290KX Hammond Netztrafo Marshall JCM900 50W

Hammond 290KX – 130 VA power transformer for Marshall JCM 900 50 W amplifiers

The Hammond 290KX is a bobbin-wound power transformer for valve guitar amplifiers. Two separate 120 V primary windings support parallel connection for 120 V or series connection for 240 V at 50/60 Hz.

The secondary windings provide 332 V at 300 mA, 41 V CT at 100 mA and 6.6 V CT at 4 A. Hammond lists the 290KX for JCM 900 50 W amplifiers but does not provide a verified original Marshall part number.

Suitable applications

• Repair of compatible Marshall JCM 900 50 W amplifiers
• Valve amplifiers requiring a single 332 V high-voltage winding rated at up to 300 mA
• Circuits requiring a centre-tapped 41 V auxiliary winding rated at up to 100 mA
• Amplifiers requiring a centre-tapped 6.6 V heater winding rated at up to 4 A
• Service projects requiring selectable 120 or 240 V primary wiring

Technical data

Product type

Power transformer for valve guitar amplifiers

Manufacturer application

Marshall JCM 900 50 W

Original Marshall part number

Not specified and identified as unknown by Hammond

VA rating

130 VA

Winding construction

Bobbin wound

Primary windings

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

High-voltage winding

332 V at 300 mA; not centre-tapped

Auxiliary winding

41 V CT, equivalent to 20.5-0-20.5 V, at 100 mA

Heater winding

6.6 V CT, equivalent to 3.3-0-3.3 V, at 4 A

Separate 5 V winding

Not provided

No-load voltages

345 V, 43.46 V CT and 6.91 V CT according to the Hammond drawing

Turns ratio

1:2.88 between a primary winding and the 332 V secondary winding

Exciting current

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

Primary DC resistance

3.36 Ω between pins 1 and 2 and 3.80 Ω between pins 3 and 4, each ±20% at 25 °C

332 V winding DC resistance

20.1 Ω between pins 8 and 9, ±20% at 25 °C

Heater winding DC resistance

0.04 Ω between pins 7 and 6, ±20% at 25 °C

Auxiliary winding DC resistance

12.0 Ω between the two red flying leads, ±20% at 25 °C

Dimensions

Approx. 96.0 × 102.9 × 80.3 mm according to drawing dimensions A × B × C

Mounting-related dimensions

D approx. 80.0 mm, E approx. 64.0 mm and mounting-hole diameter approx. 5.46 mm

Connections

Solder terminals for the primary, high-voltage and heater windings plus flying leads for the 41 V CT winding

Shipping weight

Approx. 3.24 kg according to Hammond

Connection details

120 V primary connection

Connect pin 1 to pin 3 and pin 2 to pin 4 in phase and in parallel

240 V primary connection

Join pin 2 to pin 3 and apply the mains supply between pin 1 and pin 4

332 V winding

Pin 8 is marked 0 V and pin 9 provides 332 V

6.6 V heater winding

Pin 7 is marked 0 V, pin 5 is the 3.3 V centre tap and pin 6 provides 6.6 V

41 V CT auxiliary winding

The two red leads are the 20.5 V ends and the black lead is the centre tap

Comparison with related Hammond models

Hammond 290KX

JCM 900 50 W application, 120/240 V primary and 130 VA rating

Hammond 290JX

Intended for JCM 900 100 W applications and not interchangeable on the model name alone

Hammond 290GX

Intended for JCM 800 and JMP 50 W applications with different winding requirements

Selection and installation notes

JCM 900 amplifiers were produced in different versions. The amplifier family and power rating alone are therefore insufficient; compare the schematic, mains configuration, rectifier circuit, terminal arrangement and mounting dimensions with the exact amplifier.

With the primaries connected in series for nominal 240 V operation, ideal proportional scaling at exactly 230 V gives approximately 318 V, 39.3 V CT and 6.33 V CT. These are calculated estimates; measure the actual operating voltages under load.

Incorrect phasing of the parallel primary windings creates a winding short circuit. Follow the Hammond connection drawing exactly.

Mains wiring, primary fusing, protective earth, strain relief, touch protection and insulation clearances must be completed by a qualified technician. Filter capacitors can retain lethal voltages after disconnection from the mains.

Questions about selection and use

Is the Hammond 290KX suitable as a power transformer for a Marshall JCM 900 50 W amplifier?

Yes, Hammond explicitly lists the 290KX for JCM 900 50 W applications. It provides 332 V at 300 mA, 41 V CT at 100 mA and 6.6 V CT at 4 A; the exact amplifier revision and wiring must match.

Can the Hammond 290KX be operated from a European 230 V mains supply?

Yes, connect the two 120 V primary windings in series as the nominal 240 V arrangement. At 230 V the secondary voltages will be proportionally slightly lower; verify every operating voltage under load.

Can the Hammond 290KX be configured for either 120 V or 240 V operation?

Yes, connect the primary windings in phase and in parallel for 120 V, or in series for 240 V. Incorrect primary phasing can create a winding short circuit and must be prevented.