Collins R-390A/URR
Audio Output Transformer — Failure, Specification, and Substitution Options

Section 1 — The R-390A Audio Output Stage

Circuit Configuration

The R-390A audio output stage uses a single 6AQ5 beam power pentode operating in class A single-ended configuration. This is the same output tube used in millions of consumer radios and communication receivers of the era — a robust, well-documented tube with thoroughly understood operating characteristics. It is directly coupled via the audio output transformer to the external speaker output and headphone jacks on the receiver’s rear panel. The audio volume control (AF GAIN) operates upstream in the audio chain, so the 6AQ5 operates at the output power demanded by the preceding stage, not at full power continuously.

The audio output transformer performs two functions simultaneously: it provides the impedance transformation between the 6AQ5’s plate load impedance and the speaker load, and it provides DC isolation, blocking the 6AQ5’s plate supply voltage (approximately 200–250 V DC) from the speaker and headphone outputs. Both functions must be preserved in any substitute.

Why the Primary Impedance Matters

A tube amplifier stage produces maximum power and minimum distortion when the load connected to its plate (via the transformer primary) matches its optimal load impedance. For the 6AQ5 operating at the conditions specified in the TM-11, this optimal load impedance is in the range of 4,000–5,000 Ω. A transformer with primary impedance significantly above this range produces less power and more even-order distortion. A transformer with primary impedance significantly below this range loads the tube excessively, potentially increasing distortion and reducing tube life.

For communications audio — voice over a 300–3,000 Hz bandwidth — the consequences of a 20–30% impedance mismatch are small and largely inaudible. For a working receiver used in normal communications service, a substitute transformer with a primary impedance anywhere from 3,500 Ω to 6,000 Ω will produce acceptable results. The calculation in Section 3 provides the precise target; the practical tolerance is generous.

The Headphone Output

The R-390A provides a headphone output in addition to the speaker output. In the original design, this is served by a separate winding on the audio output transformer — a higher-impedance secondary designed for the crystal or magnetic earphones of the military era (typically 600 Ω nominal). This winding provides the correct impedance match for vintage high-impedance headphones and isolates the headphone output from the speaker circuit. Most modern substitute transformers do not have this third winding. The consequences and workarounds for this are addressed in full in Section 5.

Section 2 — Failure Modes and Diagnosis

Failure Mode 1: Primary Winding Open Circuit (Most Common)

The primary winding is a long length of fine enamel-insulated magnet wire wound on the transformer bobbin. After 60–70 years of thermal cycling, the insulation enamel can crack at a point of mechanical stress and the wire develops a corrosion break, producing an open circuit. A primary open circuit means the plate circuit of the 6AQ5 is broken: there is no DC path for plate current, the tube does not operate, and there is no audio from either the speaker or headphone outputs.

This is occasionally misdiagnosed as a 6AQ5 tube failure. The symptom — no audio despite audio signal present at the driver stage and correct voltages elsewhere — is identical whether the 6AQ5 has failed or the transformer primary is open. Always measure the primary DC resistance before pulling and testing the 6AQ5.

Failure Mode 2: Shorted Turns in the Primary

A winding short — where adjacent turns of the primary winding make electrical contact due to insulation breakdown — produces a different and more subtle failure pattern than an open circuit. The transformer still conducts; the plate circuit still has a DC path. But the effective inductance of the primary is reduced (shorted turns reduce inductance), and the shorted turns act as a resistive loss within the transformer, converting signal power into heat rather than useful output.

Symptoms: reduced audio output level (often attributed to a weak 6AQ5 before the transformer is suspected), early onset of distortion at moderate volume levels, and in severe cases a transformer that runs warm to the touch under operation. The transformer may produce a faint burned smell if the shorted turns are dissipating significant power.

Diagnosis: measure primary DC resistance (will be at or below nominal, not above, for a shorted turns failure) AND measure primary inductance with an LCR meter. A transformer with shorted turns shows significantly reduced primary inductance compared to a good unit. If two transformers are available for comparison, the difference is immediately apparent on an LCR meter. A shorted transformer cannot be repaired in the field and requires replacement.

Failure Mode 3: Secondary Winding Failure

Secondary winding failure (open or shorted secondary) is less common than primary failure but produces mode-specific symptoms: the speaker output is absent if the speaker secondary fails, the headphone output is absent if the headphone winding fails, or both are absent if there is a common failure at the secondary end. Diagnose by measuring secondary DC resistance (typically very low — a few ohms for a 4–8 Ω speaker secondary) with the speaker disconnected. An open secondary reads infinite; a secondary short reads near zero even without a speaker load.

