290PX Hammond Netztrafo Vox AC15 Replacement

Hammond 290PX – power transformer for classic AC15 circuits

The Hammond 290PX is a bobbin-wound power transformer for valve guitar amplifiers. Two separate 120 V primary windings allow parallel connection for 120 V or series connection for 240 V at 50/60 Hz. Its secondary windings provide 305-0-305 V at 120 mA, a separate 5.1 V winding at 3 A and a centre-tapped 6.66 V winding at 4 A.

Hammond lists the 290PX as its power-transformer reference for the classic VOX AC15. The amplifier revision, schematic, mounting pattern and terminal arrangement must still be checked before installation. Hammond lists the 290CEZ separately for the AC15C1, so the 290PX should not be selected for every AC15 version on the model name alone.

Suitable applications

• Repair and restoration of classic AC15 amplifiers with matching electrical and mechanical requirements
• New AC15-style valve guitar amplifier builds using a pair of EL84 valves and a compatible power-supply circuit
• Valve amplifiers requiring a 305-0-305 V high-voltage winding plus separate heater windings
• Service projects requiring a configurable 120 or 240 V primary arrangement

Technical data

Product type

Power transformer for valve guitar amplifiers

Manufacturer application reference

Listed by Hammond for the VOX AC15; no original VOX part number is specified

Winding construction

Bobbin wound

Primary windings

Two separate 120 V windings for 120 or 240 V operation at 50/60 Hz

High-voltage winding

610 V CT, or 305-0-305 V, at 120 mA

Separate heater winding

5.1 V at 3 A between pin 10 and pin 11

Main heater winding

6.66 V CT at 4 A, with a 3.33 V centre tap provided by a black flying lead

No-load voltages

669 V CT, 5.47 V and 7.29 V CT according to the Hammond drawing

Exciting current

142 mA maximum at 120 V and 60 Hz, measured between pin 1 and pin 2

Primary DC resistance

6.784 Ω between pins 1 and 2 and 8.055 Ω between pins 3 and 4, each ±20% at 25 °C

Secondary DC resistance

236.863 Ω between pins 7 and 9, 0.054 Ω between pins 5 and 6 and 0.079 Ω between pins 10 and 11, each ±20% at 25 °C

Dimensions

Nominally approx. 96.0 × 76.2 × 80.3 mm; drawing tolerance approx. ±1.6 mm on each dimension

Mounting

Nominal mounting centres approx. 79.4 × 63.5 mm; mounting holes approx. 5.6 mm

Connections

Solder terminals for the primary and secondary windings plus a black flying lead for the 6.66 V winding centre tap

Lead length

Black centre-tap lead at least 11 in, or approximately 279 mm

Shipping weight

2.04 kg according to Hammond

Connection details

120 V primary connection

Connect the two 120 V windings in phase and in parallel: join pin 1 to pin 3 and pin 2 to pin 4

240 V primary connection

Connect the two 120 V windings in series: join pin 2 to pin 3 and apply the mains supply across pin 1 and pin 4

High-voltage winding

Pins 7 and 9 are the two 305 V ends; pin 8 is the centre tap

5.1 V winding

Pin 10 is marked 0 V and pin 11 is the 5.1 V connection

6.66 V heater winding

Pin 6 is marked 0 V, the black lead is the 3.33 V centre tap and pin 5 is the 6.66 V connection

Selection and installation notes

The specified 610 V CT, 5.1 V and 6.66 V CT figures are loaded secondary ratings. No-load voltages are higher, while the final heater and high-voltage rails also depend on the actual mains voltage, rectifier, filter circuit and current demand. Heater voltages should therefore be checked under normal operating load in the completed amplifier.

The electrical-data line in the manufacturer drawing describes the 5.1 V winding as centre-tapped, but the connection diagram shows only two accessible terminals at pins 10 and 11. An external centre tap on this winding should not be assumed. The black flying lead is solely the centre tap of the 6.66 V heater winding and is not a protective-earth conductor.

Connecting the parallel primary windings with incorrect phase creates a winding short circuit. Mains wiring, primary fusing, protective earth, strain relief, terminal protection and insulation clearances must be completed by a qualified technician. Filter capacitors can retain lethal voltages after the amplifier has been disconnected from the mains.

Questions about selection and use

Is the Hammond 290PX suitable as a power transformer for a classic VOX AC15?

Yes, Hammond lists the 290PX as the power transformer reference for the AC15. It is suitable when the 305-0-305 V at 120 mA, 5.1 V at 3 A, 6.66 V CT at 4 A, 120/240 V primary arrangement, mounting and connections match the exact amplifier.

Can the Hammond 290PX be connected to a European 230 V mains supply?

Yes, the two 120 V primary windings are connected in series as the nominal 240 V configuration. At 230 V the secondary voltages will be proportionally slightly below the 240 V figures, so heater and high-voltage rails should be checked under load in the completed amplifier.

Can the Hammond 290PX be used in a custom EL84 valve amplifier?

Yes, provided the rectifier circuit, total heater demand, required high-voltage current, filtering and mechanical installation all match the transformer ratings. The AC15 label alone does not replace a full power-supply calculation.