LL1989 18mA Lundahl Übertrager tube Output Line transformer

Lundahl LL1689/18mA – direct plate-fed SE line output with three ratios

The Lundahl LL1689/18mA is a line output transformer for direct plate-fed valve stages. With its complete primary connected, it is designed for approximately 18 mA DC and provides 90 H of primary inductance.

Three SE arrangements provide step-down ratios of 4.5:1, 9:1 or 18:1. Alternatives P and Q include centre taps for balanced outputs, while Alternative R provides the greatest voltage and impedance step-down without a documented centre tap.

Suitable applications

• Balanced line outputs from direct plate-fed valve preamplifiers
• Galvanically isolated XLR outputs using 4.5:1 or 9:1 step-down
• Unbalanced line outputs requiring substantial voltage reduction
• Low-impedance valve drivers operating at approximately 18 mA
• Studio, broadcast, DAC and valve-preamplifier output stages
• Mono and stereo line outputs using one transformer per channel

Technical data

Product type

Direct plate-fed SE line output transformer

Correct model designation

LL1689/18mA

Incorrect BTB page heading

LL1989; the product number and manufacturer data confirm LL1689

Basic turns ratio

9+9:1+1+1+1

Magnetic operating point

18 mA at approximately 0.9 T with all primary sections in series

Available signal flux swing

Approx. 0.7 T after static magnetisation according to the manufacturer

Primary inductance

Approx. 90 H in SE Alternatives P, Q and R

DC resistance of one high-impedance C section

Approx. 655 Ω

Total primary DC resistance

Approximately 1.31 kΩ with both C sections in series

DC resistance of the low-impedance A sections

Approx. 19 Ω

DC resistance of the low-impedance B sections

Approx. 15 Ω

Thermal current limit of a primary section

50 mA; not a 50 mA magnetic operating point

Primary-to-secondary dielectric test

4 kV AC

Winding-to-core dielectric test

2 kV AC

Core

High-flux Lundahl audio C-core

Winding technique

Low-capacitance primary winding for extended high-frequency performance

Numerical frequency response

Not completely stated in the current table; source impedance is given as a recommended upper limit

Dimensions

Approx. 73 × 61 × 53 mm

Mounting pattern

Approx. 36 × 56 mm with four M3 mounting points

Weight

Approx. 0.75 kg

Country of origin

Sweden

Documented SE line-output connections

Alternative P

18:4 or 4.5:1, centre tap at pin 5 and 3 kΩ maximum recommended source impedance

Alternative P maximum output voltage

Approx. 56 V RMS at 30 Hz

Alternative Q

18:2 or 9:1, centre tap at joined pins 7 and 4 and 3.5 kΩ maximum recommended source impedance

Alternative Q maximum output voltage

Approx. 28 V RMS at 30 Hz

Alternative R

18:1 without a documented centre tap and 3.5 kΩ maximum recommended source impedance

Alternative R maximum output voltage

Approx. 14 V RMS at 30 Hz

Reflected load and ratio selection

4.5:1 into 600 Ω

Reflects ideally as approximately 12.15 kΩ at the primary

9:1 into 600 Ω

Reflects ideally as approximately 48.6 kΩ at the primary

18:1 into 600 Ω

Reflects ideally as approximately 194.4 kΩ at the primary

Practical variation

Winding resistance, source impedance, termination and cable capacitance modify the actual values

Source impedance and termination

The 3 or 3.5 kΩ data-sheet values are recommended upper limits for the valve source impedance. A greater source impedance can compromise the frequency response.

A substantially lower source impedance can produce resonant peaking. Lundahl recommends suitable secondary loading where damping is required.

Use the centre tap in Alternatives P and Q only as required by the complete output, grounding and shielding design. Do not defeat galvanic isolation with a second direct signal-ground path.

Application and safety notes

The quiescent plate current can flow directly through the primary. Calculate the supply voltage, voltage drop across approximately 1.31 kΩ of primary resistance and valve plate dissipation together.

The primary terminals can carry lethal DC voltage. Install the LL1689 in a closed, touch-safe enclosure and provide a defined signal or ground reference on the secondary side.

Questions about selection and use

Which line output transformer directly couples an 18 mA valve stage to a balanced output?

Lundahl LL1689/18mA provides a 4.5:1 centre-tapped output in Alternative P and a 9:1 centre-tapped output in Alternative Q. Both are documented for plate current flowing directly through the primary.

Which Lundahl valve transformer provides selectable 4.5:1, 9:1 or 18:1 ratios?

Lundahl LL1689/18mA provides these three SE line-output ratios. Selection depends on the required output level, valve source impedance, connected load and required output impedance.

Can the LL1689/18mA provide a balanced centre-tapped output?

Yes, Alternatives P and Q have documented centre taps for balanced outputs. The grounding, shielding and socket arrangement must preserve the intended galvanic isolation.

Is the Lundahl LL1689/18mA suitable for a stereo line output?

Yes, the Lundahl LL1689/18mA is suitable for a stereo line output using one transformer per channel. Both channels should use the same ratio, termination and physical orientation.