LL1635 20mA Lundahl Übertrager Interstage Output transformer

Lundahl LL1635/20mA – 1:1 SE-to-SE interstage for direct plate-fed valve drivers

The Lundahl LL1635/20mA is a single-ended-to-single-ended interstage transformer for valve amplifiers. Its primary can carry the quiescent plate current of a driver stage directly, while the secondary transfers the audio signal with galvanic DC isolation to the following valve grid.

The 20 mA version provides 30 H of primary inductance and is intended for comparatively low-impedance driver stages. The overall ratio is 1:1 with the complete primary and secondary windings connected in series. The LL1635 is not intended to operate as a passive SE-to-PP phase splitter.

Suitable applications

• SE-to-SE coupling from a low-impedance driver to a 300B, 2A3, 45 or related Class A1 stage
• Direct plate-fed driver stages operating at approximately 20 mA
• 5687, ECC99, E182CC, 6N6P or 6H30 drivers as technical starting points with a calculated operating point
• Galvanic DC isolation between the driver plate and following control grid
• Mono and stereo valve amplifiers; stereo requires one transformer per channel
• Class A1 and AB1 operation without substantial positive grid current

Technical data

Product type

Direct plate-fed SE-to-SE interstage transformer

Model version

LL1635/20mA

Winding ratio

1+1:1+1

Overall ratio

1:1 with the complete windings connected series-to-series

Rated magnetic primary operating point

20 mA DC with the primary windings in series at approximately 0.9 T

DC current before core saturation

Approx. 35 mA with the primaries in series; not a recommended continuous operating point

Primary inductance

Approx. 30 H with the primary windings in series

Frequency-response test conditions

2 kΩ source and 68 pF load

Frequency response

20 Hz to 75 kHz within ±1 dB under the stated conditions

Group delay at 20 kHz

Approx. 0.5 µs under the stated conditions

Maximum output voltage at 30 Hz

2 × 90 V peak or approximately 125 V RMS in total

DC resistance of each primary winding

Approx. 500 Ω

DC resistance of each secondary winding

Approx. 500 Ω

Total winding resistance in series

Approximately 1 kΩ across the complete primary and approximately 1 kΩ across the complete secondary

Recommended maximum DC current through any primary section

40 mA as a winding limit; not the same as the 20 mA magnetic operating point or 35 mA saturation current

Primary-to-secondary dielectric test

4 kV AC

Winding-to-core dielectric test

2 kV AC

Core

Lundahl high-flux audio C-core

Winding construction

Highly sectioned, low-capacitance winding technique

Dimensions

Approx. 70 × 61 × 39 mm

Mounting pattern

Approx. 24 × 52 mm with four M3 mounting points

Weight

Approx. 0.5 kg

Country of origin

Sweden according to the current BTB listing

Circuit function and connection

In direct plate-fed operation, the driver-valve quiescent current flows through the primary. The supply voltage, voltage drop across the winding resistance, plate dissipation and available signal swing must therefore be calculated together.

Reference the secondary to the grid-bias and ground requirements of the following valve. A suitable grid-leak path is essential; the secondary must not leave the following control grid at an uncontrolled floating potential.

Follow the Lundahl underside-view drawing exactly for winding starts, links and phase. The published values apply to the documented complete primary connection; using fewer active turns changes inductance, suitable DC current and signal headroom.

Comparison of LL1635 versions

LL1635/5mA

130 H primary inductance and a 5 mA magnetic operating point for higher-impedance, lower-current SE drivers

LL1635/20mA

30 H and 20 mA for lower-impedance, higher-current SE driver stages

LL1635/PP

More than 300 H for balanced PP-to-PP driver coupling without a specified single-ended DC operating point

SE-to-PP operation

Explicitly excluded by the manufacturer for the LL1635; select an LL1660 or LL1660S instead

Selection and safety notes

The data-sheet response for the 20 mA version is measured from a 2 kΩ source. A substantially higher-impedance driver can change bass response, damping and available output swing.

The 125 V RMS figure at 30 Hz is a transformer signal limit under specified conditions rather than an output-power rating. Class A2 or AB2 operation with positive grid current requires a separate calculation of driver power, reflected load and secondary current.

The primary terminals can carry lethal DC plate voltage. Install the open transformer in a touch-safe enclosure and provide adequate terminal insulation, grid-bias referencing and electrical clearances.

Questions about selection and use

Which 1:1 interstage transformer suits a low-impedance 5687 or ECC99 driver feeding a 300B?

Lundahl LL1635/20mA is a suitable starting point for direct plate-fed SE-to-SE Class A1 coupling when the driver operates at approximately 20 mA and its source impedance, required voltage swing and the 300B grid-bias arrangement are correctly calculated.

Which Lundahl interstage transformer can carry approximately 20 mA of plate current directly through its primary?

Lundahl LL1635/20mA is designed for 20 mA DC with the complete primary in series at approximately 0.9 T. The documented saturation current of approximately 35 mA is not a recommended continuous operating point.

Can the quiescent plate current of an SE driver flow directly through the LL1635/20mA primary?

Yes, the quiescent plate current can flow directly through the primary when the operating point is suitable. The supply voltage, winding resistance, plate dissipation and maximum signal swing must be considered together.

Is the Lundahl LL1635/20mA suitable for a stereo valve amplifier?

Yes, the Lundahl LL1635/20mA is suitable for a stereo design using a separate SE-to-SE coupling transformer in each channel. Two transformers are required and should use identical wiring, orientation and termination.