LL2777B -70 Lundahl Übertrager 3,5k SE output transformer

Lundahl LL2777B/70 compact high-end SE transformer for desktop amplifiers

The Lundahl LL2777B/70 is a compact single-ended output transformer for high-quality low-power valve amplifiers. At approximately 0.75 kg, it is particularly suitable for desktop, nearfield and compact stereo systems.

Its primary load is 3.5 kΩ into 5 Ω or 3.4 kΩ into 8 Ω. A 50% ultralinear tap extends its use to suitable pentodes and beam tetrodes.

Suitable applications

• 2A3, PX4 and AD1 single-ended amplifiers
• 6C19P and triode-connected EL34 output stages
• Compact desktop and nearfield valve amplifiers
• Small ultralinear amplifiers for efficient loudspeakers

Technical specifications

Transformer type

Compact single-ended valve-amplifier output transformer

Core material

Grain-oriented silicon-steel Lundahl C-core

Primary and secondary impedances

3.5 kΩ to 5 Ω or 3.4 kΩ to 8 Ω

Ultralinear tap

50% from the B+ connection

Nominal operating point

70 mA of primary DC current for this air-gap version

Primary inductance

60 H for the 70 mA version

Transformer capability

3.5 W at 30 Hz

Frequency-response example

15 Hz to 45 kHz within +0/−2 dB using a 1 kΩ source and 8 Ω load

Insertion loss

Approximately 1 dB

Primary DC resistance

175 Ω

Secondary DC resistance

0.4 Ω at the 5 Ω tap or 0.5 Ω at the 8 Ω tap

Primary leakage inductance

11 mH in the recommended connection

Maximum primary signal voltage

110 V RMS at 30 Hz

Thermal winding limit

150 mA at 4 W winding dissipation; this is not the intended magnetic idle-current rating

Insulation

4 kV between primary and secondary windings and 2 kV between windings and core

Dimensions

Approximately 61 × 53 × 73 mm

Weight

Approximately 0.75 kg

Mounting

Four M3 holes on an approximately 36 × 56 mm pattern

Manufacturer and origin

Lundahl Transformers AB, made in Sweden

Selection notes

The approximately 3.4 to 3.5 kΩ primary load is slightly higher than some traditional 2A3 or PX4 operating points. This can support a more conservative condition with lower current and slightly reduced output.

A 4 Ω loudspeaker does not produce the same reflected load as a 5 Ω speaker. When connected to the 5 Ω tap, the reflected primary impedance falls to approximately 80% of the nominal value and must be considered in the load line.

The LL2777B/70 is intended for modest output and efficient loudspeakers. A larger output transformer is required when substantially more than approximately 3.5 W of full-power bass capability is needed.

Questions about choosing a compact desktop output transformer

Which compact high-end output transformer suits a 2A3, PX4 or AD1 amplifier?

The Lundahl LL2777B/70 is an attractive choice for compact 2A3, PX4 and AD1 single-ended amplifiers using an approximately 3.4 to 3.5 kΩ primary load. Its 70 mA operating point suits many classic triode circuits, while 60 H primary inductance provides a strong basis for a small wide-band output transformer.

Which small output transformer suits a triode-connected EL34 or 6C19P amplifier?

The Lundahl LL2777B/70 suits correctly designed triode-connected EL34 and 6C19P output stages requiring approximately 3.5 kΩ, up to about 70 mA of idle current and a few watts of output. A direct 3.4 kΩ connection is available for an 8 Ω loudspeaker.

Is the LL2777B/70 suitable for a high-quality desktop valve amplifier?

Yes, its compact construction, approximately 0.75 kg weight and 3.5 W transformer capability make it particularly attractive for desktop, nearfield and small stereo amplifiers. Combined with efficient loudspeakers, it supports a compact high-quality audio system.

Can I use the LL2777B/70's 50% ultralinear tap?

Yes, the screen grid of a suitable pentode or beam tetrode can be connected to the 50% tap. This supports pentode, triode-connected or ultralinear output stages. The UL tap is not required when using a directly heated triode such as the 2A3, PX4 or AD1.