Workshop notes · HMV “Little Nipper” Super 5 · 1950s valve radio

Line in,
valve warmth out.

How a mid-century Australian five-valve mantel radio gained a 3.5 mm AUX input and a switchable line output to a Yamaha amplifier. The original tuner and speaker still work as before, and two toggle switches choose the source and where the sound goes.

2 × 1 kΩ + 100 nFStereo-to-mono summing and a 630 V DC-blocking cap
2 × SPDTS1 picks radio or AUX, S2 picks speaker or line out
8.2 Ω · 10 WDummy load that keeps the output transformer safe
150–300 VTypical HT inside the chassis, so read the safety section
Cream HMV Little Nipper radio with an iPad on a stand on top, a toggle switch fitted below the two knobs Finished: iPad → AUX → valves
01

What you’re building

There are two taps into the radio’s existing audio path. Neither replaces any original part, and both can be undone with a soldering iron.

TAP A · INPUT

AUX into the volume control

Music from a phone, laptop or Bluetooth receiver joins the radio just before the volume pot, which is the same place the AM detector feeds. From there it goes through the 6AV6 preamp, the output valve and the output transformer, just like a broadcast would.

TAP B · OUTPUT

Speaker or line out

A switch on the output transformer’s secondary sends the signal either to the original speaker or to a dummy load plus a line output converter (LOC). The LOC feeds RCA line level to a Yamaha amplifier’s AUX input.

WHY BOTHER

A good learning project

One small build covers signal levels, impedance, DC blocking, RC filters, grounding and ground loops, inductive kickback, and how a superhet radio actually works. You also get a lovely-sounding speaker dock at the end.

What you’ll hear

The valve stages add the “warmth” people talk about. A single-ended output stage adds mostly second-harmonic distortion, which sounds musical and full rather than harsh. The small output transformer softens the top end and gently compresses peaks, and the radio’s audio stages were voiced for AM broadcasts, so the highs roll off early. The result is mellow, mid-forward and easy to listen to. It isn’t hi-fi, and that’s the appeal.

Tip: set the source to about 70–80% volume and use the radio’s own knob. A source at full volume can overdrive the 6AV6 grid and make it clip harshly.

02

How the radio works

The Super 5 is a five-valve superheterodyne. To pick the right tap points, you need to know the chain from the aerial to the speaker.

Inside the chassis: glass valves, aluminium IF transformer cans, tuning gang, ferrite rod aerial and the speaker 1 2 3 4 5 6
  1. 16AV6: twin diode (AM detector) and triode (first audio stage) in one bottle.
  2. 26BA6: IF amplifier pentode at 455 kHz.
  3. 3IF transformer cans: tuned filters that set the radio’s selectivity.
  4. 4Tuning gang: the variable capacitor the tuning knob turns.
  5. 5Ferrite rod aerial: the coil on the rod picks up the medium-wave signal.
  6. 6Speaker (Rola): the voice coil is fed by the output transformer.
The chassis from above. These are the parts you can name from the photo. The converter, output valve and rectifier are standard parts for this kind of set (e.g. 6BE6, 6AQ5/6M5, 6X4/6V4). Check the markings on yours.
Ferrite aerial 530–1700 kHz Converter mixer + oscillator (6BE6) IF amplifier 6BA6 · 455 kHz Detector 6AV6 diodes → audio Volume pot audio, tens of mV–~1 V Audio preamp 6AV6 triode Output valve single-ended pentode Output transformer ~5 kΩ → ~8 Ω Speaker speaker level, a few W Power supply transformer → rectifier → filter caps → HT 150–300 V DC HT to every valve A B RF → IF → audio: the signal gets lower in frequency and bigger in voltage at each stage
Signal chain with the two tap points. A is the volume pot. The detector has already turned radio into audio here, so line-level audio from a phone fits right in. B is the output transformer secondary: low voltage and low impedance, and the only place it’s safe to take speaker-level audio out.

Superhet in one paragraph

The converter mixes the incoming station with a local oscillator that tracks the tuning knob. Whatever station you tune, the result lands on one fixed intermediate frequency (455 kHz). The IF stage can then be tuned once at the factory for good gain and selectivity. The detector diode strips off the 455 kHz carrier and leaves the audio envelope. That audio goes through the volume pot to the triode, then the output pentode, and the output transformer steps the valve’s high-impedance drive down to suit an 8 Ω-ish speaker.

03

Safety first

⚠ Lethal voltages

Valve radios run at 150–300 V+ DC, and the big filter capacitors can stay charged after the set is unplugged. Many old radios have also been badly repaired over the years. If you’re not confident working around mains, get help from someone who is.

