmeter before proceeding.
Alternative Method — RF Voltmeter: If an oscilloscope is not available, connect an RF voltmeter (HP 410B or equivalent VTVM with RF probe) to the antenna output jack instead of using a scope at the plate pin. Adjust C184 for minimum RF voltage at the antenna output. The scope method is superior because it allows you to distinguish between feedthrough at the operating frequency and spurious VHF parasitic energy, but the RF voltmeter method works well for the basic adjustment.
What You Should See on the Oscilloscope: Before adjustment, the scope shows a clear RF sinewave at the plate pin — this is the feedthrough. As C184 approaches the correct value, the sinewave amplitude decreases sharply. At the null point, the trace should drop to the noise floor of the oscilloscope. Any residual signal at the null should be examined for frequency — if it is at the operating frequency, neutralisation is close but not perfect; if it is at VHF, you may have a parasitic oscillation issue independent of the neutralisation adjustment.
⚠ Parasitic Oscillation Test: VHF parasitic oscillation may not be visible on a standard HF wattmeter. Use a grid-dip meter in diode (wavemeter) mode or a spectrum analyzer to sniff for VHF energy near the PA compartment. Parasitic oscillation at VHF produces high plate current, excessive tube heating, and can damage the neutralising components — all symptoms that are easily mistaken for a neutralisation problem when they are actually a parasitic suppression problem (L15/L16 spacing).[2]
The 6CL6 driver tube (V7 in the 32S-3, V8 in the KWM-2) operates in class A at near maximum rated plate dissipation. If its shield does not make perfect ground contact, the driver will oscillate at VHF, applying full — actually, greatly excessive — drive to the 6146 final amplifier tubes. This condition will destroy 6146 tubes in minutes and can also damage the neutralising circuit components.[5]
Do not substitute a generic tube shield for the 6CL6. Standard replacement shields reflect heat back into the tube instead of conducting it away, leading to premature tube failure from overheating. The original Collins shield is designed for thermal management of this high-dissipation application. A shield that merely “fits” is not adequate — it must make reliable, low-resistance contact to chassis ground around its entire base.[5]
Diagnostic Tip: If the 6146 tubes glow red at idle with MIC GAIN at OFF and the PA correctly biased, suspect the 6CL6 driver shield before suspecting the neutralisation adjustment. Remove the 6CL6 shield, clean both the shield base and the tube socket ground ring, and reinstall. Verify that idle plate current returns to the specified value.
The neutralisation procedure described here applies to all Collins transmitters and transceivers using push-pull 6146 PA tubes with the Collins neutralisation circuit. The circuit component values differ between early and late production but the procedure is identical. The air-variable upgrade kit is interchangeable between the 32S-3/3A, KWM-2/2A, and 32S-1 — the mounting hardware and phenolic board dimensions are the same across the family.[4]
Neutralisation must be rechecked whenever 6146 tubes are replaced, as each tube pair has slightly different internal capacitance characteristics. This is one of the strongest arguments for the air-variable upgrade — ceramic trimmers may not survive repeated adjustment cycles, while air variables are inherently robust.[3]
- Antique Radio Forums. Collins 32S-3 Grid Current — KB6GM analysis. 32S-3 marginal neutralising circuit from factory; 6146B/W parasitic oscillation at VHF; grid-dip meter wavemeter diagnostic technique. March 2019. antiqueradios.com — 32S-3 Grid Current
- Antique Radio Forums. Neutralizing Final Tubes — WQ9E and KB6GM analysis. Parasitic oscillation diagnosis, plate current dip vs. maximum output coincidence test, VHF wavemeter technique, L15/L16 parasitic suppressor spacing. January 2020. antiqueradios.com — Neutralizing Final Tubes
- VK3KCM. 6146 Variants and Neutralization. Ceramic trimmer (1.8–8.7 pF) vs. air variable (8–50 pF) circuit comparison; KWM-2 Section 7 schematic differences; compatibility analysis. angelfire.com/de/vk3kcm — 6146 Variants
- Borowski, Gerd DJ7HS. Collins KWM-2: Installing New Parts for PA Neutralization. Complete photographic step-by-step of air variable upgrade kit installation; W7KSG kit; Johnson air variable capacitor; phenolic board mounting. qsl.net/dj7hs — KWM-2 Neutralisation
- Antique Radio Forums. Neutralizing Final Tubes — 6CL6 Driver Shield. 6CL6 class A dissipation, shield grounding requirement, shield substitution warning, driver oscillation as root cause of 6146 failure. WQ9E analysis. antiqueradios.com — Driver Shield
- Groups.io / Heathkit. 6146 Tubes — Neutralisation Technique. Screen voltage removal method; feedthrough null procedure; rough neutralisation on 20 meters, touch-up on 10 meters. January 2020. groups.io/heathkit — 6146 Neutralisation
- Collins Reflector. KWM-2A Problems — Neutralisation Dual Null. Erie ceramic trimmer behaviour: two nulls expected in sweep; single null indicates degraded trimmer; C123 feedthrough capacitor replacement. October 2004. Collins Reflector — Neut Circuit
- Antique Radio Forums. Collins PA Neutralizing Set Up. Air variable upgrade parts sourcing (RF Parts Johnson capacitor); space constraints; phenolic mounting board fabrication. December 2015. antiqueradios.com — PA Neutralizing
- Collins Collectors Association — RX For Your Collins. Master index including: “Repairing the 32S-3 — Carns and Bud, K7RMT,” “32S-3 Trouble Shooting Voltage Table,” and related PA maintenance articles. collinsradio.org — RX For Your Collins
- Collins 32S-3 Instruction Book, 7th Edition (June 1969). PA feedback neutralizing procedure, tube complement, circuit description. Collins Radio Company / Rockwell International. collinsradio.org — 32S-3 Manual (PDF)
- Collins KWM-2/2A Instruction Book, 9th Edition (January 1978). Section 4: Feedback Neutralizing procedure; Section 7: Equipment Differences and schematic changes documenting neutralisation circuit evolution. collinsradio.org — KWM-2/2A Manual Section 4 (PDF)
Gerd Borowski, DJ7HS — For the definitive photographic step-by-step guide to installing the air-variable neutralisation upgrade kit in the KWM-2, which applies equally to the 32S-3 family.
J A Call, W7KSG — For producing the air-variable neutralisation upgrade kit with the correct Johnson capacitor and mounting hardware.
Jim T., KB6GM — For the analysis of the original Collins neutralisation circuit’s marginal design and its incompatibility with later 6146 variants.
Rodger WQ9E — For the detailed 6CL6 driver shield analysis explaining how a poorly grounded shield produces symptoms easily mistaken for neutralisation failure.
VK3KCM — For compiling the 6146 variant and neutralisation circuit comparison from Collins service documentation.
Collins Collectors Association (CCA) — For maintaining the “RX For Your Collins” technical article library and the archived instruction manuals that document the neutralisation circuit evolution.