LL2741 Lundahl Netztrafo 250VA 290V 0,55A 4x6,3V 3,1A

Lundahl LL2741 – 250 VA C-core mains transformer with a flexible high-voltage secondary

The Lundahl LL2741 is a C-core mains transformer for valve amplifiers. It combines a substantial high-voltage secondary with four separate 6.3 V heater windings, supporting several galvanically isolated heater circuits.

The high-voltage secondary consists of two equal sections. They can provide 290 V in series, approximately 145-0-145 V with a centre tap or 145 V with twice the current capability when correctly connected in parallel.

Suitable applications

• Bridge-rectified valve-amplifier supplies using 290 V AC
• Centre-tapped full-wave rectification from approximately 145-0-145 V
• Lower-voltage, higher-current supplies using 145 V AC from parallel windings
• Mono and stereo amplifiers requiring several separate heater circuits
• Power supplies for low-impedance, high-current valves such as the 6C33C-B after complete power-budget calculation
• AC or following DC heater supplies for preamplifier, driver and output valves

Technical data

Product type

C-core mains transformer for valve amplifiers

Model

LL2741

Total rating

Approx. 250 VA shared by all secondary windings

Primary connection for 230 V

Two primary windings connected in phase and in series

Primary connection for 115 V

Two primary windings connected in phase and in parallel

Primary DC resistance in series

Approx. 7.5 Ω

Primary DC resistance in parallel

Approx. 1.9 Ω

Main secondary in series

290 V AC no-load at 0.55 A maximum recommended RMS current

Main secondary with centre tap

Approximately 145-0-145 V AC at 0.55 A maximum recommended RMS current

Main secondary in parallel

145 V AC at 1.1 A maximum recommended RMS current with correct winding phase

DC resistance of the complete high-voltage secondary

Approx. 16 Ω

Heater windings

Four separate windings, each providing 6.3 V AC no-load

Maximum recommended current per heater winding

3.1 A RMS within the shared 250 VA power budget

DC resistance per heater winding

Approx. 0.1 Ω

Calculated power per heater winding

Approx. 19.5 VA at 6.3 V and 3.1 A

Calculated power of all four heater windings

Approx. 78.1 VA when fully loaded

Rectifier guidance with capacitor input

The manufacturer indicates approximately 63% of the stated AC RMS current as a DC-current guide before applying the complete power budget

Rectifier guidance with choke input

The manufacturer indicates approximately 95% of the stated AC RMS current as a DC-current guide before applying the complete power budget

Core

Lundahl C-core with a small balancing air gap to resist mains DC imbalance

Magnetic stray field

Particularly low when loads are distributed as evenly as practical between both coils

Dimensions

Approx. 118 × 85 × 78 mm

Mounting pattern

Approx. 53 × 90 mm with four M4 mounting points

Weight

Approx. 2.5 kg

Country of origin

Sweden

Power budgeting and rectification

The 290 V/0.55 A and four 6.3 V/3.1 A figures are winding ratings within one shared total. Fully loading all four heater windings uses approximately 78 VA and leaves correspondingly less capacity for the high-voltage circuit and transformer losses.

A 290 V/0.55 A load represents approximately 160 VA. Combined with four fully loaded heater windings, the calculated total is about 238 VA and therefore already close to the 250 VA design budget.

All listed voltages are no-load values. Loaded voltage depends on mains voltage, winding resistance, rectifier topology and current draw. Measure the heater and plate supplies in the completed amplifier under real load.

Safety notes

The primary and high-voltage secondary carry immediately lethal voltages. Provide suitable primary fusing, double-pole isolation, protective earth, strain relief and complete touch protection.

The chassis must safely support the 2.5 kg transformer and transport shock. Thin chassis material can require a reinforcing plate and large washers.

Questions about selection and use

Is the Lundahl LL2741 suitable for a bridge-rectified valve supply using 290 V AC?

Yes, the Lundahl LL2741 provides 290 V AC at up to 0.55 A recommended RMS current with its high-voltage sections in series. The rectifier, filter, required B+ voltage and complete 250 VA power budget must suit the circuit.

Can the LL2741 high-voltage winding also provide 145-0-145 V or 145 V/1.1 A?

Yes, the two equal sections can provide approximately 145-0-145 V with a centre tap or 145 V/1.1 A when connected in phase and in parallel. Incorrect phase in the parallel connection would short the windings.

Is the LL2741 suitable for four separate valve-heater circuits?

Yes, the LL2741 has four separate 6.3 V/3.1 A windings. Verify the actual heater voltage under the connected load, and keep all heater and high-voltage loads within the shared 250 VA rating.