1 · Unplug, then discharge

Pull the plug, not just the switch. Wait, then use a meter to check the voltage across the main filter capacitors before you touch anything. Bleed them through a resistor (e.g. 10 kΩ 5 W on insulated leads) if they still hold charge.

2 · Is the chassis live?

Some sets were “AC/DC” with no power transformer, so the chassis itself is connected to mains. Most Australian mantel sets like this one have a proper mains transformer. Confirm yours does before you connect it to a phone or an amplifier. If it’s live-chassis, stop: every external connection would need isolation transformers.

3 · One hand, insulated tools

When testing a powered set, keep one hand off the chassis, use insulated probes and clip leads, and keep the bench dry and uncluttered.

4 · Never run without a load

A valve output stage must always see a speaker or a dummy load. Without one, inductive kickback in the output transformer can arc over and destroy it. This rule shapes the whole output design.

04

System design

Two selector switches, two small circuits. Blue marks new parts; the rest is original.

Source phone · iPad · MacBook or Bluetooth receiver Sum + DC block 2 × 1 kΩ → 100 nF 630 V stereo → mono, HT-proof S1 Radio / AUX Detector AM radio audio Volume → valves 6AV6 triode → output valve the “warmth” happens here Output transformer secondary: blue (hot) yellow (ground) S2 Speaker / Line Internal speaker original Rola 8.2 Ω load + LOC speaker level → line level (RCA) Yamaha amp AUX / CD (not PHONO) 3.5 mm stereo pos. 2 pos. 1 S1 · up = radio, down = AUX S2 · 1 = speaker, 2 = line out ground is common and never switched
After the mod. S1 decides what reaches the volume control. S2 decides where the amplified signal goes. Ground is shared everywhere and never goes through a switch, because switching ground creates hum and loud pops.
05

Concepts behind each part

Every part in this build exists for a reason you can work out. Here they are, with numbers.

Speaker level vs line level

Line level is what phones and amplifier AUX inputs use: about 0.3–2 V into an input of 10 kΩ or more, carrying almost no current. Speaker level is what the output transformer delivers: a few volts at hundreds of milliamps into about 8 Ω. A few watts into 8 Ω is about 4–6 V RMS, several times too hot for a line input, and the radio’s ground may not match the amp’s. A line output converter (LOC) steps it down (and often isolates it). Never feed speaker wires straight into an AUX socket, and never into PHONO, which has around 40 dB of extra gain and RIAA equalisation.

Why stereo can’t just be twisted together

A phone’s left and right outputs are two separate amplifiers. Tie them together and, whenever L and R differ, each one tries to drive the other. That means distortion, wasted current and stressed output chips. Put 1 kΩ in series with each channel and join the far ends, and the difference current is limited to a few mA. The 1 kΩ resistors cost almost nothing in level, because the next load is the radio’s volume pot of several hundred kΩ. A cheap “stereo to mono” cable is usually hard-wired with no resistors (the one checked during this build was), so don’t use it.

The DC-blocking capacitor is a high-pass filter

The capacitor passes AC (music) and blocks DC. That protects your phone if a fault ever puts HT on the volume pot, which is why it’s rated 630 V. Together with the pot’s resistance, it also forms a high-pass filter with corner frequency fc = 1 / (2π·R·C). Bigger C or bigger R gives deeper bass. Try it:

High-pass calculator

R is the volume pot’s full track, which loads the capacitor. Measure it between the top and bottom lugs with the set unplugged.

Capacitor
Volume pot
Corner fc–
Loss at 41 Hz (bass low E)–
Loss at 20 Hz–

Second opinion on the AI advice

During the build, the AI assistant said a 10 nF or 22 nF cap would “cut off all your bass”. The calculator shows it depends on the pot. With a typical 500 kΩ pot, 22 nF gives fc ≈ 14 Hz, which is fine. 100 nF is still the right buy, because it leaves margin if your pot is smaller than expected.

Reading capacitor codes

Cap code decoder

Film caps are marked like 104K2J: two digits, then a multiplier (in pF), then the tolerance letter, then the voltage code.

Why a dummy load, and why 8.2 Ω is fine

SAFE

Speaker + LOC

8 Ω ∥ ~1 kΩ ≈ 7.9 Ω. The LOC barely loads anything, so the transformer sees what it was designed for.

DANGER

LOC alone

~1 kΩ or more is close to an open circuit. On loud peaks, the energy stored in the transformer has nowhere to go, the voltage spikes, and the windings or the valve can arc over.

