LL1688 70mA Lundahl SE OPT AÜ 5,5k 9,2k Übertrager transfor.

Lundahl LL1688/70mA – documented high-voltage SE output transformer with 70 H

The Lundahl LL1688/70mA is a large single-ended output transformer for high-voltage power triodes. The 70 mA version has a documented 240 µm air gap and approximately 70 H of primary inductance.

Primary loads of approximately 5.5, 9.2 or 20.5 kΩ support several 211, 845 and related high-voltage operating points. Select the load from plate voltage, quiescent current, signal swing and permitted valve dissipation.

Suitable applications

• 211 single-ended stages operating at approximately 70 mA with a suitable load line
• Conservatively operated 845 stages with suitable plate current and voltage swing
• Other high-voltage triodes requiring 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 transformer per channel
• Output stages requiring high signal voltage and substantial primary insulation

Technical data

Product type

Large single-ended valve output transformer

Model version

LL1688/70mA

Magnetic SE operating point

70 mA at approximately 0.9 T

Air gap

Approx. 240 µm

Primary inductance

Approx. 70 H

Primary impedances

Approx. 5.5, 9.2 or 20.5 kΩ

Secondary impedances

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

Turns ratio

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

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 the magnetic operating point

Maximum primary signal voltage

Approx. 530 V RMS at 30 Hz in single-ended operation

Magnetic power limit at 20.5 kΩ

Approx. 15 W at 30 Hz

Magnetic power limit at 9.2 kΩ

Approx. 30 W at 30 Hz

Magnetic power limit at 5.5 kΩ

Approx. 60 W at 30 Hz

Winding loss

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

Frequency response under the manufacturer test conditions

10 Hz to 25 kHz at +0/-1 dB or 5 Hz to 33 kHz at +0/-3 dB

Frequency-response test conditions

2.2 kΩ source, 10 Ω load, primary in series, secondary Alternative C, secondary not grounded and approximately 10 V primary signal

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 low-capacitance winding

Construction

Open, unpotted frame

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 connections

20.5 kΩ plate load

8 Ω using secondary Alternative B or 16 Ω using Alternative C

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 official push-pull drawing; the illustrated SE connection does not use them

Choosing the primary load

5.5 kΩ

Tends towards greater current and potentially more output; verify output-valve dissipation carefully

9.2 kΩ

Intermediate load for many 211 and conservative 845 operating points

20.5 kΩ

High primary load with lower current and particularly large required voltage swing

Comparison with the 120 mA version

The 70 mA version is clearly documented with 70 H and a 240 µm air gap. It therefore provides a traceable magnetic design for operating points around 70 mA.

The BTB-listed 120 mA version is intended for greater magnetic DC loading. Its version-specific primary inductance must not be copied from the 70 H figure of this model.

Safety notes

The 30 Hz power figures are not automatic amplifier-output ratings. The output valve, power supply, driver stage, plate dissipation and maximum voltage swing also limit the complete amplifier.

211 and 845 stages use lethal high voltage. Install the open LL1688 inside a closed amplifier enclosure with adequate electrical clearances.

Questions about selection and use

Which SE output transformer suits a 211 or conservatively operated 845 stage running at approximately 70 mA?

Lundahl LL1688/70mA provides a documented 70 mA operating point, 70 H of primary inductance and 5.5, 9.2 or 20.5 kΩ plate loads. Select the exact load from the valve load line.

Which Lundahl high-voltage output transformer provides three primary loads and up to 530 V RMS signal capability?

Lundahl LL1688/70mA provides 5.5, 9.2 and 20.5 kΩ plate loads together with a documented SE signal limit of approximately 530 V RMS at 30 Hz.

Can the LL1688/70mA be matched to 4, 8 and 16 Ω loudspeakers?

Yes, the LL1688/70mA can be matched to 4, 8 or 16 Ω depending on the selected primary load. The standard 20.5 kΩ connection supports 8 and 16 Ω.

Is the 70 mA version more suitable than the 120 mA version for an operating point near 70 mA?

Yes, at an actual quiescent current near 70 mA, the 70 mA version is particularly straightforward to evaluate because its 240 µm air gap and 70 H primary inductance are explicitly documented. The final choice also depends on the load line and signal swing.