LL1627 185mA Lundahl SE OPT 650Ohm 1,2k output transformer

Lundahl LL1627/185mA – high-current SE output transformer with 650 Ω, 1.2 or 2.3 kΩ

The Lundahl LL1627/185mA is a single-ended output transformer for low-impedance, high-current valve stages. The 185 mA version uses a 250 µm air gap and provides approximately 9 H of primary inductance.

Primary loads of 650 Ω, 1.2 kΩ or 2.3 kΩ support correctly designed 6C33C/6S33S, 6AS7/6080, 6528, 6336 and related stages. The divided secondary windings provide several loudspeaker-matching alternatives.

Suitable applications

• 6C33C-B or 6S33S single-ended output stages
• 6AS7G, 6080, 6H13C, 6528 and 6336 high-current stages
• Triode-connected EL509, EL519 and related power valves
• Primary loads of 650 Ω, 1.2 kΩ or 2.3 kΩ
• Single-ended amplifiers with a magnetic quiescent current around 185 mA
• 4, 8 and 16 Ω speakers with the documented winding combinations
• Experimental 300 Ω operation following the separate BTB connection note

Technical data

Product type

Single-ended high-current output transformer

Model version

LL1627/185mA

Rated magnetic operating point

185 mA DC at approximately 0.9 T

Air gap

Approx. 250 µm

Primary inductance

Approx. 9 H for the 185 mA version

Standard primary impedances

650 Ω, 1.2 kΩ or 2.3 kΩ plate load

Secondary impedances

4, 8 or 16 Ω at 650 Ω and 1.2 kΩ; 8 or 16 Ω at 2.3 kΩ in the standard connections

Winding ratio

4 × 8.5:8 × 1

Primary DC resistance

Approx. 56 Ω with all sections in series, comprising four sections of approximately 14 Ω

Secondary DC resistance

Approx. 0.4 Ω per secondary section

Primary leakage inductance

Approx. 1.9 mH

Maximum primary signal voltage

Approx. 170 V RMS at 30 Hz in single-ended operation

Magnetic power limit at 2.3 kΩ

Approx. 13 W at 30 Hz; not a guaranteed amplifier-output rating

Magnetic power limit at 1.2 kΩ

Approx. 25 W at 30 Hz; not a guaranteed amplifier-output rating

Magnetic power limit at 650 Ω

Approx. 50 W at 30 Hz; not a guaranteed amplifier-output rating

Winding loss

Approximately 0.2, 0.5 or 0.8 dB depending on the primary load

Thermal primary-current limit

350 mA according to the 7 W loss criterion; not a recommended single-ended quiescent current

Winding-to-winding dielectric test

3 kV AC

Winding-to-core dielectric test

1.5 kV AC

Core

Lundahl audio C-core

Dimensions

Approx. 118 × 85 × 78 mm

Mounting

Approx. 53 × 90 mm with four M4 mounting points

Weight

Approx. 2.5 kg

Special 300 Ω connection documented by BTB

BTB documents a special connection for an approximately 300 Ω application: connect the high-voltage supply to joined pins 2 and 8, connect the plate to joined pins 3 and 5, and join pins 1, 4, 6 and 7.

This arrangement uses a different number of active primary turns from the standard alternatives. The standard 9 H, 185 mA and 170 V RMS figures must therefore not be assumed to remain unchanged. Verify the operating point, bass reserve, core flux and signal voltage for the exact circuit.

Comparison of current versions

LL1627/90mA

125 µm air gap and approximately 18 H; greater primary inductance at lower quiescent current

LL1627/140mA

190 µm air gap and approximately 12 H; intermediate high-current version

LL1627/185mA

250 µm air gap and approximately 9 H; the highest documented magnetic single-ended operating point in the standard range

Selection and safety notes

A lower plate load tends to allow greater current but places greater stress on the output valve. A higher plate load tends to reduce current but requires greater voltage swing and can reduce maximum output.

The 185 mA figure is not a universally suitable quiescent current for every listed valve. Plate voltage, dissipation, load line, heater and power-supply capacity and cooling must be designed together.

At a substantially lower quiescent current, a 90 or 140 mA version can be preferable because of its greater primary inductance. The 350 mA thermal limit does not describe the optimum magnetic single-ended operating point.

Lethal voltages may be present on the open transformer. Mounting, enclosure, terminal insulation and adequate clearance require suitable technical expertise.

Questions about selection and use

Which single-ended output transformer suits a 6C33C or 6S33S stage operating at approximately 185 mA?

Lundahl LL1627/185mA is designed around a magnetic operating point of approximately 185 mA and provides 650 Ω, 1.2 kΩ or 2.3 kΩ plate loads. Select the load from the plate voltage, quiescent current, dissipation and load line.

Which Lundahl single-ended output transformer provides low plate loads for 6AS7, 6080 or similar high-current stages?

Lundahl LL1627/185mA provides standard plate loads of 650 Ω, 1.2 kΩ and 2.3 kΩ. It suits correctly designed low-impedance, high-current stages when the magnetic operating point and valve dissipation are respected.

Can the 185 mA version also operate at approximately 140 mA?

Yes, the LL1627/185mA can operate at approximately 140 mA when the load line and signal swing are suitable. The dedicated 140 mA version provides approximately 12 H rather than 9 H and can therefore offer greater bass reserve.

Is the special 300 Ω connection documented by BTB usable?

Yes, the special 300 Ω connection can be used in a correspondingly designed high-current stage. Because it differs from the standard winding arrangements, the primary inductance, core flux, maximum signal swing and suitable quiescent current must be verified separately.