SAFE

8.2 Ω 10 W + LOC

A power resistor stands in for the speaker. 8.2 Ω is the nearest standard value, and real speakers swing well above and below 8 Ω anyway. 10 W covers a few-watt output stage with plenty to spare.

06

Bill of materials

Everything came from a local Jaycar plus parts already on hand. Prices change, so none are listed.

PartQtySpecJob
A · Input (AUX → volume pot)
Metal-film resistor21 kΩ, ¼ W or ½ W (1.2–2.2 kΩ also fine)
brown-black-black-brown-brown		Stereo-to-mono summing without shorting L and R
Film capacitor1100 nF 630 V DC, polyester “greencap” or MKT, marked 104K2JDC block / HT protection for the source
Toggle switch S11Sub-miniature SPDT (ON-ON), or one half of a DPDT such as Jaycar ST0310Radio / AUX selector
3.5 mm stereo lead1Any plain AUX cable, cut to expose L, R and groundInput lead
B · Output (transformer → Yamaha)
Line output converter12-channel, high-to-low, 7-wire (with power and remote leads)Speaker level → RCA line level
Power resistor18.2 Ω (or 8 Ω), 10 W or more, ceramic cement or aluminium-cladDummy load when the speaker is switched out
Toggle switch S21Sub-miniature SPDT (ON-ON)Speaker / line out selector
RCA lead1Stereo RCA–RCALOC → Yamaha AUX/CD
C · Consumables
Hook-up wire~2 mStranded, ~22 AWG, a couple of colours (shielded cable for the input run is better)Links
Heat-shrink1 pack2–5 mm assortedInsulate every joint and lug
Solder, tape, cable tiesResin-core electronics solder
D · Optional
Stripboard1Small offcut, 16 × 11 holesTidier home for the input network (see Board)
Ground-loop isolator13.5 mm inline audio transformerKills buzz from laptops on chargers and mains-powered Bluetooth receivers
Bluetooth receiver15 V USB-powered, 3.5 mm outWireless input

Shop-counter mistakes caught during the build

  • 223K2J is 22 nF. It’s usable with a big pot, but 104 (100 nF) is the safer pick.
  • 100 V caps: too low a rating for the inside of a valve chassis.
  • X2 “across-the-line” caps: made for mains suppression, not signal paths.
  • Hard-wired stereo→mono adaptor: no resistors inside, so it shorts L to R.
  • ST0310 is ON-ON with no centre-off. For S2 that’s exactly what you want, since “off” would mean no load.
07

Input circuit (tap A)

Lift the detector wire off the top lug of the volume pot and put S1 in its place. Flip the switch below to follow the signal.

S1 position
3.5 mm TIP · L RING · R SLEEVE R1 1 kΩ R2 1 kΩ C1 100 nF 630 V mono sum from detector (6AV6 diodes) original wire, lifted off the top lug S1 pin 3 · AUX pin 1 · RADIO pin 2 · COM top lug · in middle lug · wiper bottom lug · ground → 6AV6 triode grid (existing shielded lead) VOL L & R grounds twisted together chassis ground: same point, never through a switch
active signal pathgroundnew part
Schematic, tap A. R1 and R2 meet first to make the mono sum, and C1 comes after the junction. A cap between the two resistors would block one channel. The ground goes straight to the pot’s bottom lug or chassis, not through S1.

Which lug is which?

One observation from the build: the middle and bottom lugs share one cable. That’s the original shielded lead to the 6AV6 grid, with the core on the wiper (middle) and the braid on ground (bottom). The single wire on the top lug comes from the detector, and that’s the one you lift. Check with a meter, set unplugged:

  1. Ground lug: reads 0 Ω to the chassis.
  2. Wiper: its resistance to ground changes as you turn the knob, and reaches 0 Ω fully anticlockwise.
  3. Top (input) lug: reads the full track resistance to ground whatever the knob position. Write it down; that’s your R for the calculator above.
VOL top · in wiper ground original shielded lead → 6AV6 grid (core = wiper, braid = ground) detector wire (green), moved from the top lug to S1 pin 1 COM → top lug S1 1 radio 2 COM 3 AUX 104 1 kΩ 1 kΩ 3.5 mm AUX lead L (white) R (red) AUX ground (copper braid) → pot ground lug
Physical layout as built. The network was built point-to-point in free air between S1 and the pot (photo below). It works, but cover every bare lead in heat-shrink, and keep the run short and away from the mains and heater wiring. Wire colours are illustrative; go by function.
08

Output circuit (tap B)

S2 sits in the hot lead of the output transformer secondary. Position 1 feeds the speaker; position 2 feeds the dummy load and the LOC together, so the transformer always has a load.

