LL1688 120mA Lundahl SE OPT AÜ 5,5k 9,2k Übertrager transfor.

Lundahl LL1688/120mA – high-voltage SE output transformer with three primary loads

BTB lists the Lundahl LL1688/120mA as the higher-current single-ended version of the large LL1688 family. It provides primary loads of approximately 5.5, 9.2 or 20.5 kΩ and is structurally intended for high-voltage output stages such as correctly operated 845 circuits.

The public LL1688 data sheet confirms the common winding and mechanical construction but gives air-gap, primary-inductance and frequency-response data only for the LL1688/70mA. Those three figures are therefore not transferred to the 120 mA version.

Suitable applications

• High-current 845 single-ended output stages with a calculated load line
• Other high-voltage power triodes whose current and signal swing suit the confirmed 120 mA version
• Primary loads of approximately 5.5, 9.2 or 20.5 kΩ
• 4, 8 and 16 Ω loudspeakers using the documented secondary arrangements
• Mono and stereo high-voltage projects using one output transformer per channel
• New designs with verified insulation, cooling, load line and mechanical support

Version-specific data status

BTB model designation

LL1688/120mA

Current figure stated by BTB

120 mA

Publicly confirmed air gap for the 120 mA version

Not stated in the currently accessible LL1688 data sheet

Publicly confirmed primary inductance for the 120 mA version

Not stated in the currently accessible LL1688 data sheet

Publicly confirmed frequency response for the 120 mA version

Not stated in the currently accessible LL1688 data sheet

Required verification

Check the component marking, delivery documentation or a version-specific manufacturer confirmation before final circuit design

Common LL1688 family data

Product type

Large single-ended valve output transformer

Turns ratio

50+50:1+2+2+1+1+2+2+1

Primary impedances

Approx. 5.5, 9.2 or 20.5 kΩ plate load

Secondary impedances

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

Primary DC resistance

Approx. 260 Ω with all primary sections in series

Secondary DC resistances

Approx. 0.3, 0.7, 0.7 and 0.4 Ω for the individual sections

Primary leakage inductance

Approx. 7 mH

Thermal primary-current limit

200 mA according to the 10 W winding-loss criterion; not a substitute for the magnetic version rating

Maximum primary signal voltage in SE operation

Approx. 530 V RMS at 30 Hz with the complete primary in series

Magnetic power limit at 20.5 kΩ

Approx. 15 W at 30 Hz in single-ended operation

Magnetic power limit at 9.2 kΩ

Approx. 30 W at 30 Hz in single-ended operation

Magnetic power limit at 5.5 kΩ

Approx. 60 W at 30 Hz in single-ended operation

Winding loss

Approximately 0.15, 0.25 or 0.5 dB depending on the primary load

Primary-to-secondary dielectric test

4 kV AC

Winding-to-core dielectric test

2 kV AC

Core

Lundahl audio C-core

Winding construction

Highly sectioned, harmonically proportioned winding with low capacitance

Construction

Open, unpotted frame for installation inside a closed amplifier enclosure

Dimensions

Approx. 140 × 95 × 95 mm

Mounting pattern

Approx. 58 × 118 mm with four M4 mounting points

Weight

Approx. 4 kg

Primary loads and loudspeaker matching

20.5 kΩ plate load

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

9.2 kΩ plate load

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

5.5 kΩ plate load

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

33% ultralinear taps

Shown in the manufacturer drawing for push-pull operation; the documented standard SE connection does not use these taps

Comparison with the 70 mA version

The LL1688/70mA is publicly documented with a 240 µm air gap and 70 H of primary inductance. Where the actual operating point is near 70 mA, that version can be easier to evaluate because its magnetic design is fully stated.

A circuit operating around 120 mA requires the actual 120 mA version. Its primary inductance and air gap must not be inferred from the 70 mA data.

Selection and safety notes

The 30 Hz power figures are magnetic transformer limits. Actual output is also limited by the output valve, plate voltage, quiescent current, driver swing, permitted dissipation and power supply.

845 and related output stages use immediately lethal voltages. Install the open LL1688 inside a closed enclosure with adequate creepage and clearance distances.

The chassis must safely support approximately 4 kg together with transport loading. Establish core orientation relative to the mains transformer and the second output transformer before drilling.

Questions about selection and use

Which large SE output transformer suits a high-current 845 stage operating around 120 mA?

Lundahl LL1688/120mA is the BTB-listed version for this current range and provides 5.5, 9.2 or 20.5 kΩ plate loads. Confirm the air gap and primary inductance of the delivered 120 mA version before finalising the load line.

Which Lundahl SE output transformer provides 5.5, 9.2 and 20.5 kΩ for high-voltage triodes?

Lundahl LL1688/120mA provides these three loads within the LL1688 winding family. Select the load from the output valve, plate voltage, quiescent current and required signal swing.

Can the 120 mA version operate at a lower quiescent current?

Yes, the 120 mA version can generally operate at a lower current when the load line and signal swing are suitable. Where the operating point is much closer to 70 mA, the documented LL1688/70mA can provide more predictable bass reserve from its confirmed 70 H.

Is a 9.2 kΩ to 8 Ω connection available with the LL1688/120mA?

Yes, the documented secondary Alternative C provides approximately 9.2 kΩ with an 8 Ω loudspeaker. Follow the Lundahl underside-view connection diagram exactly.