LL1663 50mA Lundahl Übertrager 5,0k SE 8W output transformer

Lundahl LL1663/50mA – 5 kΩ single-ended output transformer for 8 Ω speakers

The Lundahl LL1663/50mA is a single-ended valve output transformer with a fixed nominal match of approximately 5 kΩ to 8 Ω. The 50 mA version provides approximately 35 H of primary inductance.

Its four-section dual-coil construction uses a Lundahl audio C-core and insulation foil between individual copper layers. The documented 8 W at 30 Hz is a magnetic transformer limit rather than a universal amplifier-output rating.

Suitable applications

• Single-ended stages requiring a primary load around 5 kΩ
• Correctly designed 2A3, 45, PX4 and related power-triode stages
• Conservatively operated 300B circuits at approximately 50 mA and a suitable plate voltage
• Triode-connected EL34, 6V6GT or EL84 stages with a suitable load line
• Mono and stereo SET amplifiers; stereo requires one transformer per channel
• 8 Ω loudspeakers where an approximately 5 kΩ primary load is required

Technical data

Product type

Single-ended valve output transformer

Model version

LL1663/50mA

Nominal primary impedance

Approx. 5 kΩ plate load with an 8 Ω loudspeaker

Nominal secondary impedance

8 Ω

Winding ratio

12+12:1 or (4+8)+(4+8):1

Magnetic SE operating point

50 mA according to the model version

Primary inductance

Approx. 35 H

Maximum primary signal voltage

Approx. 200 V RMS at 30 Hz

Magnetic power limit

Approx. 8 W at 30 Hz into an 8 Ω load

DC resistance of each primary half

Approx. 102 Ω

Calculated total primary DC resistance in series

Approx. 204 Ω

Secondary DC resistance

Approx. 0.4 Ω

Maximum DC current through one primary winding

160 mA as a winding limit; not a recommended SE quiescent current

Winding-to-winding dielectric test

4 kV AC

Winding-to-core dielectric test

2 kV AC

Core

Lundahl audio C-core

Winding insulation

Insulation foil between individual copper layers

Dimensions

Approx. 95 × 75 × 50 mm

Mounting pattern

Approx. 35 × 75 mm with four M4 mounting points

Weight

Approx. 1.35 kg

Country of origin

Sweden according to the current BTB listing

Single-ended connection

In the documented SE connection, the two primary halves operate in phase and in series. The manufacturer drawing shows B+ on pin 1, a link from pin 5 to pin 11 and the plate on pin 7. The secondary is connected to pins 13 and 17; the drawing uses the underside view.

The 33% taps in the LL1663 family are shown for push-pull operation. They must not be assumed to provide an identical single-ended ultralinear connection without a separate winding and load analysis.

Other loudspeaker impedances

The LL1663 has no switchable 4, 8 and 16 Ω secondary taps. Connecting a 4 Ω speaker instead of 8 Ω reflects approximately 2.5 kΩ to the primary. A 16 Ω speaker reflects approximately 10 kΩ.

A different speaker impedance is therefore useful only where the resulting primary load is deliberately required by the output-valve load line.

Comparison with the 100 mA version

The 50 mA version provides approximately 35 H, more primary inductance than the approximately 17 H of the 100 mA version. A substantially greater quiescent current requires the air-gap version intended for that current.

Selection and safety notes

A lower plate load tends to increase current and valve stress. A higher plate load requires greater voltage swing and normally reduces maximum output. Plate voltage, quiescent current, dissipation and load line must be verified together.

Lethal plate voltage can be present on the open transformer. Install it with touch protection and ensure that the chassis safely supports its weight.

Questions about selection and use

Which 5 kΩ single-ended output transformer suits a 2A3, 45 or PX4 stage operating at approximately 50 mA?

Lundahl LL1663/50mA provides approximately 5 kΩ with an 8 Ω load, 35 H of primary inductance and a 50 mA magnetic SE operating point. The exact load line must suit the valve's plate voltage and dissipation.

Which Lundahl SE output transformer provides 8 W at 30 Hz and a fixed 8 Ω output?

Lundahl LL1663/50mA is documented for approximately 5 kΩ to 8 Ω, 200 V RMS primary signal at 30 Hz and an 8 W magnetic limit.

Can the LL1663/50mA version operate at a somewhat lower quiescent current?

Yes, the LL1663/50mA version can operate at a lower quiescent current when the load line and signal swing are suitable. The operating point must still provide adequate primary inductance and remain within the valve's plate dissipation.

Is the Lundahl LL1663/50mA suitable for a stereo SET amplifier?

Yes, the Lundahl LL1663/50mA is suitable for a stereo SET amplifier using one transformer per channel. Both channels should use the same valves, load, winding connection and core orientation.