S2 position
OT secondary blue · hot yellow · “S” lug S2 COM 1 2 Internal speaker original Rola, ≈ 8 Ω R3 8.2 Ω 10 W LOC high → low in+ in− RCA → Yamaha AUX / CD red · black · blue: taped off common ground junction at the speaker’s “S” lug
active signal pathgroundnew part
Schematic, tap B. The LOC sits on contact 2 together with R3, so it only gets signal in “Yamaha” mode. That gives you one output or the other, which is what this build wanted. If you’d rather the LOC always had signal, connect its in+ to COM instead.

Flip S2 at low volume

A toggle breaks before it makes. For a few milliseconds during the flip, neither the speaker nor R3 is connected. At low volume that’s harmless; at full volume it’s exactly the unloaded kick you’re trying to avoid. Turn the radio down before you switch.

The 7-wire LOC

WireCar-audio meaningIn this build
WhiteLeft in +Twist with gray → S2 contact 2 (with R3)
White/blackLeft in −Twist with gray/black → ground junction
GrayRight in +Bridged with white, so mono reaches both RCA outputs
Gray/blackRight in −Bridged with white/black
Red+12 V powerInsulate, not connected
BlackPower groundInsulate (try it on chassis only if you get noise)
Blue+12 V remote turn-on outInsulate, not connected

If the RCA outputs stay silent with the power leads unconnected, your LOC has an active (powered) signal path. Feed red/black from a small 12 V DC plug-pack.

Rola speaker (rear) + S (gnd) yellow from OT: stays on “S” blue from OT: moved to S2 COM S2 1 COM 2 new link → speaker + 10W 8R2 J ceramic cement all grounds meet at the “S” lug LOC mounted on the back panel RCA white + gray → contact 2 white/blk + gray/blk → ground run red · black · blue taped
Physical wiring at the speaker. The blue wire moves from the speaker lug to S2’s middle pin. A new link runs from S2 pin 1 back to the speaker’s + lug. Pin 2 takes R3 and the LOC’s positive inputs. Every ground lands on the speaker’s “S” lug. Check that “S” really is ground: it should read 0 Ω to the chassis.
09

A tidier input board

The free-air build works. If you’d like something sturdier, the whole input network fits on a stripboard offcut of 16 × 11 holes.

View
copper striptrack cutoff-board wire
Stripboard layout, 0.1″ (2.54 mm) pitch, drawn at 28 px per hole. Board ≈ 41 × 28 mm. Strips run horizontally. R1 and R2 stand across strips to land on the SUM strip (row E). C1 lies along that strip, so the strip is cut under it at hole E11; otherwise the copper would short the capacitor out. The copper view is mirrored, as you see it when you flip the board to cut and solder.
StripNetConnections
AL inTip wire at A2 · R1 top lead at A5
E (left of cut)SUMR1 at E5 · R2 at E7 · C1 lead at E9
E (right of cut)OUTC1 lead at E13 · wire to S1 pin 3 at E15
IR inRing wire at I2 · R2 bottom lead at I7
KGNDSleeve wire at K2 · wire to the pot’s ground lug at K15
10

Build, step by step

Tick as you go; your progress is saved in this browser. Don’t move on from a phase until its gate passes.

0 of 0 done

0 · Make it safe and document it

bench · 20 min
Gate: HT reads near 0 V, and you have “before” photos.

1 · Identify the lugs and wires

multimeter · 15 min
Gate: the pot lugs and the speaker hot/ground are labelled with tape flags.

2 · Build the input network

bench · 30 min
Gate: tip→SUM reads ≈ 1 kΩ, tip→ring ≈ 2 kΩ, and SUM→OUT reads open (briefly charging, then ∞).

3 · Fit S1 at the volume pot

chassis · 30 min
Gate: in RADIO, the detector wire → top lug reads 0 Ω. In AUX, the cap leg → top lug reads 0 Ω.

4 · Output switch, load and LOC

chassis · 45 min
Gate: with S2 in position 1, pin 2 → ground reads ≈ 8 Ω (R3 ∥ LOC).

5 · Power-up tests

powered · careful
Gate: all four combinations of S1 and S2 work, and the background is quiet with the source paused.
11

Going wireless

A USB Bluetooth receiver plugs straight into the AUX network. The harder question is where its 5 V comes from.