Failure Mode 4: Lamination Buzz

Small audio output transformers of the R-390A era use silicon-steel laminations stacked and clamped together to form the core. After decades of service, the varnish impregnation that binds the laminations can soften and fail, allowing individual laminations to vibrate at audio frequencies. Symptoms: a mechanical buzzing or rattling that follows the audio signal frequency, clearly audible when the receiver is held near the ear (not through the speaker), and that persists even with the output load removed. This is a mechanical failure, not an electrical one. A buzzing transformer is still electrically functional but increasingly annoying and eventually disruptive to audio quality. The fix is either core re-varnishing (possible but messy) or replacement.

Diagnosis Procedure at a Glance

• Step 1 — Power off, verify B+ at zero.
• Step 2 — Disconnect one lead of the primary winding (the lead that goes to the plate of the 6AQ5). Measure primary DC resistance. Nominal: approximately 80–250 Ω depending on the specific winding design (verify from TM-11). Open circuit (infinite resistance): primary open, transformer requires replacement. Significantly below nominal: suspect shorted turns, proceed to inductance measurement.
• Step 3 — If primary DC resistance is below nominal: measure primary inductance at 1 kHz with an LCR meter. Compare to a known-good transformer if available. Significantly reduced inductance confirms shorted turns.
• Step 4 — Disconnect speaker load. Measure secondary DC resistance. Nominal for a 4/8 Ω secondary: less than 1 Ω. Open circuit: secondary open.
• Step 5 — Mechanical buzz test: with receiver powered and audio present, place a finger gently on the transformer body. Buzz that follows the audio signal and is transmitted through the chassis is lamination buzz.

Section 3 — The Operating Point Calculation: What You Actually Need

Selecting the correct substitute primary impedance requires knowing the 6AQ5’s operating point in the R-390A. The optimal load line for a triode or pentode is the primary impedance that produces maximum undistorted output power at the specified plate and screen voltages. The TM-11 specifies the operating conditions; the tube data sheet provides the load line tools; the calculation gives the target primary impedance for any substitute.

Section 4 — Substitute Options: Comparative Assessment

The following table summarises every substitute option that the R-390A community has documented with sufficient detail to evaluate. Verdict codes: PREFERRED = best available option; RECOMMENDED = fully suitable; ACCEPTABLE = works with minor compromise; MODIFIED = requires circuit or mounting modification; AVOID = unsuitable or unreliable.

Notes on Specific Options

NOS Collins original: always the preferred choice when available. Source from the r-390a.net community parts exchange or a cannibalised R-390A chassis. Verify by measuring DC resistance before installation; an NOS transformer that has developed an open winding in storage (possible, though uncommon) must be rejected despite appearing original.

Hammond 1608 series: Hammond Manufacturing (Guelph, Ontario) is the most frequently cited substitute in the R-390A community. The 1608 series are compact single-ended output transformers designed for tube applications including the 6AQ5, EL84, and similar. Hammond’s data sheets are publicly available and specify primary impedance, DC resistance, and secondary taps clearly. The primary concern with Hammond substitutes is physical size: the Hammond 1608 is smaller than the original Collins transformer, requiring a mounting adapter or a small sub-plate. Several community members have designed and shared mounting adapter templates for the Hammond in an R-390A audio deck position; these are available through the r-390a.net forum. Audio quality from the Hammond substitute is indistinguishable from original in normal communications use.

Edcor Electronics CXSE series: Edcor (Albuquerque, NM) provides custom-wound audio output transformers with specified primary impedance and configurable secondary taps. The significant advantage over Hammond is that Edcor can wind a third (headphone) winding to the customer’s specification at order time, solving the headphone winding problem described in Section 5. The lead time for Edcor custom orders is typically several weeks. Edcor transformers are slightly larger than Hammond 1608 units and require a mounting adapter. Cost is higher than Hammond but below a specialist custom winder.

Generic Chinese substitutes: the community consensus on low-cost generic audio output transformers from Chinese sources is clear and consistent: do not use them in a quality R-390A restoration. The primary concern is specification accuracy — the stated primary impedance, typically derived from a label rather than measurement, has been found to differ from the actual measured inductance by 30–50% in community testing. A transformer labelled “5 kΩ primary” that measures as 2,500 Ω loads the 6AQ5 outside its design range and cannot be correctly substituted in the R-390A audio stage without significant circuit modification.

