LL1650 Lundahl Netztrafo 250VA 2x 275V 125mA 4x6,6V 3,1A

Lundahl LL1650 – 250 VA C-core mains transformer with a 350 V high-voltage winding

The Lundahl LL1650 is a C-core mains transformer for substantial valve-amplifier power supplies. It provides a 350 V/0.63 A main secondary together with four separate 6.6 V/3.1 A heater windings.

The two primary windings connect in series for 230 V mains or in parallel for 115 V mains. All secondary voltages are no-load figures; actual voltage depends on mains voltage, winding resistance, load and rectifier topology.

Suitable applications

• High-voltage supplies for substantial hi-fi and valve power amplifiers
• Bridge-rectified B+ supplies from a 350 V AC secondary
• Push-pull output stages requiring several separate heater circuits
• Mono and stereo projects using four galvanically separate 6.6 V windings
• AC or following DC heater supplies for preamplifier, driver and output valves
• Power supplies whose complete high-voltage and heater load remains within 250 VA

Technical data

Product type

C-core mains transformer for valve amplifiers

Model

LL1650

Estimated total rating

250 VA, shared by all secondary windings

Primary connection for 230 V

Both primary windings in series; manufacturer range 210 to 250 V

Primary connection for 115 V

Both primary windings connected in phase and in parallel; manufacturer range 105 to 125 V

Primary DC resistance in series

Approx. 7.5 Ω

Primary DC resistance in parallel

Approx. 1.9 Ω

Main secondary Sec 1

350 V AC no-load voltage in the standard series connection

Maximum recommended RMS current for Sec 1

0.63 A as a winding figure; total power and heater loads limit the simultaneously usable current

DC resistance of Sec 1

Approx. 20 Ω

Sec 1 construction

Two equal winding halves providing 350 V total or approximately 175-0-175 V at the midpoint

Secondaries Sec 2 to Sec 5

Four separate windings, each providing 6.6 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 of one heater winding

Approx. 20.5 VA at 6.6 V and 3.1 A

Calculated power of all four heater windings

Approx. 81.8 VA when fully loaded

Manufacturer power-distribution guidance

For favourable utilisation, take approximately 75% of the total power from Sec 1

Rectifier current with capacitor input

Manufacturer guidance is approximately 63% of AC RMS current; 0.63 A therefore corresponds theoretically to about 0.40 A DC before applying the total power budget

Rectifier current with choke input

Manufacturer guidance is approximately 95% of AC RMS current; 0.63 A therefore corresponds theoretically to about 0.60 A DC before applying the total power budget

Core

Lundahl C-core with a small balancing gap to resist saturation caused by mains DC imbalance

Magnetic stray field

Particularly low when secondary loads on the two coils are distributed as equally as practical

Dimensions

Approx. 118 × 85 × 78 mm

Mounting pattern

Approx. 53 × 90 mm with four M4 mounting points

Weight

Approx. 2.5 kg

Connections

Open PCB or solder pins shown in the underside-view drawing

Country of origin

Sweden according to the current BTB listing

Primary and secondary connections

First primary winding

Pins 1 to 2

Second primary winding

Pins 4 to 3

230 V primary connection

Join pin 2 to pin 3 and apply mains between pins 1 and 4

115 V primary connection

Join pins 1 and 4 together and join pins 2 and 3 together with the correct phase

Main secondary

Sections on pins 5 to 6 and pins 8 to 7; for 350 V join pins 6 and 7 and take the output from pins 5 and 8

Sec 2

Pins 13 to 11

Sec 3

Pins 14 to 12

Sec 4

Pins 17 to 15

Sec 5

Pins 18 to 16

Earlier pin positions

Older production can use positions 14a, 13a, 16a and 15a instead of the current positions

Power budgeting

A 350 V/0.63 A load already represents approximately 220.5 VA. Loading all four heater windings at 3.1 A in addition would push the total well beyond 250 VA. The individual winding currents are therefore not simultaneously available at their maxima.

With all four heater windings fully loaded, approximately 168 VA remain within a 250 VA budget for Sec 1. This corresponds to approximately 0.48 A at 350 V. It is an approximate total-power calculation rather than a new winding rating.

Lundahl's guidance to take approximately 75% from Sec 1 corresponds to about 187.5 VA or approximately 0.54 A at 350 V. The remaining approximately 62.5 VA must cover every heater winding in use.

Rectification and obtainable B+ voltage

With bridge rectification, 350 V AC has a theoretical peak of approximately 495 V before diode, copper and load losses. Actual B+ can be substantially lower depending on current draw, mains voltage, rectifier and filter.

A centre-tapped full-wave rectifier uses the two Sec 1 halves as approximately 175-0-175 V. The current data sheet does not document a standard 275-0-275 V or 550 V connection.

A capacitor-input supply produces high charging-current peaks. Rectifiers, first filter capacitance, fuses and inrush limiting require suitable ratings. A choke-input supply requires critical inductance and a continuous minimum load.

Safety and installation notes

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

The four 6.6 V figures are no-load voltages. Check heater voltage and any following DC regulator under the actual valve load and maximum mains voltage.

The 2.5 kg transformer requires a sufficiently rigid chassis. Use suitable distance and core orientation between the mains transformer, output transformers and sensitive input transformers to reduce magnetic coupling.

Questions about selection and use

Which Lundahl mains transformer provides 350 V/0.63 A and four separate 6.6 V heater windings?

Lundahl LL1650 provides a current-data-sheet Sec 1 rating of 350 V/0.63 A together with four separate 6.6 V/3.1 A windings. All windings share the 250 VA total rating and must not be fully loaded together without calculation.

Which 250 VA C-core mains transformer suits a high-power valve amplifier requiring elevated B+ voltage?

Lundahl LL1650 is a useful starting point for correctly designed high-voltage valve supplies using its 350 V main secondary. The rectifier, required B+ voltage, plate current and heater-power demand must be designed together.

Can the LL1650 operate from nominal 115 V and 230 V mains?

Yes, the LL1650 can operate from 115 V with its primaries in parallel or 230 V with its primaries in series. The windings must be connected with the correct phase and full mains-safety provisions.

Is the LL1650 suitable for bridge rectification and four separate heater circuits?

Yes, the LL1650 is primarily suited to bridge rectification and provides four separate heater windings. Their actual voltage and loading must remain within the shared 250 VA budget together with Sec 1.