BEST

Separate USB supply or power bank

Use a good-quality, fully isolated 5 V USB plug-pack outside the radio, or a power bank. There’s no new mains wiring inside the cabinet and no extra load on the 70-year-old transformer. If it buzzes, add a ground-loop isolator.

OK, CAREFULLY

USB charger wired inside

A small certified USB charger wired after the radio’s switch and fuse turns on with the set. Mains wiring must be insulated, strain-relieved and enclosed. If you’re not sure, don’t.

AVOID

Tapping the 6.3 V heater winding

It can be done (rectify, filter, regulate), but the heater circuit is usually grounded on one side or centre-tapped. A bridge rectifier then fights the audio ground, and the hum often gets worse.

The no-solder alternative: an AM transmitter

A tiny AM transmitter (with a built-in Bluetooth receiver, or a separate one) broadcasts on a free spot of the dial, about 1–2 m range. Tune the radio to it. Nothing inside the set changes, and the sound still passes through every valve. Expect AM bandwidth (roughly 5 kHz) and some interference. A car FM Bluetooth transmitter can’t be converted: it works at 88–108 MHz with FM modulation, about 100 times higher in frequency.

12

Hum, buzz and other noise

On first power-up this build had a loud “current noise”. Start with two questions: does it change with the volume knob, and is it there on RADIO as well as AUX?

SymptomLikely causeFix
Loud 50 Hz roar in AUX that rises with the volume knobAUX ground not connected, or the input wire open, so the grid is floatingRe-solder the sleeve/ground to the pot’s ground lug. Test: clip the AUX ground to the chassis and see if the roar stops.
Buzz or whine only when the laptop is charging, or the BT receiver is on mainsGround loop: radio ↔ source ↔ mains earth (↔ Yamaha)Run the source on battery to prove it, then fit a 3.5 mm ground-loop isolator before the 1 kΩ resistors.
Hum appears only with the Yamaha connectedGround loop through the RCA shieldTry the LOC’s black lead on chassis, or not. Use an LOC with isolated outputs, or an RCA ground-loop isolator.
Hum in AUX, quiet on RADIO, with a long plain wire runUnshielded high-impedance wiring picking up the heater or mains fieldShorten the runs, or use shielded cable (braid grounded at the pot end only). Route away from the heater and mains wires.
Hum on RADIO and AUX, unaffected by volumeTired electrolytic filter caps (100 Hz ripple). This was there before the mod.Recap the power supply. That’s a separate, worthwhile restoration job.
Thin, bass-light soundDC-block cap too small for your potCheck with the calculator and use 100–220 nF.
Harsh, clipped soundSource too hot for the 6AV6 gridTurn the source down to about 70% and use the radio’s knob.
Nothing from the YamahaS2 in position 1, wrong input chosen, or an active LOC with no powerCheck S2 and the AUX input selection, or power the LOC from 12 V.
13

Glossary

Valve (tube)
A vacuum device where a heated cathode emits electrons and a grid controls the current to the anode. Small grid voltage changes produce big anode changes, which is amplification.
HT / B+
The high-tension DC supply (150–300 V) that feeds the valve anodes. It’s the dangerous part.
Superheterodyne
A receiver that converts every station to one fixed intermediate frequency before amplifying and detecting it.
IF
Intermediate frequency: 455 kHz in Australian AM sets.
Detector
The diode stage that recovers audio from the AM carrier. Here it’s inside the 6AV6.
Output transformer
Matches the output valve’s high impedance (kilohms) to the speaker’s low impedance (ohms).
Impedance
AC “resistance”. Sources want to see the load they were designed for.
Line level
About 0.3–2 V signal into a high-impedance input. The language of AUX, CD and tape inputs.
Speaker level
The amplifier’s output: several volts at real current into roughly 4–16 Ω.
LOC
Line output converter. Steps speaker level down to line level, usually with RCA outputs.
Dummy load
A power resistor that stands in for the speaker so the output stage is always loaded.
Flyback / inductive kick
The voltage spike from an inductor when its current has nowhere to go. It’s why a valve amp needs a load.
DC-blocking cap
A series capacitor that passes AC audio but blocks DC voltage.
High-pass corner
The frequency where an RC filter is 3 dB down: f = 1/(2πRC).
Ground loop
Two or more ground paths between devices, forming a loop that picks up mains hum.
SPDT / DPDT
Single or double pole, double throw: one or two switches, each choosing between two contacts. ON-ON means no centre-off.
Greencap / MKT
Polyester film capacitors. Codes like 104K2J mean 100 nF, ±10%, 630 V.