Section 5 — The Headphone Winding Problem

The original R-390A audio output transformer has three windings: the primary (6AQ5 plate load), the speaker secondary (4 Ω and 8 Ω taps), and a separate headphone secondary winding for the rear-panel headphone output jacks. The headphone winding is designed for the crystal or magnetic earphones of the military era, with a nominal impedance of approximately 600 Ω or higher. This winding provides two functions: impedance matching between the low-impedance speaker secondary and the high-impedance vintage headphones, and DC isolation of the headphone output from the audio supply rails.

Almost every modern substitute transformer — Hammond 1608, Hammond 125, Triad A-series — has only two windings: primary and speaker secondary. There is no headphone winding. When one of these transformers is installed, the rear-panel headphone jacks have no signal from the transformer and effectively go dead.

Workaround Options

Section 6 — Mounting Dimension Compatibility

The original Collins audio output transformer is mounted on the audio deck of the R-390A with specific mounting bolt hole spacing. Modern substitute transformers from Hammond, Edcor, and Triad have different physical dimensions and mounting configurations that do not match the original bolt pattern.

The Mounting Adapter Approach

The most common and practical solution is a simple mounting adapter plate — a small piece of aluminium or steel sheet drilled with holes on one side matching the R-390A audio deck mounting holes, and holes on the other side matching the substitute transformer’s mounting points. The adapter sits between the deck and the transformer, filling the dimensional gap. Community members have designed and shared mounting adapters for specific Hammond and Edcor models through the r-390a.net forum; searching for “R-390A output transformer mount” or “Hammond 1608 R-390A” retrieves these designs.

The adapter does not need to be precision-machined: a simple hand-filed aluminium plate cut from hardware store sheet stock is adequate. The only critical requirement is that the transformer body does not contact the audio deck and that the transformer leads can reach their connection points without excessive tension.

Lead Dress After Substitute Installation

When a substitute transformer is installed with a mounting adapter, the lead dress (the routing of the transformer primary and secondary leads to their connection points) may differ from the original. Take care to route the primary lead away from the audio input circuit leads: the primary carries the 6AQ5 plate signal at relatively high impedance, and coupling between the primary and the audio input grid circuit can introduce instability or oscillation at audio frequencies. Maintain the original lead separation geometry as closely as possible, using the substitute transformer’s lead positions as constraints.

Section 7 — Installation and Verification

• 1
  Measure the substitute before installation Power off, B+ verified at zero. Measure the substitute transformer’s primary DC resistance (confirm it falls in the acceptable range for the target operating point). If an LCR meter is available, measure primary inductance at 1 kHz and compare against the manufacturer’s specification. Verify the secondary tap configuration matches your required loads (4 Ω or 8 Ω speaker). Reject any substitute that does not match its stated specification.
• 2
  Install the substitute with mounting adapter If using a Hammond or Edcor substitute, fabricate or source the mounting adapter. Mount the transformer on the adapter, then mount the adapter on the audio deck. Route the primary lead to the 6AQ5 plate pin connection; route the secondary leads to the speaker output terminals. If implementing a headphone winding workaround (Option A or B from Section 5), complete that wiring at this step.
• 3
  First powered check: B+ and plate voltage on 6AQ5 With the substitute installed, apply power via Variac. After 5 minutes warm-up, measure DC plate voltage on the 6AQ5. Compare against the TM-11 specification for your contract year. The plate voltage will differ slightly from the original if the substitute has a different primary DC resistance; this is expected and acceptable within ±10% of the TM-11 nominal. A plate voltage significantly above nominal (more than 15%) suggests the primary DC resistance is lower than expected; significantly below nominal suggests higher DC resistance. Both are acceptable if within the tolerance range.
• 4
  Audio output test: frequency response and level Inject a 1 kHz tone at −40 dBm at the audio input (or tune to a modulated carrier). Verify audio is present at the speaker output. The level should be appropriate for normal communications listening. If audio is absent, recheck the primary wiring. If audio is distorted at all volume levels, recheck the primary impedance and plate voltage. Verify the headphone output produces audio (via whichever headphone winding workaround is installed).
• 5
  Buzz and thermal check After 30 minutes of operation, gently touch the top of the substitute transformer. It should be barely warm — an output transformer operating correctly in a receiver-level audio application dissipates very little power. A hot transformer (too hot to hold a finger on) indicates excessive dissipation from a shorted turn in the substitute or an incorrect operating point. Confirm no mechanical buzzing from the laminations; some new transformers require a brief burn-in period before the core varnish sets fully.

