LL2752 PP Lundahl Übertrager 2,0k 1,2k output transformer

Lundahl LL2752/PP – flexible high-power push-pull output transformer

The Lundahl LL2752/PP is a large push-pull output transformer for substantial valve power amplifiers. Its highly sectioned primary and secondary windings provide plate-to-plate loads of 1.2, 2.0 or 4.6 kΩ.

It can therefore support both one output-valve pair and correctly designed parallel push-pull circuits. Select the secondary arrangement only after establishing the required plate-to-plate load and actual loudspeaker impedance.

Suitable applications

• Push-pull output stages using EL34, 6550, KT88, KT120, KT150 or related valves
• Parallel push-pull stages using four or more output valves per channel
• Plate-to-plate loads of 1.2, 2.0 or 4.6 kΩ
• Class A and Class AB1 amplifiers without substantial positive grid current
• 4, 8 and 16 Ω loudspeakers in the documented combinations
• Mono and stereo high-end DIY projects using one transformer per channel

Technical data

Product type

Push-pull valve output transformer

Model version

LL2752/PP

Primary impedances

Approx. 1.2, 2.0 or 4.6 kΩ plate-to-plate

Secondary impedances

4, 8 or 16 Ω, depending on primary load and secondary arrangement

Turns ratio

12+12+12+12:2+1+1+2+1+1

Primary DC resistance

Approx. 92 Ω with all four primary sections in series

DC resistance per primary section

Approx. 23 Ω

DC resistance per secondary section

Approx. 0.7 Ω

Primary leakage inductance

Approx. 1 mH

Thermal primary-current limit

280 mA according to the 7 W winding-loss criterion; not a permitted DC imbalance

Maximum primary signal voltage in PP operation

Approx. 480 V RMS at 30 Hz

Magnetic power limit at 4.6 kΩ

Approx. 45 W at 30 Hz

Magnetic power limit at 2.0 kΩ

Approx. 105 W at 30 Hz

Magnetic power limit at 1.2 kΩ

Approx. 180 W at 30 Hz

Winding loss

Approximately 0.2, 0.4 or 0.7 dB depending on the primary load

Primary inductance of the PP version

Not stated separately as a numerical value in the current public data sheet

PP balancing air gap

Not stated separately as a numerical value in the current public data sheet

Primary-to-secondary dielectric test

3 kV AC

Winding-to-core dielectric test

1.5 kV AC

Core material

Silicon-iron audio C-core; an amorphous-core version is available as a separate configuration

Winding construction

Harmonically sectioned, low-capacitance primary and secondary windings

Dimensions

Approx. 118 × 85 × 78 mm

Mounting pattern

Approx. 53 × 90 mm with four M4 mounting points

Weight

Approx. 2.5 kg

Primary loads and loudspeaker connections

4.6 kΩ plate-to-plate

8 Ω using secondary Alternative B or 16 Ω using Alternative C; no documented standard 4 Ω connection

2.0 kΩ plate-to-plate

4 Ω using Alternative B, 8 Ω using Alternative C or 16 Ω using Alternative D

1.2 kΩ plate-to-plate

4 Ω using Alternative C or 8 Ω using Alternative D; no documented standard 16 Ω connection

Ultralinear or CFB taps

Not documented as standard LL2752/PP features

Valve count and primary-load selection

A 4.6 kΩ load can suit one output-valve pair or other higher-load operating points. The 2.0 and 1.2 kΩ alternatives are relevant to lower total loads and often to several parallel output valves.

The correct load depends on valve type, valve count, plate and screen voltage, quiescent current, operating class and intended output. The valve designation alone does not determine the transformer connection.

In parallel push-pull amplifiers, each valve should have an individual control-grid stopper and normally an individual screen-grid stopper where applicable. Verify every quiescent current separately.

Core versions and safety

The amorphous-core version can be a constructive alternative where the circuit requirements match. Compare core material, saturation margin and full-power behaviour for the exact version rather than assuming that amorphous material is universally superior.

The high 30 Hz power figures are magnetic transformer limits rather than guaranteed amplifier output. The open transformer carries lethal voltage and requires a closed, mechanically secure installation.

Questions about selection and use

Which push-pull output transformer suits four KT88 or 6550 valves in a parallel PP amplifier?

Lundahl LL2752/PP is a technically useful starting point with 1.2 or 2.0 kΩ plate-to-plate loads for correctly designed parallel push-pull stages. Valve count, plate voltage, quiescent current and intended output determine the connection.

Which Lundahl output transformer provides 1.2, 2.0 and 4.6 kΩ plate-to-plate loads?

Lundahl LL2752/PP provides these three primary loads and documented secondary arrangements for 4, 8 or 16 Ω. Not every plate-to-plate load supports all three speaker impedances.

Can the LL2752/PP be matched to 4, 8 and 16 Ω loudspeakers?

Yes, the LL2752/PP provides suitable 4, 8 or 16 Ω arrangements depending on the selected primary load. The 4.6 kΩ connection supports 8 and 16 Ω, 2.0 kΩ supports all three and 1.2 kΩ supports 4 and 8 Ω.

Is the LL2752/PP suitable for high-power parallel push-pull amplifiers?

Yes, the LL2752/PP is suitable for correctly designed parallel push-pull amplifiers. Current sharing, individual bias verification, driver capability, power-supply capacity and the thermal loading of every valve must be considered.