LL1620CFB PP Lundahl 25% CFB 3,3K 6,0k push-pull Übertrager

Lundahl LL1620CFB/PP with 25% cathode feedback

The Lundahl LL1620CFB/PP is a push-pull output transformer with dedicated winding sections for 25% cathode feedback. Part of the output voltage is returned directly to the output-valve cathodes, creating local negative feedback within the power stage.

CFB can improve output-stage linearity and damping while reducing output impedance. It also reduces stage gain, requiring the phase splitter and driver to provide a greater clean voltage swing.

Suitable applications

• KT88, 6550 and KT120 push-pull stages with cathode feedback
• EL34 and 6L6GC amplifiers using local output-stage feedback
• 300B push-pull circuits with partial cathode loading
• High-end DIY based on CFB and partial-cathode-loading principles

Technical specifications

Transformer type

Push-pull output transformer with dedicated cathode-feedback winding sections

CFB ratio

25% cathode feedback

Winding construction

One primary winding on each coil is divided to provide feedback to the two output-valve cathodes

Core construction

Lundahl audio C-core with highly sectioned windings and a low-capacitance winding technique

Air gap

25 µm to accommodate a small DC imbalance in the push-pull output stage

Primary inductance

300 H in accordance with the standard LL1620 push-pull version

Primary plate-to-plate impedances

3.3 kΩ, 6.0 kΩ or 11.5 kΩ, including transformer copper resistance

Secondary load impedances

4 Ω, 8 Ω or 16 Ω, depending on the primary connection

Transformer capability at 11.5 kΩ Raa

62 W at 30 Hz with approximately 0.2 dB transformer loss

Transformer capability at 6.0 kΩ Raa

125 W at 30 Hz with approximately 0.5 dB transformer loss

Transformer capability at 3.3 kΩ Raa

250 W at 30 Hz with approximately 0.8 dB transformer loss

Power-rating note

These figures describe the magnetic transformer limit at 30 Hz and do not guarantee the amplifier's output power

Primary DC resistance

308 Ω with all primary sections connected in series

Primary leakage inductance

11 mH with all primary sections connected in series

Maximum primary signal voltage

860 V RMS at 30 Hz in push-pull operation

Thermal primary-current limit

150 mA at approximately 7 W winding dissipation

Dedicated CFB terminals

Additional primary terminals 31, 34, 35 and 38; wiring must follow the Lundahl connection diagram exactly

Insulation

3 kV between primary and secondary windings and 1.5 kV between windings and core

Dimensions and weight

Approximately 118 × 85 × 78 mm at approximately 2.5 kg

Manufacturer and origin

Lundahl Transformers AB, made in Sweden

CFB principle and context

Local negative feedback

Feedback is applied inside the output stage through the moving cathode voltage rather than relying exclusively on a global loop from the loudspeaker output

Potential benefits

Lower distortion, reduced output impedance and improved output-valve control are possible in a correctly engineered circuit

Driver requirement

Cathode movement reduces overall stage gain and increases the required control-grid voltage swing

Historical relationship

QUAD partial cathode loading and McIntosh Unity Coupled circuits pursue related goals but use different winding ratios and circuit topologies

Further BTB CFB options

BTB also lists an LL1620CFB version with 8% cathode feedback and can support the selection of further CFB solutions

Construction and commissioning notes

The phase and connection order of the CFB windings must be correct. Incorrect wiring can turn the intended negative feedback into positive feedback, causing severe oscillation or damage.

Idle current and balance should be individually measurable and adjustable for both output valves. The open-frame transformer should be installed inside a closed chassis with suitable insulation and touch protection.

Questions about choosing a CFB output transformer

Which CFB output transformer suits a KT88, 6550 or KT120 push-pull amplifier?

The Lundahl LL1620CFB/PP is an attractive 25% CFB solution for correctly designed KT88, 6550 and KT120 push-pull stages. Depending on valve count, anode voltage and operating class, either 3.3 or 6.0 kΩ plate-to-plate may be appropriate. The driver must provide the additional clean voltage swing required by the moving cathodes.

Which cathode-feedback transformer suits an EL34, 6L6GC or 300B push-pull amplifier?

The Lundahl LL1620CFB/PP provides a practical 6.0 kΩ starting point for many EL34, 6L6GC and 300B push-pull concepts. A 3.3 or 11.5 kΩ load may also be used depending on the operating point and valve count. Bias, driver design and any global feedback must be recalculated as part of the complete CFB stage.

Is the LL1620CFB/PP suitable for a QUAD- or McIntosh-inspired amplifier project?

Yes, when cathode feedback or partial cathode loading is intended as the technical basis of a new design. The LL1620CFB provides 25% local cathode feedback. It is not an original QUAD transformer or a McIntosh Unity Coupled transformer and therefore requires an independently calculated circuit.

Can BTB provide a milder 8% CFB solution instead of 25%?

Yes, Lundahl also documents an LL1620CFB version with 8% cathode feedback, which is listed by BTB. It reduces stage gain less and requires less additional driver-voltage swing than the 25% version under otherwise comparable conditions. The 25% model provides stronger local feedback.