Section 8 — Audio Stage Circuit Reference

  ┌──────────────────────────────────────────────────────────────────────────┐
  │   R-390A AUDIO OUTPUT STAGE — TRANSFORMER CONNECTIONS AND TEST POINTS   │
  │   Exact pin numbers and component designators from TM-11 for your year  │
  └──────────────────────────────────────────────────────────────────────────┘

  [AUDIO DRIVER] ──► [AF GAIN POT] ──► [6AQ5 AUDIO OUTPUT] ──►[PRIMARY]──►
                                        Control grid G1                     │
                                        (~−10 V DC bias)                    │
                                        Screen G2 (~200 V)           B+ in  │
                                                                            │
   TRANSFORMER ANATOMY (original Collins — 3-winding type):                │
   ┌──────────────────────────────────────────────────────────────────────┐ │
   │  PRIMARY winding (plate to B+)                ◄─────────────────────┘ │
   │  ├── One end to 6AQ5 plate (pin varies)                               │
   │  └── Other end to B+ supply rail                                      │
   │  Nominal primary DC resistance: verify from TM-11 (~80–250 Ω)        │
   │  Nominal primary impedance: ~4,000–5,000 Ω                           │
   │                                                                        │
   │  SPEAKER SECONDARY                                                     │
   │  ├── 4 Ω tap ──► Speaker terminals (rear panel)                      │
   │  └── 8 Ω tap ──► Speaker terminals (alternate)                       │
   │                                                                        │
   │  HEADPHONE SECONDARY (original only — absent in most substitutes)    │
   │  └── 600 Ω nominal ──► Headphone jacks (rear panel)                 │
   └──────────────────────────────────────────────────────────────────────┘

  DIAGNOSIS MEASUREMENT POINTS (power off, B+ verified zero):
  ┌────────────────────────────────────────────────────────────────────────┐
  │  Measurement              │  Normal         │  Fault indication        │
  │  Primary DC resistance    │  80–250 Ω       │  ∞ = open winding        │
  │  (one lead disconnected)  │  (verify TM-11) │  Low = shorted turns     │
  │  Primary inductance @1kHz │  Per mfr spec   │  << spec = shorted turns │
  │  Secondary DC resistance  │  < 1 Ω          │  ∞ = open secondary      │
  │  Plate voltage (powered)  │  Per TM-11      │  High = open primary     │
  │  Transformer temperature  │  Barely warm    │  Hot = shorted turns/    │
  │  (after 30 min operation) │                 │  wrong impedance         │
  └────────────────────────────────────────────────────────────────────────┘

  HEADPHONE WINDING WORKAROUNDS (when using Hammond/Edcor substitute):
  ┌────────────────────────────────────────────────────────────────────────┐
  │  Option A (simplest):                                                  │
  │  8 Ω tap ──[series resistor 100–470 Ω]──► headphone jacks            │
  │  Resistor value = headphone Z (typical 32–600 Ω) as a fraction        │
  │  of the speaker impedance × estimated sensitivity adjustment           │
  │                                                                        │
  │  Option B (most correct for standard substitutes):                    │
  │  8 Ω tap ──[600 Ω : 8/32 Ω matching transformer]──► headphone jacks │
  │  Small UTC or Hammond matching transformer, catalogue item             │
  │                                                                        │
  │  Option C (Edcor custom — best engineering solution):                 │
  │  Order Edcor CXSE with 3rd winding specified at 600 Ω                 │
  │  Eliminates all headphone compromise; same lead time as standard Edcor │
  └────────────────────────────────────────────────────────────────────────┘
  NOTE: All winding specifications must be verified against the TM-11
  edition for the specific contract year before ordering any substitute.
  Early and late R-390A production use slightly different audio stage
  operating points that affect the optimal primary impedance selection.

R-390A audio output stage reference — transformer connections, diagnosis measurement points, and headphone winding workarounds. Verify all component designators and pin assignments from the TM-11-5820-357-34&P for the specific contract year before any work.